S Gousset

E Le Coarer

N Guérineau

L Croizé

T Laveille

Y Ferrec

NANOCARB-21: A MINIATURE FOURIER-TRANSFORM SPECTRO-IMAGING CONCEPT FOR A DAILY MONITORING OF GREENHOUSE GAS CONCENTRATION ON THE EARTH SURFACE

I. INTRODUCTION

Earth's positive radiative forcing is significantly accelerated by massive anthropogenic emissions of greenhouse gases, like carbon dioxide and methane [START_REF] Stocker | Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change[END_REF]. Since 2014 the concentration of CO2 exceeds 400 parts per million (ppm), and the concentration of CH4 climbs up to 1900 parts per billion (ppb) [START_REF] Dlugokencky | Global atmospheric methane: budget, changes and dangers[END_REF] with a constant annual increase. Thereby it is a major issue to quantify the impact and potential disturbances induced on the geophysical natural carbon cycle. To better constrain climate models, it is important to improve our knowledge of CO2 fluxes between terrestrial reservoirs (biosphere, atmosphere and ocean) and to be able to distinguish natural and anthropogenic sources and sinks of carbon. Currently, a few space missions are dedicated to the monitoring of greenhouse gas concentration, such as OCO and GOSAT, potentially soon relayed by ESA mission CarbonSat and CNES mission MicroCarb. This spatial constellation is backed on ground by a network of stations. The ultimate goal is to be able to monitoring the greenhouse gas emission from local (city) to global scale (Earth) systematically at any time [START_REF] Ciais | Current systematic carbon-cycle observations and the need for implementing a policyrelevant carbon observing system[END_REF]. Nevertheless, the revisit frequency is not yet sufficient to reach a daily coverage. There is also not real continuous sampling of the surface (only obtain by averaging data over time).

Within this context we propose the NanoCarb-21 space mission, to complete the observational data flows with high spatially resolved daily data, additionally to the MicroCarb mission. This goal requires a large constellation of satellites in orbit at a given moment. We can realistically achieve this by considering nanosatellites, cheaper to develop, product and launch. It is a huge challenge to make science with a cubesat, considering miniaturization issues of the payload. Thus we cannot aim to be independent of MicroCarb mission and of the existing constellation, for example concerning the limited on-board calibration systems.

We present in this paper the design of an ultra-compact imaging spectrometer for the NanoCarb-21 mission, based on the last developments about µSPOC (Spectrometer on Chip) technology at the ONERA and IPAG. We firstly describe the preliminary mission elements in the section II. Then the section III presents the payload, by detailing µSPOC technology, conversion into imaging-spectrometer, and the current optical/implementation design. We finally expose in the sections IV and V our pre-phase-0 studies, consisting in optimizing the design, maximizing the sensitivity, and predicting some performances, according to numerical simulations presented in the section IV.

II. FIRST MISSION ELEMENTS

The NanoCarb mission has been designed complementary to the CNES MicroCarb mission, in order to complete the current global greenhouse gas survey network by a specific data production. At data product level 1 (e.g. calibrated spectra), the missions will provide to temporally and spatially increase the data flow about CO2 and CH4 absorption in the atmosphere, and also about geophysical biases, such as scattering effects due to aerosols, water vapor and clouds. At level 2, we expect to enhance spatial coverage and temporal resolution of the specie concentration maps retrieved by model inversion. Therefor the mission elements will be based on the other greenhouse dedicated missions such as MicroCarb. We have considered a LEO sun-synchronous orbit, probably at an altitude of 594 km, and a constant local time of 11:30. Due to precession, this kind of orbit allows to cover the Earth's surface with a monthly delay (10-25 days). By placing each of the 21 nanosatellites in this orbit with an interval of approximately 5 minutes, the NanoCarb mission provides a daily coverage of the Earth. The Earth's surface will be sampled continuously by a spectro-imager embedded in each unit of the constellation, providing a high spatial resolution of 9 km 2 for a 150 km-wide swath, which is consistent with emission or sink phenomena at local scale. The science instrument is also based on actual/scheduled missions, by observing the solar light reflected on the Earth's surface and scattered by the atmosphere. We expect to implement two modes: nadir observations above emerged land and glint mode above the oceans (low albedo) in order to increase collected light. The required sensibility is better than 3 ppm of CO2 with a goal at 1 ppm, and 12 ppb of CH4. In the same way of MicroCarb, CarbonSat or GOSAT, three main spectral bands are currently considered: 1.6 µm, 2.06 µm and 760 nm. The 1.6 and 2.06 µm bands present some absorption lines due to atmospheric CO2 and CH4. The 760 nm band is dedicated to the measure of atmospheric pressure (O2 concentration), scattering (aerosols) and water

ICSO 2016

Biarritz, France International Conference on Space Optics 18 -21 October 2016 vapor effects. The main issue of the NanoCarb mission is to design a compact imaging spectrometer. We describe such a payload in the next section.

