Carlo Cecchetto

Caterina Donati

Unprobed movement and labeling: tree trimming as a key to understand successive cyclicity 1

Introduction: the status of unprobed movement

The status of unmotivated movement in syntax is a delicate issue. On the one hand, very general methodological considerations invite us to adopt a restrictive theory in which movement should always have a clear motivation, since in the absence of constraints no theory can make predictions that can be supported or falsified. On the other hand, assuming that movement must be motivated presupposes that it is inherently costly and this assumption is questionably. For example, Chomsky (2008: 137) claims that Merge must be costless, otherwise production of isolated lexical items should be preferred to complex linguistic structures. If movement is defined as Internal Merge, namely an ordinary case of Merge in which the merged category is taken from the derivation itself rather than from the lexicon, movement should also be costless, and in principle should be able to apply freely. This claim is repeated by [START_REF] Chomsky | The UCLA Lectures[END_REF]. If movement is really free, then why does it quite systematically require a trigger?

Although ontological discussions on the status of basic syntactic operations are interesting and even necessary, we believe that they can be productive only if they are implemented in a concrete framework that ultimately allows the consequences of the opposing views to be evaluated empirically.

This is what we will try to do in this paper. We will investigate the notion of unmotivated movement, which, following current minimalist terminology, we will call 'unprobed movement'. We will show that unprobed movement is allowed but severely constrained. Importantly, this approach to unprobed movement will offer a new perspective on two classical topics in syntactic theory: successive cyclic movement and, to the extent that we can deal with this issue in this paper, head movement.

This paper is organized as follows. Section 2 introduces the framework, namely a theory of labels which underlines their role as syntactic devices, and an algorithm for determining labels at each step of the derivation when such a step is probed. Consistently with this system centered on syntactic labels, unprobed movement, which by definition yields an unlabeled syntactic object, is predicted to be very restricted and possess very specific properties. In section 3 a simplification of the Phase Impenetrability Condition (PIC) where only the label remains visible once a phase is completed is defended. This simplified definition can dispense with the spurious notion of "edge" and is consistent with the central role of labels in defining connectedness phenomena. The intermediate steps of successive cyclic movements are reinterpreted as instances of unprobed movement, which allow the relevant phrase to get "tree-trimmed" out of the phase and go back into the working space, ready to be probed by some further probe. Section 4 highlights a consequence of the proposed analysis for the operation by which V exits the vP phase and gets to be probed by T. A general analysis of head movement in terms of unprobed movement is sketched. Section 5 concludes the paper.

Labels and how they are determined

In this section we summarize our assumptions about labeling. Although labels are not always defined, we believe that an explicit definition is useful and we adopt the following definition from Cecchetto & Donati's (2015a):

(1)

Labels. When two objects  and  are merged, a subset of the features of either  or  become the label of the syntactic object {, }. A label: (i) can trigger further computation (ii) is visible from outside the syntactic object {, }.

Under (1), a label plays a crucial role inside the derivation. A label defines which of the features that are merged during a given derivation "stay alive", namely remain active (in Minimalist terms: can act as Probes) and visible (in Minimalist terms: are Goals that can be targeted by a Probe). We call the definition of label in (1) 'internal', since it stresses the role of labels inside the derivation. An important property of the definition in (1) is that labels are not assigned any special role at the syntax/semantics interface. This differs from the view of labels according to which they provide instructions to the semantic component on how to interpret syntactic objects (see recently [START_REF] Chomsky | The UCLA Lectures[END_REF]. We call this the 'external' definition of label, since it stresses the role of labels at the interface, mostly at the C-I interface. In fact, core syntax would be label-free according to the supporters of the external definition (these include [START_REF] Collins | Eliminating labels[END_REF][START_REF] Seely | Merge, derivational c-command and subcategorization[END_REF][START_REF] Narita | Endocentric structuring of projection-free syntax[END_REF]Chomsky 20015;[START_REF] Chomsky | The UCLA Lectures[END_REF], cf. Stockwell 2016 for a state-of-the-art article on this debate). We believe that the main reason to reject an external definition of labels is that syntactic categories with the same label can correspond to different semantic types (a DP can be an individual, a generalized quantifier or a property; vice versa, a proposition can be a CP or a DP; a predicate can be a VP, an NP, an AP, and so on and so forth). This makes the role of labels as instructions for the semantic interface dubious at best. On the contrary, there are good reasons to assume an internal definition: first labels do play an important role in determining derivations in dimensions such as locality and intervention (see Cecchetto & Donati 2015a for a more extensive discussion and Frampton & Gutman 2002). Second, as we will explain in Section 3.1, the external definition of label inevitably implies a non-negligible computational cost.

Recently [START_REF] Chomsky | The UCLA Lectures[END_REF] has proposed a new conceptual argument against labeling in syntax. He argues that the proper definition of Merge should obey a 'third principle' requirement (Restrict Resources) which dictates that the number of objects in the Working Space should be augmented by at most one by any application of Merge2 . The Restrict Resources principle would be violated if when  and  are merged, the output is the object   plus the label of this object. The reason is that before Merge there are two objects in the Working Space ( and ) while after Merge there are four, namely ,   ) and the label of  . We are sensitive to the conceptual argument but we believe that it does not apply to the definition of label in (1). What (1) says is that each application of Merge is a case of 'feature stripping', namely (1) states that only a subset of the features of one of the two merged objects remains active at the next level up. We believe that, as a process of reduction of available information, labeling obeys the Restrict Resources principle and can even be seen as a response to it.

No matter whether labels are external or internal to syntax, any theory of labeling must be integrated with the formulation of the proper algorithm(s) that determines a label for each syntactic object that is being formed.

There are two main configurations to consider, namely Merge of an atom of the syntactic derivation (call it Lexical Item, LI) with an already formed syntactic object (call it a phrase), as in (2), and Merge of two phrases, as in (3):3

(2) LI, XP (3) XP, YP For the case in (2), although the technical implementations may vary, there is a general consensus that the LI provides the label. One simple way to capture this is [START_REF] Chomsky | Problems of Projection[END_REF] Minimal Search principle. When LI and XP are inspected for label determination, LI is immediately detectable, being simpler.

The case in (3) is more complex because Minimal Search principle cannot return a label, as both XP and YP are equally complex in the relevant sense. In the labeling theory literature, there are two families of approaches to handle the problem raised by the configuration in (3). The first approach builds on the fact that Merge is not the only operation that takes place in core syntax. The other operation, namely Agree, is inherently asymmetric, since a Probe looks for a Goal that can value some of the unvalued features of the Probe (a paradigmatic example is T which probes the phi-features of the external argument). Therefore, the proposal is that when a Probe and a Goal are merged, it is the Probe that provides the label. If selection, qua asymmetric operation, is reduced to a special case of Probing, this approach can also derive the fact that the selector provides the label. Proponents of this approach include Adger 2003; [START_REF] Pesetsky | Probes, Goals and Syntactic Categories[END_REF]Cecchetto & Donati 2015 a.o.). Of course, when Merge is dissociated from probing, which is possible if Merge can apply freely, the object created by Merge is predicted to remain without a label.

The second approach to deal with the labeling problem associated to the configuration in (3) is the one developed by [START_REF] Chomsky | Problems of Projection[END_REF][START_REF] Chomsky | Problems of projection. Extensions[END_REF]. It can be illustrated with the case of the external argument (EA) of a transitive verb. In this configuration no structural asymmetry can lead Minimal Search into finding a label. As Chomsky assumes that an object can remain without a label until it accesses the C-I interface, he considers unproblematic that the EA, v object remains without a label when it is created. However, the label-less configuration EA, v is destroyed when the external argument is internally merged with T (incidentally, this requires the non-innocent assumption that the copy of EA is not visible for labeling). As for the EA, T object created by Internal Merge, it will be labeled by a mechanism of feature sharing: T and EA share phi-features, therefore the EA, T object will have a label <ϕ, ϕ>. This approach can extend to all cases in which the moved category and the target share features (these include Criterial features like Q, Topic, Focus etc., as developed by [START_REF] Rizzi | Cartography, criteria, and labeling[END_REF]2016).

In this paper, we will adopt the first approach, the one that builds on the asymmetric character of Probing, because we believe that assuming a syntactic object can freely remain label-less as long as it will be eventually destroyed later on in the derivation introduces a significant computational burden and is in sharp contrast with Third Factor principles that aim at minimizing computational costs (we go back to this in Section 3.1).

Unprobed movement and successive cyclic movement

The internal and external definitions of label briefly discussed above make opposite predictions concerning the type of syntactic objects that can remain label-less (if any). If labels are needed internally for the computation to proceed (as we will assume here), the expectation is that every syntactic object will need a label, except the root, where the computation ends. On the other hand, under the external definition, labels are needed at the interface, so a label is certainly needed at the end of the cycle (the root), when the structure is sent to interpretation. Let's us keep this in mind while discussing successive cyclic movement.

In the generative tradition, there is a conceptual motivation for why long movement must proceed by intermediate steps: the computational load is alleviated, as desirable, if, once the domain that contains a sub-link is completed, its internal details can be "forgotten".

To be concrete, consider (4), in which the wh-phrase must move from the embedded VP to the matrix CP. Assuming no successive cyclic movement, all the information about the entire structure must be retained until the very end of the derivation. However, if movement proceeds as shown in (4'), each time the wh-phrase escapes from a given domain by moving to its edge, the details concerning that domain exit the working space and stop being accessible (they can be "forgotten"). Supposedly, this alleviates the computation burden. In (4') we hypothesize, as is common practice, that successive cyclic movement of the wh-phrase targets the positions Spec,vP and Spec,CP on the path from its base position to its final landing site. Notice however that the conceptual argument is independent from the specifics of the way successive cyclic movement is implemented within a more fine-grained analysis of the CP system4 .