III. A MICROSPOC-BASED IMAGING FOURIER TRANSFORM SPECTROMETER

A. µSPOC principle and assets

We designed the NanoCarb payload to take advantage of the latest joint developments between ONERA and IPAG about compact spectrometry. Therefor it will be based on the integrated Fourier Transform Spectrometer (FTS) family so-called SPOC (SPectrometer On Chip) [START_REF] Coarer | SWIFTS-LA: An Unprecedently Small Static Imaging FT Spectrometer[END_REF]. This concept consists in introducing a continuous modulation of the Optical Path Difference (OPD -δ -) in the chip in one direction, forming 2-waves interferometric fringes directly on the photo-sensitive surface. In the simple way the OPD modulation is introduced by an interferometric element, such as a glass slide or a prism glue to the sensor. The Fig. 1 show such a spectrometer, and we can see to the right an example of 2D-interferogram obtain with a monochromatic and uniform illumination (912nm strong line of Argon). A Fourier transform provides to obtain the incident light spectrum. By opposite to the SIFTI FTS [START_REF] Hébert | From the Concept to the Definition of the SIFTI Instrument: Static Infrared Fourier Transform Interferometer[END_REF], a µSPOC is a fully passive bulk spectrometer without mobile components. Hence it is potentially lesser sensitive to thermal and mechanical biases. It is also very simple to thermalized it with a drastic reduced power consumption due to its extreme compactness, which is promising for on-board spectrometry. The design Fig. 1 has been adapted in this way for the incoming ATISE nanosat mission of the Grenoble Spatial Center University (CSUG) to detect auroral emission lines in the upper atmosphere [START_REF] Coarer | ATISE: a miniature Fourier transform spectro-imaging concept for surveying auroras and airglow monitoring from a 6/12U[END_REF]. Parallel developments affecting the NanoCarb mission consist of an adaption of the concept to the spectro-imagery, with drastic miniaturization issues.

B. Conversion into micro-imaging spectrometer for NanoCarb

We describe in the following paragraphs the SPOC imaging spectrometer of NanoCarb, giving its principle and the relevant design issues. Then we detail its implementation in a cubesat. To introduce some imagery feature we proceed to a discretization of the initially continuous OPD modulation. Thus the sensor is now divided into thumbnails or sub-apertures, each one coding for only one OPD of the interferogram. The field is imaged over the thumbnails and modulated according to the OPD. To retrieve the interferogram contrast at a given OPD for one point of the imaged field, we take advantage of the satellite scrolling over the time, inducing a pixel displacement on the thumbnail and so an intensity modulation according to the introduced OPD. A pixel line of the thumbnail corresponds to the ground satellite swath.

In the most up-to-date design, we expect to implement a 1024x1024 NGP sensor from SOFRADIR [START_REF] Delannoy | NGP, a new large format infrared detector for observation, hyperspectral and spectroscopic space missions in VISIR, SWIR and MWIR wavebands[END_REF], sensitive from 400 nm to 3-4 µm with a Read Out Noise (RON) about 150 electrons. The photo-sensitive chip area will be divided into four 500x500 pixels independent sub-areas. Each one will be dedicated to the specific spectral bands described in the previous section, then divided into 100 50x50 pixels thumbnails (e.g. 100 OPD) as shown Fig. 2right. The channel number 3 is not yet currently allocated and allows to adapt potentially the instrument to another spectral band for complementary measures as required (bias mitigation for example). One pixel represents a 3x3 km resolution element of the imaged field.

C. Optical implementation

The Fig. 2-left shows the current optical design of the four-band imaging spectrometer. A very small 3 mmdiameter telescope is required in order to respect the Jaquinot criterion about aperture [START_REF] Coarer | SWIFTS-LA: An Unprecedently Small Static Imaging FT Spectrometer[END_REF] and the field constraints. The device (including the chip) is easily contained in a 150x40x40 mm 3 very compact area, which is also simple to be thermally regulated at 200 K. Such a compact instrument is fully compatible with a 6U cubesat (30x20x10 cm 3 ) as shown Fig. 3. About 2U are allocated to the optical bloc including the chip and the cooling device. The remaining space could be dedicated to the control electronic boards, communication and navigation systems, with most of the components available off-the-shelf. The estimated mass of the payload is about 6-7 kg, for a total power consumption of 10 W including cooling. We estimate a data flow about 21 Go/day with compression, and 3.6 Go/day with on-board data processing.