(4)

Which student did Mary say that John thinks that Bill met? (4') Which student did Mary [v [which student]

say [C [which student] that John [v [which student] thinks [C [which student] that Bill [v [which student] met [which student]]]]]]?

The Phase Impenetrability Condition: the problem with intermediate steps

In current minimalist frameworks, the principle that forces long movement to take place in a successive cyclic fashion is the Phase Impenetrability Condition in ( 5).

(5)

Phase Impenetrability Condition

The complement of a strong phase  is not accessible to operations at the level of the next highest strong phase , but only the head and the edge of  are [START_REF] Chomsky | Derivation by phase[END_REF].

The definition of Phase Impenetrability Condition must be complemented by the list of domains that act as phases. These include vPs and CPs (the status of DPs and PPs is more controversial, see Matushansky 2006; Abels 2012 a.o. for a discussion). Phase Impenetrability Condition explains why movement must be successive cyclic by postulating a "time bomb mechanism", where the "explosion" moment is when a given phase stops being accessible. For example, in (6) the complement of vP (namely VP) stops being accessible by the moment C enters the derivation. Therefore, 'which student' must leave the VP before that moment, or remain otherwise buried within the phase. This is schematically shown in (6'), where we indicate in grey the structure that becomes inaccessible.

(6) Which student did you meet?

(6') a. [T you [T T [v [V meet which student ]]]] b. [C did [T you T [v which student [v [V meet [which student]]]]]] c. [C [which student] [C did [T you T [v [which student [v [V meet [which student]]]]]]
The same logic applies to the case in ( 7), but this time there are three copies of 'which student' in intermediate positions, since three phases have been crossed (embedded vP, embedded CP and matrix vP).

(7)

Which student do you think that John met?

(7') [C [which student] do [T you [v [which student] [V think [C [which student] [C that [T John [v [which student][V met [which student]]]]]]]]]]
While the definition of Phase Impenetrability Condition in (5) successfully explains why (long) movement is successive cyclic, it suffers from some drawbacks, all of them widely discussed in the literature (cf. Lahne 2008 a.o.). The first concerns the identification of the categories that behave like phases. The initial motivation was that CPs and vPs are phases because they have a special status at the interface(s), for example they are 'propositional' (sentences are CPs and vPs are the domains where theta roles are assigned). However, under the definition of Phase Impenetrability Condition in ( 5) what exits the derivation and gets to the interface is not the phase (the object with a special status), but its complement (hence, by definition, an object without any special status at the interface). A second reason makes the definition of Phase Impenetrability Condition in (5) cumbersome: the category that is allowed to escape from the phase is spurious. In fact, what is taken to survive inaccessibility under ( 5) is two quite different objects: in traditional terms, they can be identified as the specifier and the head of the projection that counts as a phase. While there is an intuitive sense in which the Spec is at the edge, this is much less obvious for the head. In general, what is hidden under the concept of 'accessibility' in ( 5) is really a negative definition: what remains accessible is whatever is not a complement. Negative definitions are problematic because they reveal the incapacity to define a theoretical object by identifying (some of) its core features. There is a reason for this loose definition of 'edge': it is precisely designed to allow head movement and A'-movement out of phase (we go back to this in Section 4). However, the biggest problem with the definition of Phase Impenetrability Condition in ( 5) is that it does not explain what motivates the movement to the edge of the phase. Certainly, this movement is ultimately beneficial, because it allows a wh-phrase (or any other category that undergoes long distance movement) to escape the phase that becomes inaccessible. However, unless unprobed movement is allowed, a motivation (a Probe) is necessary for each movement step. For certain languages and for certain types of movement a motivation for each step may be available. This holds for languages like Chamorro, or Irish or Asante Twi, where intermediate steps have a morpho-syntactic reflex, and it is thus reasonable to assume that they are morpho-phonologically triggered, hence probed (cf. Henry 1995 in Belfast English; [START_REF] Chung | The design of agreement: Evidence from Chamorro[END_REF]Lahne 2008 for Chamorro;[START_REF] Mccloskey | Transformational Syntax and Model Theoretic Semantics: A Case Study in Modern Irish[END_REF][START_REF] Mccloskey | Quantifier Float and Wh-Movement in an Irish English[END_REF] for Irish; Korsah & Murphy 2019 for Asante Twi). But in many (most?) other languages, embedded CPs and vPs do not show any morpho-phonological feature associated to intermediate steps of successive cyclic movement.

In order to deal with this problem, [START_REF] Chomsky | Derivation by phase[END_REF] assumes that intermediate Cs (and vs) can be endowed with an abstract (EPP) feature which probes the category that undergoes successive-cyclic movement. However, this move is far from being satisfactory. First, this EPP feature suspiciously looks like a deus ex machina. Furthermore, as stressed by [START_REF] Bošković | On Successive Cyclic Movement and the Freezing Effect of Feature Checking[END_REF], we are forced to assume that an intermediate C has the relevant EPP feature if and only if C happens to be on the path of a category that undergoes long movement. In other cases, the intermediate C cannot have the EPP feature, otherwise sentences like (8) should be grammatical, with 'which student' moved to the intermediate C endowed with a probing EPP feature.

(8) *Who thinks which student that John met? Therefore, the EPP feature solution is totally ad hoc.

A way to handle the problem of the lack of motivation for successive cyclic movement is simply dropping the requirement that Internal Merge is motivated and that every syntactic object must be labeled (Chomsky 2013: 44). As we say in Section 2, the idea defended by [START_REF] Chomsky | Problems of Projection[END_REF][START_REF] Chomsky | Problems of projection. Extensions[END_REF] is that structures of the form {XP, YP} that are not associated with any type of feature sharing (including the intermediate step of successive-cyclic-movement) have no label and are tolerated, as long as they are destroyed by the end of the derivation. For example, the structure in ( 9) is tolerated if by the end of the derivation (the step illustrated in 9') 'which student' and the dedicated head in the COMP area where it eventually sits share a Q feature and hence make up a labeled syntactic object.

(9) [ [D which student] [C that John met]] (9') [Q [D Which student] [C do you think [ [D which student] [C that John met]]]]?
This analysis requires adopting the external definition of label, discussed above, according to which labels are needed only at the interface. But we believe that this analysis suffers from a major drawback. For the sake of the argument, assume that that the structure in ( 9) has been created and can be temporarily tolerated. Still, the derivation must proceed in order to reach the resolutive step in (9'). The intended derivation is one in which the label-less object is selected by 'think' but it is not clear how selection can take place in absence of a label. Selection requires a label, but a label is not (yet) present when it is needed. It seems that the no-labeling approach to successive-cyclic-movement requires that any sort of merging and probing operations be allowed freely, at least until the interface is accessed. This amounts to assuming that multiple alternative derivations are allowed, including those that will be filtered out at the interface. Allowing 'ungrammatical' sub-derivations to run freely, although temporarily, seems to us a clear case of computational inefficiency.

This problem is recognized by proponents of the idea that label-less objects are not problematic for the derivation. For example, [START_REF] Ott | Grammaticality, interfaces, and UG[END_REF] stress that if access to the C-I interface is done cyclically the deviance will detected quickly (at the phase level). However, at most this minimizes the problem, it does not solve it. Even more importantly, there is another issue pointed out by Stockwell (2016:139-140):

"if Merge is entirely free, there is nothing that could force a phase head to be merged. This means that a derivation could continue through an infinite number of derivational steps without ever being transferred to the interfaces. Rather than ensuring that a fatal problem is spotted reasonably early, a problem may never be spotted at all…. Second, even merging a phase head would never necessarily resolve the issue. The specifier of the phase head is not transferred to the interfaces in cyclic spell out; only its complement is. It seems that on the free-merge approach Internal Merge can operate as 'interface bounceback'. The edge feature allows any SO with an unvalued feature that is rejected by the interfaces to be merged as a specifier of the phase head. This would allow any deviant or crashing element to escape from the phase, allowing the derivation to continue through infinitely many phases without the deviance being noticed. Far from identifying deviance early on, phases could in fact prolong the agony."

Given this consideration and in agreement with so-called crash-proof syntax [START_REF] Frampton | Crash-proof syntax[END_REF], we maintain that labels play a pivotal during the derivation, also by determining selection.

However, we do think that the intuition that the intermediate steps of successive cyclic movement involve the creation of an unlabeled node is correct. In the next section we propose a novel implementation of this idea that is fully compatible with the internal definition of label and does not suffer from the drawbacks we just mentioned.

The tree trimming mechanism

As anticipated, in order to fix the problem of the absence of a motivation for intermediate steps of successive cyclic movement, we take the bull by the horns and assume that the intermediate step of successive cyclic movement in languages like English, where it has no overt morphological reflex, is what it looks like: an instance of unprobed merge. Before getting there, we also propose the version of Phase Impenetrability Condition in (10), which is much simpler than the canonical cumbersome definition in (9) and is consistent with the central role that labels have in syntax in the system we are defending here.

(10) Simplified Phase Impenetrability Condition.

When a phase is concluded, only its label remains accessible to further syntactic computation.