The main issue of the instrument design is now to identify some interest fringes on the interferogram, that will be sampled at the OPD δk. Thus we have to optimize the instrumental sensitivity to a given variation of the CO2 or CH4 concentration. We have also to identify in the interferogram some geophysical bias (aerosols) signatures. This study is based on numerical simulations, presented in the next section.

IV. NUMERICAL SIMULATIONS

We simulated atmospheric absorption spectra at the ONERA using MODTRAN (Release 5.2.1) [START_REF] Berk | MODTRAN 5: a reformulated atmospheric band model with auxiliary species and practical multiple scattering options: update[END_REF] radiative transfer code, in several typical observation scenarios. We use these synthetic spectra at IPAG to compute theoretical interferograms and derive sensibility and design assessments. In the following paragraphs we firstly give the simulation parameters, then the observational scenario. We finally present the theoretical interferogram in the interest spectral bands in order to discuss about measurement strategy for concentration retrieval.

MODTRAN 5 reaches a spectral resolution of 0.1 cm -1 which is consistent with atmospheric absorption line width. In this paper we focus only on the measurement of CO2 concentration in the averaged column density. We

ICSO 2016

Biarritz, France International Conference on Space Optics 18 -21 October 2016

have not yet consistent simulated data with different profiles of concentrations of CH4. The atmosphere composition model is the US standard. We simulated spectra with CO2 concentration from 400 to 430 ppm. The nominal concentration is taken at 400 ppm, near to the actual annual and global averaged value. Different kind of aerosol models have been tested as reported in Tab. 1. We have reported also the averaged luminance in the spectral bands 1 and 2, obtained with a surface albedo of 0.3, and a solar zenithal angle of 20°. We compared these values to a CNES scenario and we could report it as a medium flux condition, such as above emerged land at a tempered latitude with vegetation for example.

Tab. 1. Simulation parameters for MODTRAN radiative transfer code. We present on Fig. 4 the simulated 1.6 µm (left) and 2.06 µm (right) bands without aerosol, for a nominal CO2 concentration of 400 ppm. The 1.6 µm band presents several regularly spaced unsaturated absorption lines, allowing to measure the CO2 concentration. On the opposite the lines at 2.06 µm are saturated. Nevertheless, they can potentially bring some information about scattering effects as we will discuss in the next section.

Range of CO2 concentration

Finally, we computed on Fig. 5 the corresponding theoretical interferogram for the full 1.6 µm band. Given the limited number of OPD per spectral band on the spectrometer, we can't sample all the interferogram and thus it is not possible to retrieve the full spectrum. Nevertheless, the FTS allows to recognize some specific motif in the spectrum. In our case we can observe Fig. 5 some bumps at 5.5 mm then 11 mm and 16.5 mm of OPD, corresponding in the dual space to the absorption line comb (period ~1.8 cm -1 ). The fringe contrast in these region is directly proportional to the energy absorbed by CO2, and thus to the CO2 concentration (assuming small variations of concentration). Our samples will be placed between 5 and 6mm for the 1.6 µm band. In order to maximize the contrast in this region and the sensibility to the concentration of CO2, we have to optimize the spectral window, that will be study in the next section. To conclude, we can also notice that a 1 ppm variation of column density CO2 (goal) causes a luminance variation about 10 -11 W/cm 2 /sr/cm -1 for a mean level about 10 -7 W/cm 2 /sr/cm -1 at 1.6 µm. We study also in the next section the capability of the instrument to measure such a small variation. V. SENSIBILITY STUDY We present in this section a preliminary sensibility study based on the previous numerical simulations. First we optimize the NanoCarb design. Then we derive its effective performances with photometric considerations. To finish, we will quantify the bias causes by aerosols, and give some ways to mitigate it. For this study we consider partial differential interferograms, that we call Jacobian in the following of this paper. These are very useful tools to observe specific effects of tiny variations of the spectrum over the interferogram.

A. Sub-band optimization for CO2 concentration measurement

The first work consists in optimizing the spectrometer sensibility to the concentration of CO2. To do that we fit a sub-band of the 1.6 µm spectral band in order to maximize the contrast of the fringes between the 5 mm et 6 mm OPD according to the previous section. More the sub-band is large, more the collected flux is important, but potentially lesser the fringes are contrasted around 5.5 mm due to jitter in the line period. We scan the 1.6 µm band with increasingly wide windows, and we compute for each the maximum value of the interferogram between 5 mm and 6 mm of OPD. The obtained figure of merit is shown Fig. 6. Each vertically-delimited area represents a given window width. The optimal value is found for a 30 cm -1 -wide window, centered around 6220 cm -1 . The corresponding Jacobian is shown Fig. 7 for a variation of the CO2 concentration of 1 ppm. We can now place our 100 samples of OPD in the fringes around the maximum of the Jacobian (excluding the unity pic at 0 mm), where the sensibility to a variation of concentration is maximal.

B. Effective sensitivity, photometric elements

We can derive now from this study some photometric elements. To do that we introduce the Sensitivity-to-Noise Ratio (SNRj), corresponding to the maximum of the Jacobian in electron compared to noise. For a Jacobian obtained with a variation of concentration of 1 ppm, a SNRj greater than 1 indicates that the instrument can detect such a variation, given photometric data. SNRj is given by:

2 max RON OPD pix e OPD pix j OPD pix j n n N n n N n n SNR σ + = (1)
With npix the number of pixel in the swath, nOPD the number of sub-aperture per band, Ne the mean number of electron per frame and per pixel (derives from the spectral luminance), σRON the sensor RON and Njmax the maximum illumination of the Jacobian. The SNRj takes into account both photon and read out noises. We report on the Tab. 2 the results of this study. Given the SNRj of 2.5 in our setup, we have demonstrated here the full capability of NanoCarb to detect a variation of the CO2 concentration lesser than 1 ppm in medium flux conditions. We can notice also the convenient mean number of electron per frame and per pixel on the interferogram Ne for 1.6 µm band, despite of the very small telescope aperture (3 mm).

Tab. 2. Photometric elements for CO2 concentration measurement.

Jacobian variation d[CO2]=1 ppm σRON [electron]

150 Ne [electron/frame/pixel] 134092 Njmax [electron/frame/pixel] 14 SNRj 2.5

C. Aerosol effects and mitigation

The previously computed intrinsic sensibility is not directly effective due to geophysical biases, affecting all the dedicated missions. The aerosols in the atmosphere are responsible for the main biases over the concentration estimation. Indeed, the induced scattering effects disrupt the optical path of light in the atmosphere, and introduced artificial variations of the measured concentration. They are mitigated only during the model inversion with priors extracted from in situe measurements. We propose in this paragraph an approach to extract information about aerosol from our data.

On the Fig. 8-top two Jacobians on the 1.6µm band are represented, the first one with a null differential of concentration, but with the introduction of rural-type aerosol. The second one without aerosol and a differential of the CO2 concentration of 20 ppm. We notice that the aerosol induced a contrast variation at the same OPD than CO2 absorption, and furthermore introduce a ~20 ppm-bias over the concentration measurement! Thus we can effectively not clearly discriminate scattering effects due to aerosols to absorption by CO2 on the 1.6 µm band. However, we notice a scattering signature on the 2.06 µm band, quasi-independent of the CO2 absorption. We can observe it on the Fig. 8-bottom, where the same Jacobians as top of figure are represented, but this time on the 2.06 µm band. We can reasonably assume that the fringe's contrast variations at the 6 mm OPD depends mainly of the scattering effects. This contrast variation is caused by the differential evolution of the edge of saturated lines of the 2.06 µm band affected by scattering, in contrast to the bottom. Thus a differential measurement of fringe contrast on 2.06 µm band at 6 mm and 12 mm (apparently not affected by scattering) allows to detect aerosol effects. This is a very promising mitigation way, with our limited number of samples over the interferogram. We are currently investigating the 760 nm band for mitigate the other potential geophysical biases. The next step will consist in studying the instrumental biases such as thermal variations. We presented in this paper a very compact imaging-FTS for a 6U nanosatellite, dedicated to the temporal completion of data from the MicroCarb CNES mission. We propose an optimization of the instrument based on our numerical simulations of radiative transfer. We expect currently to compare precisely our simulations with more up-to-date CNES observational scenarios, in order to refine the design and expand the operational case number. The drastic miniaturization constraints of such an instrument are responsible for tradeoff about spectrum retrieval. Nevertheless, we demonstrated here that it is possible to extract some very specific information from the FTS data. Thus we reach a very high sensitivity better than 1 ppm of CO2 in medium flux conditions, with only 100 samples over the interferogram. We have also the capability to detect scattering effects due to aerosols. We are currently studying on the same manner the mitigation of other kind of aerosol and geophysical biases such as water vapor. The next phase-0 studies will allow to analyze the instrumental biases induced over the measurement. We have also to determine SPOC calibration scenarios and navigation specifications.