The definition in (10) has several advantages: (i) it does not contain any reference to the spurious notion of edge;

(ii) it does not postulate a 'time bomb' mechanism, since a phase stops being accessible right when it is concluded and not at a designated point later on in the derivation; and (iii) what has a special status at the interface is the phase, not its complement. Apparently, there is a huge price to pay for this simplicity, namely (10) does not seem to allow any movement out of the phase. This is clearly wrong, as movement out of a phase does occur. In this section we deal with this problem as far as Successive Cyclic Movement is concerned. In Section 4 we shall briefly address head movement, which in some cases also targets a position outside the phase.

We can illustrate our approach to successive cyclic movement based on (10) by considering once again the derivation of the sentence (7) above, repeated as (11). For simplicity we illustrate only the step involving the intermediate step to the embedded Spec,CP. However, the same logic applies to the intermediate step involving the edge of vP.

(11) Which student do you think that John met?

Step 1: The wh-phrase is internally merged to the embedded C as a case of unprobed merge.

(11') [ [D Which student] [C John met with student]]

If the label is determined by the probe, as we assume, no Probe means no label, so an unlabeled syntactic object is created. This is temporarily OK because derivationally speaking the structure to which 'which student' merges is a root clause. By definition, a root clause does not need to enter further derivation, hence does not need a label (under the internal definition we are defending here). However, a problem arises when the main verb 'think' enters the derivation because, under the definition of label in (1), an unlabeled syntactic object is not visible and cannot be selected. Let us see what happens in the next steps.

Step 2: Under the simplified Phase Impenetrability Condition in (10), only the label C remains in the computation, not the syntactic object under the label (here we indicate an object that becomes inaccessible by using a grey color). Remember that, under the definition (1), the label C is a subset of the features of the embedded C. This must include the categorial feature C.

(11'') [ [D which student] [C John met which student]]

At this point, 'which student' and the label C do not form a constituent because the syntactic object to which 'which student' has attached in the former cycle is not accessible any more (it has abandoned the derivation). The label C and 'which student' are hence both available for further derivation, but as disconnected elements. It is convenient to give a name to the mechanism according to which two objects that were previously connected become disconnected, because one of them becomes inaccessible under the simplified PIC in (10). Let us call this mechanism 'tree trimming'. We assume that simplified PIC (and tree trimming as a result) are a consequence of the Restrict Resources principle, which minimizes the number of objects allowed in the Working Space.

After tree trimming, the label C and 'which student' still belong to the working space of the derivation. Under the reasonable assumption that by the end of the computation any material in the working space needs to be merged into the structure, both these elements need to be integrated into the new structure under construction.

Step 3: The label of the (now inaccessible) embedded clause, namely C, is probed by the selecting verb 'think':

(11''') [V think [C _ _ _]]
The structure gets a label from the probe (matrix V), under the assumption that selection is a form of probing.

Step 4: 'which student' remains in the working space until a suitable probe kicks in, with which it can merge: this is the matrix C. When matrix C probes 'which student', it labels the resulting syntactic object, which ends up having label C:

(11'''') [C [D Which student] [C do you [V think [C _ _ _ ]]]?
The advantages of the tree trimming approach to successive cyclic movement should be self-evident. First, tree trimming is a direct consequence of the minimally simple definition of the Phase Impenetrability Condition in (10). Second, the tree trimming approach correctly restricts "intermediate steps" to phase edges only, without stipulating any edge or special EPP feature attached to them5 . In any other location, an unprobed merging operation would yield an unlabeled syntactic object that would not be "trimmed" and would therefore stop the derivation. Third, it avoids the computational costs discussed above for other implementations of the idea that successive-cyclic-movement creates an unlabeled object6 .

An interesting question arises concerning those morpho-syntactic reflexes of intermediate steps attested in some languages, which we mentioned above. By the way tree trimming works, there is no proper multi-links chain in so-called successive cyclic movement, which would be composed by the copy in the landing site, the one(s) in the intermediate position(s) and the one in the base position. Only two copies are assumed to be represented through the derivation, the head and the foot, while the copy merged unprobed at the edge of the phase is immediately removed through tree trimming. But if there is no copy where the morpho-syntactic reflex of so-called successive cyclic movement arises, how can this reflex be explained?

Let us address this question by focusing on one easy case among those cited above, namely auxiliary inversion in Belfast English. The basic facts are the following: as in Standard English, auxiliary inversion is observed in direct questions (cf. 12a). Unlike Standard English, Belfast English requires auxiliary inversion in embedded questions (cf.12b) and even in the intermediate site of successive-cyclic movement (cf. 13). Crucially, no such inversion is possible in positions lower than the gap ( 14). (Belfast English, Henry, 1995: 106) In principle, this can be interpreted as evidence that it is the general mechanism of successive-cyclicmovement which triggers auxiliary inversion and that the latter is the mechanical reflex of the former. However, things are more complicated than this. In fact, other instances of successive-cyclic-movement do not trigger auxiliary inversion. This is shown by the ungrammaticality of (15) in which there is long distance movement of the relative pronoun 'who', which must have passed through the CP of the complement clause of the verb 'claim'. Still, embedded auxiliary inversion is not allowed.

(15) *This is the man who John claimed who did I see. (Belfast English, Henry, 1995: 106) As rightly stressed by [START_REF] Abels | Phases. An essay on cyclicity in syntax[END_REF], this means that auxiliary inversion is not a simple reflex of successive cyclic movement. Depending on the nature of the moving category (relative versus interrogative) and of the embedded complementizer, inversion takes place or not. This strongly suggests that the movement step that is responsible for auxiliary inversion in Belfast English is feature triggered, namely probed, hence challenging the approaches that take successive cyclic movement to involve an unprobed step (including the tree trimming analysis). However, notice that, under the tree trimming analysis, if the embedded complementizer is a suitable probe, nothing prevents it from attracting a given category, before this category further moves unprobed to the edge of this CP, ultimately tree trimming. We hypothesize that this is what happens in (13), as schematically illustrated below:

(13') Who did John hope [who [C who would he see who]]

The question remains of what is the feature which is probed by the embedded C in ( 13), and whether this kind of explanation can be extended to all the other cases where intermediate steps of successive cyclic movement have morpho-syntactic effects that have been discussed in the literature. Since this paper is dedicated to the general mechanism of successive cyclicity, a fully worked out case by case identification of the features that can probe the step preceding unprobed movement is outside our scope. However, a point is important: this feature cannot be the same as the one that is responsible for the final probing, which will allow the moved and tree trimmed category to re-enter the derivation. The reason is that we make the standard assumption that a specific feature can be checked only once in the derivation. To be concrete: in the case of (13), if the interrogative wh-feature of 'who' were checked in the embedded CP, 'who' could not re-enter the derivation because it could not be probed by matrix CP. This is of course in line with the fact that in (13) the embedded C is not interrogative: if it probed an interrogative wh-feature, there would a mismatch between its interrogative nature in syntax and its non-interrogative interpretation. We stress this because it shows how the tree trimming mechanism can capture core cases of what Rizzi (2010) calls Criterial Freezing effects, namely the fact that once a phrase enters into a criterial configuration, it normally becomes unavailable to further movement. Given tree trimming, this translates into the fact that once a category  has entered into a checking configuration for a feature , it can still move unprobed to the edge of a phase and go back to the working space. But that same feature  cannot be probed again, allowing  to re-enter the derivation (a different feature in  can be probed if available, though).

As for (15), we hypothesize that the embedded C does not probe the relative 'who', much like in Standard English, and therefore unprobed movement is not preceded by a probed movement step 7 .

Perspectives on Head Movement

Still, it is uncontroversial that a head can move out of a phase, as in V to T in Romance, or Verb Second configurations and so-called "long head movement", instantiated for example by participle fronting in Bulgarian (cf. Lema & Rivero 1990 a.o.). This is to be expected if there are no rule-specific constraints. Still, prima facie at least, the Simplified Phase Impenetrability Condition in (10) does not allow head movement out of a phase, since the notion of edge of the phase (which, by stipulation, includes the head of the phase) is dispensed with. The system outlined in the paper allows a way by which a phase head can escape the phase, though. We illustrate this with movement of V out of the phase vP: V can get out of vP via unprobed movement to its edge (cf. 15). ( 15

) [ V [v v [V V ]]] unprobed movement
At this point however a difference emerges with respect to the successive cyclic movement case discussed in section 3: V is a lexical item and under minimal search it can provide a label. Therefore, V can label the structure formed after it has moved unprobed to the edge of the vP. Informally, we can say that vP is turned into a category with label V 8 . ( 16)

[V V [v v [V V ]]]
It is only at this point that, under the simplified Phase Impenetrability Condition, the object under the label v, i.e. the phase, becomes inaccessible. 7 A rather different type of effects that are normally taken as empirical motivation for successive-cyclicmovement concern interpretation, cf. [START_REF] Barss | Chains and anaphoric dependence[END_REF]. For example, assuming that an anaphor in a picture-NP is subject to Principle A (something that, incidentally, is fairly controversial, cf. [START_REF] Reinhart | Reflexivity[END_REF], the fact that the anaphor can be bound by 'John' in (i) is taken as evidence that a copy of the picture-NP sits at the edge of the most embedded CP, where 'himself' is locally bound by 'John'. (i) Which picture of himselfi does Mary believe that Johni thinks that she is going to buy? Assuming tree trimming, we would be forced to say that the operation that moves the anaphor close to its antecedent in (i) is s a topicalization-like movement. We assume that this movement is probed and leaves the copy that counts for binding purposes and that it is followed by unprobed movement and tree trimming. Cf. Abels 2012 for a survey of the many interpretation reflexes of successive movement that have been discussed in the literature. 8 Notice that we are implicitly endorsing here an approach à la [START_REF] Matushansky | Head movement in linguistic theory[END_REF], who propose that canonical head movement configurations involve "normal" syntactic movement to a specifier position, followed by a morphological process, which she calls m-merger, by which the moved verb gets fused with its affixes. Cf. also [START_REF] Harizanov | Whither head movement? Natural Language and Linguist Theory[END_REF] for a different implementation of the same idea.