As a conclusion the NanoCarb-21 mission has the capability to bring high temporally resolved data about CO2 concentration to the MicroCarb mission. The SPOC technology is also promising for on-board compact spectrometry in science space-based applications. Indeed, we override the common weak points of the FTS, with a fully passive bulk instrument without mobile elements. Furthermore, this new kind of spectrometer allows to consider a break in terms of mass, volume and consumption power, and the possibility to make science with nanosatellites.

Fig. 1 .

 1 Fig. 1. SPOC principle. Left: OPD modulation between glass prism and chip, responsible for 2-waves interferences on the sensor surface; Right: 2D-interferogram obtains with the strong line of Argon at 912 nm and a uniform illumination.

Fig. 2 .

 2 Fig. 2. Left: optical design of the four band imaging spectrometer SPOC; Right: spectral band allocation over the chip. Each spectral band is divided into 100 square sub-apertures coding for 100 different OPD δk.

Fig. 3 .

 3 Fig. 3. Imaging micro-SPOC implementation example in a 6U cubesat plateform from ISIS.

Fig. 4 .Fig. 5 .

 45 Fig. 4. MODTRAN simulations of absorption lines of atmospheric CO2 at 1.6 µm (Left) and 2.06 µm (Right), for a concentration of 400 ppm.

Fig. 6 .Fig. 7 .

 67 Fig. 6. Maximum of the Jacobian around 5.5 mm for a variation of 1 ppm of CO2 according to the sub-band position and width in 1.6 µm band.

Fig. 8 .

 8 Fig. 8. Effects of scattering on the 1.6 µm (top) and 2.06 µm (bottom) bands. We notice a potential aerosol signature on the 2.06 µm band independent of CO2 absorption. An interesting way to mitigated the relevant biases over the measure.


Laurianne Georgeton
email: laurianne.georgeton@univ-paris3.fr

Tanja Kocjančič
email: tkocjancic@gmail.com

Cécile Fougeron
email: cecile.fougeron]@univ-paris3.fr

Domain initial strengthening and height contrast in French: acoustic and ultrasound data

Keywords: vowels, prosodic boundary, French, height contrast

This paper investigates how prosodic boundary strength (IPi vs. IPm) affects the production of the vowels /i, e, ɛ, a/, which contrast on a four degrees of height scale in French. Acoustic and tongue configuration data are examined for four speakers. Results show an expansion of the vowel space in IP-initial position that is achieved by a rising of F2 for /i, e, ɛ/ and of F1 for /ɛ, a/. Differences in tongue configuration also contribute to an expansion of the articulatory space in IP-initial position are also observed. Measurements on the highest point of the tongue show a narrowing of the constriction for /i, e, ɛ/, accompanied by fronting for /e, ɛ/, and a widening and backing of the constriction for /a/ for most speakers. These variations in IP-initial position lead to a maximization of phonetic contrasts in terms of height and frontness within the pairs /e-ɛ/ and /ɛ-a/ for most speakers, but not within the pair /i, e/, probably due to articulatory/acoustic constraints.

Introduction

Several studies have reported an effect of prosodic position on vowels, with a modification of their acoustic or articulatory properties when accented or close to a prosodic boundary (see the review by [START_REF] Cho | The phonetics-prosody interface in laboratory phonology[END_REF]andrecently Kim &Cho 2011, 2012;[START_REF] Georgeton | Domain-initial strengthening of French vowels and phonological contrasts: Evidence from lip articulation and spectral variation[END_REF]. Except for the last reference, most studies have been limited to the investigation of only a few types of vowels, usually peripheral vowels. Consequently, it is not clear whether all vowel types are modified in the same way, nor whether prosodically driven segmental variations may be modulated by the density of the phonological inventory. In order to address these questions, the purpose of this study is to investigate prosodic effect on vowels in a dense dimension of contrasts, namely the four levels of height on which the four front vowels /i, e, ɛ, a/ are contrastive in French. Variations in the lingual and acoustic properties of these vowels are tested according to the strength of the prosodic boundary, i.e. according to whether they are initial or medial in an Intonational Phrase (IPi or IPm). Few studies have investigated variations in the lingual articulation of vowels in absolute domain-initial positions (#VC sequences) and their results show that vowels are influenced by prosodic strengthening (as consonants do) with a global increase in gestural magnitude in higher prosodic constituents, which interacts with vowel in different directions. In a study investigating lingual variation of the two vowels /ɛ/ and /ɔ/ in English, Lehnert-Lehouillier and colleagues reported a greater articulatory magnitude in IP initial position, without more description on the direction of the changes in lingual configuration [START_REF] Lehnert-Lehouillier | Prosodic strengthening in American English domain-initial vowels[END_REF]. Kim andCho (2011, 2012) observed that for all the three front English vowels /i, ɪ, ae/, boundary induced variation resulted in a featural enhancement of [+/-high] properties in such a way that, in IP-initial position, the high front vowels /i, ɪ/ were higher while the low front /ae/ was lower. For the vowel /ae/, this variation in height was accompanied by more anterior tongue position. In French, prosodically driven lingual variations on vowels have been mostly investigated under focal accentuation or in domain-final position. [START_REF] Loevenbruck | An investigation of articulatory correlated of the Accentual Phrase in French[END_REF]Loevenbruck ( , 2000) ) observed a similar expansion in height contrast between /i/ and /a/ under focal accentuation, with a higher tongue body for /i/ and a lower tongue body for /a/. Tabain & Perrier investigated domain final /i/ (2005), /a/ (2003) and /u/ (2007) in different prosodic constituents. A lower tongue body before stronger prosodic boundaries was also found for the low vowel /a/, but for /i/ the effect was lesser and speaker-dependent: one of their three speakers showed a backing of the tongue but the other two tended to raise and front their tongue body. For the vowel /u/, they observed tongue dorsum backing coupled with raising or lowering depending on the speaker. The authors concluded that these different strategies concurred to a common acoustic goal: the raising of F3 for vowel /i/, and the lowering of F2 for vowel /u/ in order to prevent a perceptual confusion with /y/. These results suggest that articulatory variations induced by prosodic boundaries may depend on the language's phoneme inventory and the preservation of vowel contrasts (see also [START_REF] Cho | Domain-initial strengthening as featural enhancement: Aerodynamic evidence from Korean[END_REF] for consonantal contrasts). In a recent study [START_REF] Georgeton | Domain-initial strengthening of French vowels and phonological contrasts: Evidence from lip articulation and spectral variation[END_REF], we also observed contrastdependent variation in domain-initial position, by looking at the labial articulation and acoustic properties of the 10 oral vowels of the French system. While all vowels showed an increase in lip area in IP-initial position, this effect was found to be larger for the unrounded vowels. Consequently, the contrast between front rounded and unrounded vowels was found to be maximized in IP-initial position. In the present paper, we address further this question by investigating changes in tongue configuration and acoustic property for the four front (unrounded) French vowels /i, e, ɛ, a/. Within these four levels of height, density-dependent limitations on phonetic variation may be at play (as suggested by [START_REF] Manuel | The role of contrast in limiting vowel-tovowel coarticulation in different languages[END_REF] for example). According to the literature cited above, an enlargement of the oral constriction can be expected for the lower vowel /a/, while predictions are not clear for the vowel /i/, and absent for the non-peripheral vowels, the mid-closed /e/ and mid-open /ɛ/. We will therefore examine how prosodic effect modifies the articulation of these domain-initial vowels and whether it interacts with vowel contrasts in this dense system.

Material and method

The lingual configuration and acoustic properties of the four front oral vowels /i, e, ɛ, a/ have been investigated in two prosodic conditions: in an Intonational phrase-initial (IPi) vs. a IP-medial (IPm) position. For the IPm position, vowels were initial in the second word of a fake compound first name. The four vowels were produced in controlled sentences in a [ip#VC] context. C is /p/ for /i, e, a/ and /v/ for mid-open /ɛ/, in order to prevent its pronunciation as mid-closed (see [START_REF] Georgeton | Domain-initial strengthening of French vowels and phonological contrasts: Evidence from lip articulation and spectral variation[END_REF] for the description of a similar corpus). Four female speakers were recorded with a Midray DP600 ultrasound (60 i/sec) with head stabilization (Articulate Instruments Ltd, 2008). Each sentence was produced 10 times in a random order but repetitions with un-exploitable tongue contours have been discarded from the analysis. Table 1 summarizes the number of renditions analyzed per vowels and prosodic positions (IPi, IPm) for the 4 speakers (SA, SC, SL, SZ). Target vowels were segmented and labeled in Praat in order to extract acoustic duration and formant values. F1 and F2 were taken at three successive points in the middle of target vowels and then averaged. For lingual configuration, one to three (depending on vowel duration) successive tongue contours were traced manually in the middle of the vowel with Articulate Assistant Advanced (Articulate Instruments Ltd, 2012) and then averaged for each vowel/position condition. An estimation of vowel height and place of articulation was done by quantifying the height (y-axis) and front-back location (x-axis) of the highest point of the tongue, which was automatically extracted from individual contours.