(17

) [V V [v _ _ _ _ _ _ ] ]
This means that there is a difference between what happens after unprobed movement of a head and after unprobed movement of a phrase. In the latter case, tree trimming takes place and both the label of the phase and the moved phrase re-enter the working space, where they remain until a suitable probe attracts each of them back into the derivation. However, we do not assume that tree trimming takes place after a head, say V, moves unprobed to the edge of a phase, say vP. The reason is that V, as a lexical item, can provide the label to the resulting structure, even if the movement by itself is unprobed. Therefore, for tree trimming to take place at that level would involve destroying the node labeled V created on the top of vP. We assume that this is blocked by the No-Tampering Condition, which says you should not destroy a structure that has been created. Of course, in case of unprobed movement of a phrase, the No-Tampering Condition is irrelevant, since there is no label to destroy, given that unprobed movement has created no label to begin with. There is some empirical evidence suggesting that V does not re-enter the working space. Consider the ungrammaticality of sentences like (18), traditionally explained as a violation of the Head Movement Constraint [START_REF] Travis | Parameters and effects of word order variation[END_REF]. If V could go back to the working space after its unprobed movement to v, the sentence in (18) would be predicted to be OK, contrary to facts. This is so because V could be probed by C and merged there directly. On the other hand, if the head V remains in the derivation after its unprobed movement to vP, obvious locality considerations make 'will' closer to C than 'buy'. Notice an important prediction of the analysis of V-to-T that we are sketching: if the label of vP becomes V, T should select V, not v. We think that this prediction is borne out (and is at the basis of the notion of extended projection introduced by [START_REF] Grimshaw | Extended Projections[END_REF]). We offer the following piece of empirical evidence here: T is sensitive to the lexical semantics of the verb, which is expressed at the level V , not to argument structure which is expressed at the level v. We illustrate this with Italian. Consider the imperative. The inflection of imperative (expressed in C but present at T as well) selects specific lexical verbs, irrespective of their thematic properties. This is shown by the following examples. Unaccusative, unergative, and transitive verbs can be imperative (cf. 19 a-c) but only if the imperative is compatible with their lexical semantics. *Svieni! (unaccusative) Faint-IMP The pattern in (19) can be described by saying that (imperative) T must see the label V while it is not sensitive to the label v.

Conclusion

This paper addressed the issue of the status of unprobed merge in syntax, which we have claimed to be possible but severely restricted since it cannot create a labeled syntactic object. While in general we have been assuming that the presence of an unlabeled object, which is syntactically invisible, blocks the derivation, this problematic configuration can be tolerated at phase edges because it is destroyed when the phase exits the derivation, according to the mechanism which we called tree trimming. This is the direct consequence of the simplified version of the Phase Impenetrability Condition that we have proposed, which assumes that, when a phase is completed, only its label survives. As a result, any material that has merged unprobed to the phase itself gets "detached" (tree trimmed) from it when this becomes inaccessible and this material is put back into the working space. Successive cyclic movement can be modeled this way, as we have discussed in details in this article. We believe that the approach outlined in this article, which opens the way to considering successive cyclic movement and head movement as a new natural class, that of unprobed movements, can not only provide a new explanation to old problems, but indirectly makes a strong case in favor of a label-driven syntax in response to recent attempts (cf. [START_REF] Chomsky | Problems of Projection[END_REF][START_REF] Chomsky | The UCLA Lectures[END_REF] to expunge labels from syntax and relegate them to the interface.

(

  12) a. What have you done? b. She asked [who had I seen]. (13) Who did John hope [who would he see]? (14) *Who do you think [who did John tell who [did Mary leave]?

(

  18)* [C What [C buy] [T John [T will] [v buy] bread]] ?

We thank an anonymous reviewer for very useful comments. And…happy birthday Léa!

Chomsky refers in his argumentation to MERGE (in capital letter), which is an operation mapping a workspace into another (slightly bigger) workspace. MERGE replaces 'traditional' Merge in the framework sketched by[START_REF] Chomsky | The UCLA Lectures[END_REF]. However, the conceptual argument made in the text is general enough to apply to different conceptualizations of Merge.

A third configuration corresponds to step one of any derivation, namely Merge of two lexical items. For this, see[START_REF] Chomsky | Problems of Projection[END_REF] andCecchetto and Donati (2015: 2.9.1).

The second type of motivation for successive cyclic movement is empirical: in various languages there are overt reflexes of the intermediate traces of the moved wh-category in the relevant intermediate positions. We go back to some of these data later in the text (in particular Section 3.3).

Under this aspect the tree trimming approach we are defending, although it assumes no probe for intermediate steps, qualifies the path of successive cyclic movement as punctuated, not uniform, under the terminology proposed by[START_REF] Abels | Phases. An essay on cyclicity in syntax[END_REF].

The account proposed in the text is similar but not identical toCecchetto & Donati's (2015). An important consequence of the tree trimming approach is that it can explain for free the occurrence of strong island effects. We refer toCecchetto & Donati (2015: 4.4 -4.7) for an elaboration of this important point.


A Bounhir

Z Benkhaldoun

B Mougenot

M Sarazin

E Siher

L Masmoudi

Munoz Tunon

Aerosol columnar characterization in Morocco: ELT prospect

Keywords: site testing, atmospheric effects, data analysis, photometer. 1-Introduction

The work presented in this paper focuses on site testing in terms of aerosol loadings where ground based measurements are essential. In our case they are materialized by the aerosol optical thickness (AOT) and are provided by the AERONET (Aerosol Robotic Network) network from four stations, Dakhla and Marrakech in Morocco and Santa-Cruz and Izana in the Canary Islands. To fully scan all the area of the Moroccan territories, satellite measurements are certainly the most efficient way. We used the most popular and reliable products. TOMS (Total Ozone Mapping Spectrometer) aerosol index (AI) provided by both TOMS Earth Probe and TOMS OMI (Ozone Monitoring Instrument) along with aerosol optical thicknesses provided by MODIS (Moderate Resolution Imaging Spectroradiometer) and MISR (Multiangle Imaging Spectroradiometer) instruments, onboard Terra platform. The idea is to compare sensing capabilities of each instrument in the region under study, in order to know which is suitable for a given place and when. For that purpose linear regression analysis were performed between satellite data and AERONET observations. Good correlations were observed with the Pearson correlation coefficient, R, varying from 0.68 to 0.92 for MODIS, MISR and TOMS OMI. However for TOMS EP the correlations are fairly poor (from 0.54 to 0.74). A ten years analysis of the TOMS EP index has been performed with a calibration of the aerosol index into TOMS retrieved aerosol optical thickness in the area of interest (Morocco and Canary Islands) and an inter-comparison with the other products was achieved. In the frame of the ELT (Extremely Large Telescope) project prospect, once the appropriate satellite instrument have been chosen and the area scanned, the next step would be to scan aerosol loadings at higher altitude locations. Since vertical distribution of aerosol optical thickness and microphysical properties are not well understood and modelized, we used the relationships related to Izana (Izana's altitude is 2367 m), as a first attempt, to extrapolate the aerosol optical thickness at higher locations in the Moroccan mountains. Izana and Santa-Cruz very close to each other (30 Km) are located in the same satellite pixel and then have the same satellite (AOT) or (AI) whereas AERONET gives very distinct aerosol optical depths. A good linear correlation (R=0.92) has been observed between the AERONET aerosol optical depths at Izana and Santa-Cruz. The seasonal correlation coefficients are 0.85 for winter, 0.87 for spring, 0.91 for summer and 0.87 for autumn The ratio AOD Santa-Cruz /AOD Izana has a seasonal behavior, reaches the average of 4.5 in winter and 2 in summer time and the subtraction of the aerosol optical thicknesses has an average of 1.3. Finally we retrieved the aerosol optical thickness at Oukaimeden: a Moroccan observatory located at 2700 m above sea level, and about 70 km from Marrakech city. We then converted the aerosol optical depth into astronomical light extinction and compare with previous records measured at the observatory.

2006], a quantitative survey of the aerosol loadings and their microphysical and optical properties is an essential part of the site selection process. The study of sky transparency has been initiated by astronomers, who were carrying about photometric characterization of their observatories. Stellar photometric measurements are carried out in most observatories as routine astronomic observations, since 1950 (1970) for La Silla and (1980) for La Palma. Before daytime photometry being popular, atmospheric scientists took expertise from astronomers [START_REF] Laulainen | Analysis of atmospheric extinction data obtained by astronomers: II. Seasonal variations in astronomical extinction[END_REF][Laulainen et al -a, 1977-b]-b].

[P. [START_REF] Formenti | Aerosol optical depth over a remote semi-arid region of South Africa from spectral measurements of the daytime solar extinction and the nighttime stellar extinction[END_REF]2002] have used a record of atmospheric nighttime turbidity data from astronomical stellar radiation measurements, in South Africa, to study changes in aerosol concentrations over the past three decades prior to the popularization of sun photometry. Astronomical light extinction coefficient can be converted to aerosol optical depth and vice versa. Since the dilemma concerning the heating of the Earth, the new field of aerosols; their optical and microphysical properties with sun photometry, LIDAR techniques and satellite remote sensing, is emerging and growing very fast. Now astronomers can take advantage of these well developed tools to characterize sky transparency with even more deeper insight. The astronomical light extinction (A) is determined by comparing the apparent brightness, B, of the standard stars with there intrinsic luminosity at different air masses, through the Beer's law. The Langley technique relying on the plot of ln(B) versus air masses is used to extract the astronomical light extinction which is the slope of the linear regression. Daylight photometry is based on the same physical principles: I λ = I λ0 *exp(τ tot *m); I λ is the direct solar irradiance, I λ0 is the extraterrestrial irradiance, λ is the wavelength, τ tot is the total atmospheric optical depth; the sum of the optical depths of molecules (Rayleigh scattering), gazes (absorption) and aerosols (Mie scattering and absorption), m is the inverse of the cosine of the zenith angle θ. As for nighttime photometry, the linear regression Langley procedure is applied to extract the atmospheric total optical depth. The aerosol optical depth is then calculated by subtracting to τ tot the aerosol optical depth of Rayleigh scattering τ R and the Ozone absorption optical depth τ O3 in the Chappuis bands, and the absorptions of water vapor, carbon dioxide, methane and nitroxyde, depending on the wavelength used. The astronomical light extinction coefficient A is related to the total optical depth: A=1.086*τ tot . The scattering by molecules has a predictable profile depending on the wavelength, altitude and the refractive index of molecules. The optical thickness of ozone is calculated using daily averages from TOMS satellite. The rest of the paper is organized as follows. In section 2, we describe the data used in the current study, the area of interest, and the methodology adopted. In section 3, we show the details and the results of the regression analysis performed between ground measurements and the different satellite data. Section 4 deals with comparison of ground measurements of Izana and Santa-Cruz. In section 5, first the TOMS EP index is converted to aerosol optical thickness mapping the area of study, and then we made a comparison of long term satellite AOT retrievals. Finally, we retrieve the AOT at Oukaimeden observatory, by different approaches. In order to compare to previous measurements of astronomical light extinction coefficient performed at the observatory during the year 1997, we converted the AOT to astronomical light extinction coefficient over 1997 using TOMS EP records.

2-Study area, data and method

2-1 Study area

Two AERONET stations located in Morocco; Marrakech (lat=31.6°; lon=-8.15°; alt=420m) and Dakhla (lat=23.7°; lon=-15.95°, alt=12m), have delivered data during at least two years. Close to Morocco, two other AERONET stations located in the Canary Islands; Izana (lat=28.3°, lon=-16.5°; alt=2367m) and Santa-Cruz(lat=28.5°, lon=-16.25°, alt=52m) have delivered data during relatively a long period of time. The mape showing the locations of the AERONET sun-Sky radiometers used in this study is shown in figure1. These sites span desert dust aerosol predominant environment.

2-2 Data source a-AERONET aerosol optical thickness

We used data acquired by The AERONET (AErosol RObotic NETwork) program established by NASA and LOA-PHOTONS (CNRS). The instruments deployed are automatic sun and sky scanning radiometers (CIMEL). The photometer makes measurements of, direct sun and diffuse sky radiances, within the spectral range 340-1020 nm. The direct sun measurements are acquired in eight spectral channels. The 940 nm band is used to estimate total precipitable water content. The bandwidths of the interference filters vary from 2 to 10 nm. The total spectral optical depth is computed through the Beer-Lambert-Bouguer law after correction taking into account gaseous absorption. Sky measurements are performed at 440, 670, 870 and 1020 nm with two basic sky observations "almucantar" and "principal plane". The philosophy is to acquire aureole and sky radiances observations through a large range of scattering angles from the sun through a constant aerosol profile to retrieve size distribution, phase function and single scattering albedo.

Data are transmitted through the DCS Data Collection System operating the geosynchronous GOES, METEOSAT, and GMS satellites and are available at the sun photometer home page http://aeronet.gsfc.nasa.gov. The instruments are described in detail in [START_REF] Holben | AERONET-A Federated Instrument Network and Data Archive for Aerosol Characterization[END_REF]] and the inversion procedure is well documented in [Dubovic et al;1998[START_REF] Smirnov | Cloud sceening and quality control algorithms for the AERONET database[END_REF][START_REF] Smirnov | Cloud sceening and quality control algorithms for the AERONET database[END_REF]-b, 2002]. Aerosol optical depth data are computed for three data quality levels: Level 1.0 (unscreened), Level 1.5 (cloud screened), and Level 2.0 (cloud-screened and quality assured). Cloud screening procedures are detailed in [Smirnov et al;[START_REF] Smirnov | Cloud sceening and quality control algorithms for the AERONET database[END_REF].

b-TOMS aerosol index

The TOMS (Total Ozone Mapping Spectrometer) instruments allow an efficient method of space based aerosol detection: TOMS Aerosol Index [START_REF] Torres | Derivation of aerosol properties from satellite measurements of backscattered ultraviolet radiation: theoritical basis[END_REF]]. TOMS AI is a measure of the change of spectral contrast in the near ultraviolet (341 and 380 nm) due to radiative transfer effects of aerosols in a Rayleigh scattering atmosphere. The AI is defined as: where I mes is the backscattered radiance measured by TOMS and I calc the radiance calculated using a radiance transfer model for a pure Rayleigh atmosphere. TOMS data used in this work are daily gridded level-3 product at a resolution of 1° latitude and 1.25° longitude, The AI data were acquired from the NASA/GSFC TOMS ozone processing team at http://toms.gsfc.nasa.gov/ftpdata.html and are related to TOMS EP and TOMS OMI aboard Earth Probe and AURA platforms respectively. c-MISR aerosol optical thickness. MISR is one of five instruments launched into polar orbit aboard NASA' s Terra spacecraft in August 1999. The MISR instrument provides high sensitivity for a wide range of scene reflectance (0.02% to 100%) without change in gain. MISR has 9 cameras so the instrument can view successively a same point of the Earth at this number of angles simultaneously. Most of the information about aerosol and cloud particle properties, and some of the information about surface structure, come from studying observations taken at different scattering angles. In each of the nine MISR cameras, images will be obtained in four spectral bands, i.e. in four different colours, one each for blue, green, red, and near-infrared. The canter wavelength of each of these bands is 446, 558, 672, and 867 nanometers respectively. MISR has a periodic coverage between two and nine days depending on the latitude [Martonchick et al, 2002]. Its unique combination of multiple bands and angles enables it to retrieve aerosol optical thickness and additional particle properties at a resolution of 17.6 Km over both land and ocean, with no assumption about the absolute land surface reflectance or its spectral characteristics in the aerosol retrieval algorithm [Martonchick et al, 1998[Martonchick et al, , 2002]]. The bands in the red and near-infrared (670 and 865 nm) provide vegetated surface identification, and are also useful for marine aerosol studies since water is nearly black at these wavelengths. The green band at 555 nm is near the peak of the solar spectrum, and thus will be given high weight in studies to estimate broadband reflecting properties (albedos). The MISR wavelengths have been selected to avoid known ranges of strong atmospheric gas absorption and solar Fraunhofer lines. d-MODIS aerosol optical thickness: MODIS (or Moderate Resolution Imaging Spectroradiometer) is a key instrument aboard the Terra (EOS AM) and Aqua (EOS PM) satellites. Terra MODIS and Aqua MODIS are viewing the entire Earth' s surface every 1 to 2 days, acquiring data in 36 spectral bands between 0.405 and 14.385 µm. The MODIS instrument has a viewing swath width of 2,330 km and it acquires data at three spatial resolutions 250m, 500m, and 1,000m. MODIS retrieval algorithm is based on the "dark object" method for aerosol estimation [Kaufman et al, 1997[Kaufman et al, -a, 1997[START_REF] Kaufman | Relationship between surface reflectance in the visible and mid-IR used in MODIS aerosol algorithm-theory[END_REF]. The basic assumption is that if the surface is densely vegetated, the middle infrared reflectance around 2.1 µm (MODIS band 7) ρ 7 is linearly related to surface reflectance at the blue band (band 3 around 0.49 µm) ρ 3 and the red band (band 1 around 0.66 µm) ρ 1 . Since most aerosol sizes are smaller than the middle-infrared wavelength, aerosol effects in band 7 are negligible and its surface reflectance can be easily estimated. Based on surface measurements, Kaufman et al. (1997-b) established the following empirical formulae: ρ 7/ ρ 1 =0.25, and ρ 3/ ρ 7 =0.5. After calculating surface reflectance of band 3 and 1 from their linear relationships with band 7 reflectance, aerosol optical depths at band 1 and 3 can be estimated by using look-up tables. These look-up tables are pre-calculated for each aerosol model and pre-defined for a given location and time. Along with all the data from other instruments on board the Terra spacecraft and Aqua Spacecraft, MODIS data are transferred to ground stations in White Sands, New Mexico, via the Tracking and Data Relay Satellite System (TDRSS). The data are then sent to the EOS Data and Operations System (EDOS) at the Goddard Space Flight Center. After Level 0 processing at EDOS, the Goddard Space Flight Center Earth Sciences Distributed Active Archive Center (GES DAAC) produces the Level 1A, Level 1B, geolocation and cloud mask products. Higher-level MODIS land and atmosphere products are produced by the MODIS Adaptive Processing System (MODAPS), and then are parcelled out among three DAACs for distribution. This work lies within the scope of the ELT project, with the goal of prospecting the best sites for astronomical observations by means of aerosol cover. A means to determine the properties of atmospheric aerosols in all the area would be to place solar or stellar photometers in at least at several tens of places and to achieve measurements there during several months to be able to extract from substantial information. This looks hard and time and energy consuming, then a pre-selection study should be achieved before photometric measurements. The tools we have with this intention are AERONET and satellite data. As previously mentioned we have four AERONET stations; Marrakech, and Dakhla in Morocco and Santa-Cruz and Izana in the Canary Islands. We must note that in our analysis the stations of primary importance are Marakech, Izana and Santa-Cruz. That of Dakhla will be only used to validate the results of correlations between AERONET and satellite data. Izana is an important site because it is located at 2400 m of altitude and thus through its study, we will have the occasion to see the order of magnitude and the variations of the AOT in altitude. Santa-Cruz being located at 12 m above sea level and at meadows of 30 km Izana, its study will allow the quantification of the AOT disparities between the low and high altitudes. Marrakech, being located at the bottom of the High Atlas mountains, its AOT will be used for to extrapolate the optical thickness in neighbouring high altitude places, taking advantage of previous results concerning Izana and Santa-Cruz. The satellite data used are the aerosol index (AI) provided by both Earth Probe and OMI, and the aerosol optical thicknesses provided by MODIS and MISR. Linear regression analyses were performed between these satellite data and all the available AERONET AOT. At each station, we will make an inter comparison of the satellite products. The feasability of using the aerosol index (AI) derived from TOMS measured radiances to characterize the temporal and spatial distribution of tropospheric aerosols over both land and water have been demonstrated since 1996 [Hermann et al, 1997[START_REF] Hsu | Comparisions of the TOMS aerosol index with sun-photometer aerosol optical thickness: results and applications[END_REF] We summarise this paper as follows; we will establish the correlation between AERONET aerosol optical thickness and TOMS Earth Probe and TOMS OMI aerosol index individually in every station of the four sites mentioned before A linear relationship involving all AERONET data of the region of Morocco and the Canary Islands and their corresponding Earth Probe aerosol index is used to calibrate the TOMS EP AI into 'SAOT' Satellite aerosol optical thickness over the area containing Morocco and the Canary Islands. A long term record of ten years analysis of the SAOT is then provided. As MISR and MODIS instruments provide satellite aerosol optical thicknesses, we will establish the correlation between ground AOT and satellite AOT and make an inter-comparison between satellite products. As Izana is at 2370 m of altitude, all the established relationships related to it are going to be used to extrapolate the aerosol optical thickness at elevated places located in the Moroccan mountains. The derived relationships are then used to map aerosol loadings over areas where AERONET data are not provided. We will deduce optical properties over Oukaimeden: a Moroccan astronomical observatory, located in the Atlas mountain at 70 Km from Marrakech city and at 2700 m above sea level. This retrieved AOT will be then converted to light astronomical extinction and compared to previous in-situ measurements done at the observatory. This is helpful to retrieve the AOT of ELT pre selected sites in Morocco from either satellite or ground measurements.