In order to test whether F1, F2, duration and coordinates of the highest point of the tongue vary according to prosodic position, analyses by speaker are done with sample t-tests for each vowel. In order to test the interaction of boundary effects with height contrast between each pair /i-e/, /e-ɛ/ and /ɛ-a/ two-factor ANOVAs (position, vowel) were conducted for each speaker. (Note that due to space limitations, statistical details are not given here and only significant differences are reported; for speaker SA, given his small number of renditions, only tendencies are reported).

Results

Boundary effect (IPi vs IPm) on the lingual and acoustic properties per vowel types

Differences in tongue contours and in spectral properties according to prosodic positions are illustrated in Figure 1. For all speakers, an effect of prosodic position is observed, with speaker-and vowel-dependent patterns. Looking at F1 and F2 of vowels in IP-initial position compared to IP-medial position, the following variations are observed:

• /i/ has a higher F2 for all speakers except SZ, and no variation is found on F1; • /e/ has a higher F2 for all speakers and a higher F1 for one speaker (SL) • /ɛ/ has a higher F2 and a higher F1 for all speakers; • /a/ has a higher F1 for all speakers. An effect on F2 is found for speakers SL and SZ but in an opposite direction: higher F2 for SZ and lower F2 for SL. These spectral variations are not accompanied by systematic differences in acoustic vowel duration. Speakers SL, SC and SA have longer /i/ in IPi, but for the other vowels, few differences in vowel duration appear, and these differences are speaker-and vowel-dependent. A lengthening in IPi is found for /e/ for SC, a shortening is found for /ɛ/ for SL, for /a/ for SL and SZ, and for the nine other comparisons there is no change in vowel duration. Considering articulatory variations, the differences in tongue contours between the two positions presented in Figure 1 appear to be larger for speakers SL, SZ and SA than for speaker SC, and are clearly vowel dependent. As explained in the method section, a quantification of the differences in lingual configuration is made to estimate the degree and location of the constriction at the highest point of the tongue. In IP-initial position, the following variations are observed:

• /i/ has a narrower constriction with a rising of the highest point of the tongue for all speakers except SZ, who rather shows a baking of the constriction.

• /e/ has a narrower constriction for all speakers and a fronter constriction for speaker SL and SA; • /ɛ/ has a narrower constriction for all speakers and a fronter constriction for all except SC. • /a/, on the other hand, has a wider constriction for all speakers, and a backer constriction for all except SC. Overall, from a systemic perspective, the variations observed in IP-initial compared to IP-medial position contribute to an expansion of the acoustic and articulatory spaces. This expansion is achieved by an enlargement of the spaces both in the vertical dimension (constriction height, F1) and horizontal dimension (constriction location, F2).

Boundary effects on the contrast between vowels pairs

The effect of prosodic position is further tested here on the acoustic and articulatory contrast between adjacent vowels along the vowel height dimension of contrast. For this, we test whether the effect of boundary depends on the vowel identity within the three vowel pairs (/i-e/, /e-ɛ/, /ɛ-a/), and therefore whether the contrast between the members of the pairs is affected by prosodic position.

For the /i-e/ pair, a significant interaction is found on constriction height for all speakers: both /i/ and /e/ have a narrower constriction in IPi, but the amplitude of the tongue rising for /e/ is larger than that for /i/. Consequently, /e/ gets closer to /i/ and the contrast in height dimension between these two vowels is not maximized in IP-initial position. An interaction is also found on dimensions linked to place of articulation: front-back position of the constriction for SL and SC, and F2 for SZ and SA. For all speakers except SC, both vowel are modified in the same way (higher F2 for SZ and SA and fronter constriction for SL) but with a larger change for /e/. Consequently /e/ gets closer to /i/. Speaker SC is the only one showing a larger contrast in constriction location between /i/ and /e/ in IPi, with a fronting of the constriction for /i/ and a backing for /e/.