2-3 Methodology

3-Satellite measurements and ground measurements comparison.

3-1 TOMS aerosol index and AERONET aerosol optical thickness correlation.

The only available long term record of atmospheric aerosols over both oceanic and continental areas is provided by TOMS. Indeed TOMS data are delivered since 1978 until now; Nimbus 7 from November 1978 until May 1993, Meteor 3, from August 1991 until December 1994, Earth Probe from September 1996 until December 2005 and OMI from 2004 until now. The primary importance of this instrument resides in the fact that it is possible to study the variability of the aerosols over the last three decades in the area of interest as long as a satisfying relationship between sun photometer and TOMS data is derived. [START_REF] Hsu | Comparisions of the TOMS aerosol index with sun-photometer aerosol optical thickness: results and applications[END_REF] has shown that the TOMS AI measurements are linear proportional to the AOT derived independently from ground based sun photometers instruments over regions of biomass burning and regions covered by African dust. Their theoretical model simulation has demonstrated that the AI depends on the aerosol optical thickness (AOT), single scattering albedo (SSA), the aerosol layer height and viewing geometry; which makes the quantitative interpretation of the AI very difficult. However the strong dependence of the AI on the height distribution of the aerosol decreases its sensitivity to the aerosol presence at altitude below 1.5 Km. An inversion procedure that retrieves aerosol properties from TOMS radiance has been developed by [START_REF] Torres | Derivation of aerosol properties from satellite measurements of backscattered ultraviolet radiation: theoritical basis[END_REF][START_REF] Torres | A long-term record of aerosol optical depth from TOMS observations and comparision to AERONET measurements[END_REF][START_REF] Torres | Aerosol properties from EP-TOMS near UV observations[END_REF]. [START_REF] Torres | Aerosol properties from EP-TOMS near UV observations[END_REF] confirms the good agreement between TOMS AOT and AERONET data for mineral, carbonaceous and sulphate aerosols. [Ginoux et al 2003] developed an empirical relation to express the TOMS AI for the case of dust plumes, as an explicit function of four physical quantities: the single scattering albedo, optical thickness, altitude of the plume and surface pressure. For consistency we choose the 440 nm channel, because the 340 and 380 nm channels are not available in all sites. For large particles such as dust, the wavelength dependence in AOT between 340 and 440 nm channels is very small, so that the 440 nm AOT value is approximately the same as the 340 nm AOT, as previously reported by the TOMS team. First we will give an order of magnitude of the aerosol index at Dakhla, Marrakech, Izana and Santa-Cruz during a long period of time, from 1997 until 2005. We can see from figure 2 that Dakhla's index is very high, the index at Marrakech is moderate and at Izana and Santa-Cruz, it is relatively low. Santa-Cruz is located exactly at the border of two TOMS pixels. In figure 2, AI Santa-Cruz is related to the index of the joint pixel of Izana, and for the regression analysis, the index at Santa-Cruz is the average index of the two pixels. Annual behaviour of the index (figure 2) depicts evident seasonal effects with maximum in summer and minimum in winter. In summer time high AOD events occur in this region of the world due to its proximity to the Saharan desert. However with different magnitudes depending on the geographic position, thus Dakhla receives more Saharan dust than Marrakech and Santa_Cruz and Izana. Linear regression analysis has been performed, between sun photometer and TOMS data, individually in every station; the results are illustrated in table 1. The derived relationships can be applied to this long term record of the EP TOMS to convert the AI to AOT. Then we can have inter-annual and intra-annual variability of the AOT for a decade. Linear regression analysis are performed in the form :AOT AERONET =A*AI+B. The linear annual and seasonal relationships between AOT and AI are illustrated in table 1; A; is the slope, B; the intercept, N; the number of data with coincident AOT and AI, and the average AOT, and AI for the full range of data. TOMS maximum absolute error bounds are within 95% confidence level to the TOMS calculated AOT. The period studied for every site is the duration of availability of AERONET data. The AOT considered are the daily average ones, instead of overpass time values, because daily variability from daily means are less than 10% [START_REF] Smirnov | Diurnal variability of aerosol optical depth observed at AERONET sites[END_REF]. The sun photometer at Dakhla was functioning during the years 2002, 2003 and 2004, with some months of non availability during this period. Data of 2002 and 2003 are level 2 data and data of 2004 are level 1.5. The TOMS index at Dakhla is from the Earth Probe only, because the OMI index was available since the end of 2004. Since most of the AERONET data are level 2, we did not remove the days where the number of measurements is less than ten, like we did at Izana and Santa-Cruz where the data are level 1.5. At Marrakech, Izana and Santa-Cruz, the index used is from Earth Probe and OMI platforms. The annual average of the EP index is 1.90 for Dakhla (2002-2003-2004), 1.03 for Marrakech (2004Marrakech ( -2005)), 0.82 for Santa-Cruz (2005) and 0.82 for Izana (2004Izana ( -2005)), the corresponding AOT averages are, 0.33, 0.27, 0.22 and 0.09. Izana being at high altitude, its AOT is very low, 2.5 times lower than the average at Santa-Cruz. AOT at Dakhla is very high because it is often in the pass of dust storms from the desert to the Atlantic. The slope for Dakhla is 0.12, the same as the one for Santa-Cruz. The slope depends on absorption properties and the height of the aerosol layer.

If the aerosols at Dakhla are more absorbent than the aerosols at Santa-Cruz, then they should reside at higher altitudes. As the AOT at Dakhla has a maximum value of AOT higher than the one of Santa-Cruz, then the corresponding single scattering albedo should be different. We expect that the slope of Izana is lower than Santa-Cruz one because its AOT is low. The slope at Marrakech is 0.07, which is a low value. One reason for that may be because the data are related to the summer-autumn period and do not cover the whole year, or may be because the AOT versus AI scattergram for 2005 has wide dispersions. The summer mean AOT at Dakhla is very high, 0.56; contrasting with spring value, 0.25. One surprising thing is that spring value at Marrakech, 0.35; is higher than spring one at Dakhla. TOMS Earth Probe gives low correlation coefficients for Marrakech (0.53), Santa-Cruz (0.57) and Izana (0.60). However it is performing at Dakhla. We must say that the correlation can be improved if we draw some outliers from the calculation. Except for Dakhla, the TOMS Earth Probe index signal at Marrakech, Santa-Cruz and Izana, during the period studied has very few data for November, December, January and February. The retrieval of winter and autumn AOD is limited. The seasonal curve of the index (EP for Dakhla and OMI for the others), at all sites, presents minima corresponding to the winter and the autumn, a maximum in summer with for spring an intermediate value. The seasonal one for the AOT is different; for Dakhla, Santa-Cruz and Izana, winter, spring and autumn means are almost the same, with a pick in summer time. At Marrakech, seasonal curve of the AOT has minimum in winter and autumn and maximum in spring and summer. Since at all sites there is not linearity between AOT and AI seasonal curves, the annual relationship is not accurate enough for AOT retrievals. Seasonal relationships should be taken into account for the AOT retrieval over the past. An adjustment between the different TOMS instrument have to be done first. Seasonal analysis is possible with TOMS OMI index because data are available throughout the year. The correlations between TOMS OMI and AERONET at Santa-Cruz and Izana (R ranges from 0.68 to 0.89) are good, whereas at Marrakech, winter and autumn gave no specific relation. Therefore, we can ask the question why TOMS OMI is performing over the Canary Islands and not that much over Marrakech? It may be that the aerosol layer over Marrakech in winter and autumn resides at low altitudes within the sensing limits of the TOMS instrument. Indeed the average single scattering albedo (SSA) is 0.81 for Dakhla, 0.85 for Santa-Cruz and Izana and is 0.91 for Marrakech. Since Marrakech has the highest SSA then its aerosol layer may reside at lower altitudes than the other sites. Another reason for that may be the topography of the pixels; Marrakech's pixel is much contrasted with mountains as high as 4000 m, whereas the pixels for Izana and Santa-Cruz have more than 75% oceanic surfaces. Homogeneous surface reflectivity increases the sensing performances of satellite remote sensors, though it have been reported that the surface reflectivity in the near UV has a negligible influence. During high AOT events in summer time, the sensing performance is increasing despite non-homogeneities in surface topography.

TOMS data have been used from astronomers; [START_REF] Siher | Correlation between TOMS aerosol index and astronomical extionction[END_REF] found good correlation between TOMS index and light astronomical extinction over La Palma observatory, however debated [START_REF] Varela | Noncorrelation between atmospheric extinction coefficient and TOMS aerosol index at the canarian observatory[END_REF]2004]. We must say that the effect of the Mont Pinatubo volcano has been detected for more than two years from all observatories in the world and that the TOMS did not clearly detect such an event. TOMS (EP or Nimbus7) is not a very useful tool for site characterization , OMI , MODIS and MISR have better resolution than TOMS but not enough at particular abrupt topography sites, unless high level (smaller than 2) satellite data are used.

3-2 MISR-AERONET and MODIS-AERONET AOT correlation.

We will examine monthly values because MISR provides weekly values and MODIS TERRA provides one value every two days. The data were collected from the GIOVANNI MOVAS interface that provides monthly global level-3 products (MIL3MAE for MISR and MODO8_03 for MODIS) at 1*1 degree grid average. The MODIS AOT is at λ=550nm and the MISR one is at λ=557.5nm. The aerosol optical thickness from AERONET is at the wavelength λ=440nm because the 500 nm channel is not available in all sites. The results of linear regression analysis between the MODIS or MISR retrieved AOT and AERONET in the form AOT satellite =A*AOT AERONET + B, are illustrated in Table 2; R is the correlation coefficient, N, the number of months studied and corresponding means of AOT retrieved by satellite: SAOT and measured by sun photometer: AOT. We can see from figure 3 the good agreement between ground based AOT and both satellite AOT at Dakhla. This is confirmed by the good correlations; R=0.84 for AERONET-MISR linear relationship and R=0.92 for the AERONET-MODIS one. Both satellite instruments overestimate the aerosol optical thickness at Dakhla. The slope after adjusting the wavelengths (440 nm and 550 nm) is 1.2 for MODIS and 1.16 for MISR. The retrieval algorithms seem to have acceptable assumptions about surface reflectance and do not exhibit a significant calibration error. At Marrakech, MODIS aerosol optical depth is in good agreement with AERONET AOT, the correlation coefficient is R=0.85. The MISR AOT doesn't give as good correlation coefficient; R=0.77 and underestimates the AOT especially during 2005 and 2006. The slope after adjusting wavelengths is 0.98 for MODIS and 0.83 for MISR. A slope different from unity indicates that there might be some inconsistency between aerosol microphysical and optical properties used in the retrieval algorithms and that in the real situation. Slopes slower than unity indicate an underestimation of AOT by MISR with respect to AERONET retrieval. Low intercepts suggest that there are low calibration errors. According to these results, we can deduce that MODIS is the best candidate to characterize the AOT of Marrakech. At Santa-Cruz, we can see that unlike Marrakech, now the MISR data follow the AERONET curve better than MODIS. For MODIS, first the correlation coefficient is R=0.68, A=0.54 (for λ=550 nm) and B=0.12. This indicates an important underestimation of the AOT by MODIS retrieval algorithm, due to improper assumptions of the aerosol physical properties of that region. There is a bias for low aerosol loadings which indicates a calibration error or an improper assumption about surface reflectance. Indeed the MODIS retrieval algorithm is based on the "dark object method" [START_REF] Kaufman | Relationship between surface reflectance in the visible and mid-IR used in MODIS aerosol algorithm-theory[END_REF]. There are two key requirements associated with this method: (1) existence of large homogeneous dense vegetation in the scene, (2) stable empirical relationships of surface reflectance between band 7 and band 3 and 1. The first requirement precludes the accurate reflectance retrieval of surfaces with nondense vegetation canopies. To meet the second requirement, dense vegetation canopies have to be distinguished from other dark objects, such as wet soil and water. The correlation coefficient for MISR is good, R=0.88, and the slope A=1.25 (λ=550 nm), it overestimates the AOT at Santa-Cruz. This instrument is suitable for sensing aerosol loading over Santa-Cruz. The aerosol optical thicknesses measured with sunphotometers at Izana and Santa-Cruz are in very good correlation as well illustrated in the section altitude effect. We expect, like at Santa-Cruz, good correlation between sun photometer and MISR signals. Izana and Santa-Cruz are in the same MISR and MODIS pixel, they have the same satellite aerosol optical thicknesses. Izana being at high altitude, its aerosol optical thickness is very low. Satellite measurements do not see the high altitude contribution as long as the pixel topography is mostly constituted by low altitude land. MISR overestimates the AOT over ocean [Mischenko et al;[START_REF] Mishchenko | Past, present, and future of global aerosol climatologies derived from satellite observations: a perspective[END_REF] and maybe underestimates the AOT over topography contrasted areas. Meanwhile MODIS seems more performing over contrasted land surfaces in North Africa. From seasonal statistics for Santa-Cruz, the MISR AOD is overestimated in summer and underestimated in winter.

4-Altitude effect

Vertical distribution of aerosol properties is not well understood; there is no systematic way of deducing aerosol microphysical and optical properties at a given height from aerosol properties measured on the ground just below. Active sensors like LIDAR can give the average position of the aerosol layer. In this section we will focus on AERONET AOT at Izana and Santa-Cruz and make regression analysis. As the AERONET data at Santa-Cruz and Izana are level 1.5 and then are not manually inspected, we remove from the statistics the days where the number of measurements is less than ten. The AOT magnitude of Santa-Cruz is always higher than the AOT at Izana. They have the same oscillation with magnitude depending on the season. The daily mean aerosol optical thickness of Santa-Cruz and Izana exhibits a good linear correlation with a Pearson correlation coefficient of 0.92 for both years 2005 and 2006. The seasonal correlation coefficients are 0.85 for winter, 0.87 for spring, 0.91 for summer and 0.87 for autumn. The linear relationships (annual from daily means, annual from monthly means, annual from seasonal means, seasonal) are illustrated in table 3 (AOT Izana =A*AOT Santa-Cruz +B) with corresponding correlation coefficient, number of measurements and AOT means. Like for previous regressions, as the seasonal curves of Santa-Cruz and Izana (table 3) are not strictly linear, seasonal regression analysis have to be performed for accurate estimate of the AOT. From this table we can see that the annual average of Izana is 0.08 which is very low aerosol loading, we must say that the corresponding histogram is very sharp with 75% of the frequency occurring for AOT less than 0.05. At Izana, the corresponding averages for winter, spring and autumn are respectively; 0.035, 0.06 and 0.05, exhibiting good astronomical observation qualities in term of sky transparency. Santa-Cruz annual mean is 0.21; 2.6 times higher than Izana's annual average and at 0.13 from Izana's annual average. The subtraction of the seasonal AOT at Santa-Cruz and Izana is 0.165 for winter, 0.109 for spring, 0.126 for summer and 0.129 for autumn. The aerosol optical thickness at Santa-Cruz expressed with surface layer extinction coefficient α is:

( ) ( ) ( ) Izana background atmospher Top h(Izana) h(Izana) Ground atmospher Top Ground Cruz Santa AOT AOT dh h dh h dh h AOT + = σ + σ = σ = -
AOT background represents the subtraction of the aerosol optical depths at Santa-Cruz and Izana; its annual average is about 0.13; with a higher average of 0.16 in winter and a lower one of 0.11 in spring. Steady AOT values often occurs at Izana during winter and autumn and high AOT events often occur in summer time. Santa-Cruz seems to see more AOT events during all the year with a magnitude depending on the season. The plume altitude distribution should have its maximum higher than Izana's altitude in summer time and lower in winter. Figures (4-a and4-b) shows the behaviour of the monthly and seasonal mean AOT values respectively; we can see clearly very low values at Izana compared to Santa-Cruz. Seasonal behaviour is also clear with low AOT values in winter time, increasing in spring time to a maximum in the early summer and decreasing during summer and autumn to a minimum value in the following winter. Inter annual variability in the seasonal shape is visible with lower 2006 spring AOT means than winter ones. The ratio AOT Santa-Cruz /AOT Izana has seasonal variation from 2 in summer time to 4.5 in winter.

5-Retrieval of Aerosol optical thickness and light extinction coefficient at Oukaïmeden observatory.

First, we want to confront long term satellite aerosol loading previsions from different instruments. Figure 5 illustrates the average aerosol optical thicknesses of MODIS and MISR instruments from 2000 until 2006. These images (MODIS and MISR) were acquired using the GES-DISC Interactive Online Visualization and Analysis Infrastructure (Giovanni) as part of the NASA' s Goddard Earth Sciences (GES) Data and Information Services Center (DISC). MISR AOT forecast for the Canary Islands and the Moroccan coasts is between 0.2 and 0.3, and between 0.1 and 0.2 for the interior of Morocco. However, MODIS forecasts values between 0.2 and 0.3 for most of the Moroccan territories except a small area (AOT between 0.1 and 0.2) included between 7° to 5° W and 31.5° to 33.5° N corresponding to a part of the Atlas mountains. In order to compare all the satellite data we have to convert the TOMS EP index into aerosol optical depth; SAOT. Figure 6 shows the scattergramm of all the AERONET data in this area and their corresponding TOMS EP index. The corresponding linear relationship is: SAOT= 0.11(±0.01)*AI_EP + 0.11(±0.02)

The correlation coefficient is R=0.61 The mean retrieved SAOT from 1997 to 2005 is illustrated in figure 7 (a). Retrieved monthly values from AERONET measurements at Marrakech are extracted from relationships in table 3 related to monthly means. Daily values can be retrieved from AERONET and from TOMS index .To have better estimation of daily values we can use seasonal relationships. From figure 7 (a), we can see that MODIS and AERONET retrievals are in good agreement. We have already noticed that the MISR signal underestimates the AOT around Marrakech (section 3-2), whereas the MODIS signal gives a better estimation. Figure 7 (b) illustrates daily retrievals from TOMS OMI and from AERONET for the year 2005. We can notice the good agreement of the two signals for spring and summer time as previously reported. Daily values relation overestimates the AOT because the probability distribution of AOT at higher altitudes is sharp whereas it is broad at lower altitudes. Linear relationship from lower altitudes to higher altitudes does not give sharp AOT distribution. Now we will try to retrieve the astronomical light extinction coefficient over the observatory where in-situ measurements have been achieved there; in 1997 [Benkhaldoun et al;1998;Siher et al;2002]. The only way we have to go back in time is the TOMS EP index. We have already said that it gave the lowest correlation coefficients and that winter and autumn retrievals are not accurate. TOMS EP stops functioning at the end of 2005 because of calibration problems. We transformed the TOMS EP index to aerosol optical thickness first using the relation related to Izana in order to account for altitude contribution. Then, since the measurement at the observatory have been done at 589 nm wavelength, we transformed the 440 nm AOT retrieved to 589 nm AOT using climatology table of Izana from AERONET database. The result is illustrated in figure 9 where we added the astronomical light extinction coefficient of La Palma for the same year. The measurements of the light astronomical extinction coefficient have been done at La Palma using stellar photometry and at Oukaimeden using daytime photometry from IRIS (International Research of the Interior of the Sun) database. The value corresponding to September is very low, we think it is a bad data; the corresponding number of days of measurements is 9. The average of the retrieved extinction coefficient is 0.14 mag/air mass; the same as the measured one at La Palma. The average value at Oukaimeden is 0.1mag/air mass.

6-Conclusion and perspectives

In this work we have characterized sky transparency by means of aerosol optical properties trough the aerosol optical thickness parameter in the area of Morocco and the Canary Islands. Ground based measurements are provided by the AERONET Network in four locations: Dakhla, Marrakech, Santa-Cruz and Izana. TOMS Earth Probe aerosol index have been used to have long term aerosol cover in the four AERONET stations. Seasonal effects of aerosol cover have been clearly demonstrated through ten years of aerosol index variations. A relationship involving all AERONET data and their corresponding TOMS Earth Probe index have been used to transform the TOMS EP index into 'SAOT' satellite aerosol optical thickness. The area of Morocco and the Canary islands have been mapped with inter-annual variations for a long term study. We also used other satellite products like TOMS OMI aerosol index, MODIS and MISR aerosol optical thicknesses. A set of linear relationships have been established between ground measurements and satellite data, individually in every station, for all available ground measurements. Then an inter-comparison of satellite products was conducted. The Pearson correlation coefficient for TOMS OMI, MISR and MODIS vary from 0.68 to 0.92 and from 0.54 to 0.74 for TOMS Earth Probe. Izana is very close to Santa-Cruz (30 km), but 2300m higher in altitude. Their corresponding ground aerosol optical thickness are in very good correlation (R=0.92), however AOT Izana have small value with a very sharp histogram around 0.05 and AOT Santa-Cruz has wider histogram. Izana is the key station for this paper as we will use its relationships to extrapolate AOT values at elevated places. We retrieve the AOT parameter at Oukaimeden; a Moroccan observatory near Marrakech city, at 2760 m above sea level. We also retrieved light extinction coefficient for the year 1997 and compare with in-situ measurements achieved at the observatory at that time. The discrepancies between the measured and calculated light extinction coefficient in the autumn and winter months can be explained in terms of the seasonal behaviour of dust distribution and the poor spatial resolution of the TOMS EP. Satellite data with larger spatial and spectral resolutions are required to derive a reliable AOT for pre-selected ELT sites.

  Figure 7 (b) shows the inter-annual variability of the SAOT during the period 1997-2005 From figure 7, the mean SAOT during 1997-2005 in the Canary Islands and north Morocco is below 0.2 and greater than 0.15 with an inter-annual variability approximately of 0.04 for the Canary Islands and 0.06 for north Morocco. Now we will try to retrieve the AOT at Oukaimeden; a Mororccan observatory, near Marrakech city at 2760 m of altitude. To retrieve monthly values from MODIS and MISR measurements, we can use the relationships related to Izana: AOD AERONET = 0.35(±0.16)AOD MODIS -0.006(±0.035) AOD AERONET = 0.22(±0.08)AOD MISR + 0.02(±0.02)

Figure 1 :

 1 Figure 1: map showing sites under study (red arrow).

Figure 2 :Figure 3 :

 23 Figure 2: seasonal aerosol index values at Dakhla, Marrakech, Izana and Santa-Cruz, since 1997 until the end of 2005. a) b)

Figure 4 :

 4 Figure 4: Time evolution of the monthly mean (a) and seasonal AOD values at Santa-Cruz and Izana (b).

Figure 5 :

 5 Figure 5: MISR (a) and MODIS (b) mean aerosol optical thickness during the period 2000-2006.

Figure 6 :Figure 7 :Figure 8 :

 678 Figure 6: scattergramm of all the AERONET AOT and the corresponding TOMS EP AI in Morocco and the Canary Islands

Figure 9 :

 9 Figure 9: light extinction coefficient measured and retrieved at Oukaimeden observatory, with comparison with La Palma measured coefficient.

Table 2 :

 2 AERONET AOT with MODIS AOT and MISR AOT linear relationships from monthly values AOT satellite =A*AOT AERONET +B with corresponding correlation coefficient R, number of months N, and corresponding average SAOT satellite AOT and average AERONET AOT.

		Santa-Cruz/Izana AERONET AOT relationships (2005-2006)
		A	B	R	N	<AOT>	<AOT>
						Santa-Cruz	Izana
	Annual	0.59(±0.03)	-0.04(±0.01)	0.92	255	0.21	0.08
	daily values				days
	Monthly	0.29(±0.13)	0.01(±0.03)	0.71	22	0.22	0.07
	values				months
	Seasonal	0.67(±0.28)	-0.06(±0.06)	0.96	6	0.21	0.08
	values				seasons
	Winter	0.39(±0.09)	-0.02(±0.02)	0.85	33	0.20	0.035
					days
	Spring	0.46(±0.06)	-0.02(±0.01)	0.87	83	0.17	0.061
					days
	Summer	0.64(±0.06)	-0.03(±0.02)	0.91	100	0.26	0.134
					days
	Autumn	0.50(±0.09)	-0.03(±0.03)	0.87	39	0.18	0.051
					days

Table 3 :

 3 Linear relationships between AERONET AOT Santa-Cruz and Izana from daily values AOT Izana =A*AOT Santa-Cruz +B with corresponding correlation coefficient R, number of data N, and corresponding average AOT.

Acknowelogements:

We would like to thank TOMS and AERONET groups from NASA Goddard Space Flight Center for providing data. Thanks to GIOVANNI group. We thank the investigators: Emilio Cuevas, Hamad Benchekroun, Brent Holben, Philippe Goloub, and Benoit Duchemin and their staff for establishing and maintaining the sites of Marrakech, Santa-Cruz, Izana and Dakhla used in this investigation.

Bibliography:

Z. Benkhaldoun, E.A. Siher; "Photometry análisis using IRIS database"; SOHO6/Gong98 Workshop, Boston, USA, 1998, ESA SP-418, pp. Dakhla (2002-2003-2004) Marrakech (2004-2005-2006 Dakhla (2002-2003-2004) Marrakech (2004-2005-2006