For the pair /e-ɛ/, an interaction is found on the dimensions linked to vowel height: F1 for all speakers except SL, and constriction height for SL. This interaction reflects a maximization of the distinction between the mid-closed and mid-open vowels in IPi for all speakers, with a greater narrowing of the constriction for /e/ than /ɛ/ (SL), and a large rising of F1 for /ɛ/ (SC, SZ, SA). Interactions are also found on the dimensions linked to place of articulation for all speakers except SC: on F2 for SL and SA, on the front-back position of the constriction for SL and SZ. Again the contrast between the two vowels is maximized in IPi for SL and SA with a larger F2 rise for /e/ than /ɛ/ (SL and SA) and more fronting of the constriction (SL). For SZ however, the two vowels get closer in IPi with a slight backing of the constriction for /e/ and a slight fronting for /ɛ/.

For the pair /ɛ-a/, interactions are found on constriction height for all speakers, on F1 for all speakers except SA, on constriction location for all except SC and on F2 for all except SZ. The two vowels are more distinct in IPi position with a wider constriction and higher F1 for /a/ than /ɛ/ for all speakers except SA, and with a backer constriction for SL, SA, SZ, and a lower F2 for SL, SC, SZ.

Discussion and conclusion

In this study, we investigated the effect of prosodic position on the lingual and acoustic properties of the front unrounded oral French vowels /i, e, ɛ, a/ in IP-initial vs. IP-medial positions. As found in the other studies presented in the introduction, we show that prosodic position influences the articulation and acoustic properties of the vowels. In IP-initial position an expansion of the articulatory and acoustic spaces is observed in height and front-back dimensions of the lingual constriction (estimated from the highest point of the tongue) as well as in F1 and F2 dimensions. The observation of more than two degrees of vowel height provides interesting results on the direction of this boundary effect. Different sets of vowels can be distinguished. On the dimension related to tongue height (vertical position of the highest point of the tongue), the non-low vowels /i, e, ɛ/ pattern together with a narrower constriction degree in IPinitial position for most of the speakers (three for /i/, four for /e, ɛ/). On the other hand, the low vowel /a/ shows a widening of the oral constriction for all speakers, as observed in other studies. Regarding the location of the constriction in the frontback dimension, a fronting of the constriction is observed in IPi for /e, ɛ/ (two speakers for /e/, and three for /ɛ/), while /a/ has a backer constriction (three speakers). These measurements made on the highest point of the tongue appear to be well suited to quantify some of the differences in tongue contours illustrated in Figure 1. However, they do not capture the overall modifications in tongue shape and location. Modifications in the oral cavity resonators are therefore better accounted for by the variations in F1 and F2. In IP-initial position, the vowels /i, e, ɛ/ also pattern together with a rising of F2 for at least three speakers. In terms of F1, the non-high /ɛ, a/ are both characterized by a rising of F1 for all speakers (also for /e/ for one speaker). Our data further show that in a dense system of contrast, where vowels have little room for phonetic variations, prosodic boundary effects may contribute to maximize the contrast between adjacent vowels. An increase of phonetic distinctiveness is observed on our articulatory and/or acoustic measurements with differences according to speakers and vowel pairs. Overall, for all pairs but /i-e/, the distinction between the vowels is maximized in IP-initial position in at least one dimension for all speakers. The contrast between the mid-open and mid-closed vowels is mainly increased in the height dimension (F1 and constriction height), and the contrast between the mid-open and open vowels is mainly increased in height and/or frontness of the constriction. In most cases, the direction of the articulatory and/or acoustic variations in IPinitial position is similar for the two vowels of the pair, but is larger for one of them, leading to an increase of contrast between the two. A similar tendency was observed in [START_REF] Georgeton | Domain-initial strengthening of French vowels and phonological contrasts: Evidence from lip articulation and spectral variation[END_REF] for the labial configuration resulting in an increase of contrast between front rounded and unrounded vowels (see also [START_REF] Cole | Prosodic effects on acoustic cues to stop voicing and place of articulation: Evidence from Radio News speech[END_REF] for consonants). For the pair /i-e/, however, no increase of contrast is found, except for the location of the constriction for SC. Acoustic and physiological limitation can explain that the tongue rising in IPi is more constrained for /i/ and is thus smaller than that for /e/. Similarly, boundary induced variation at the lips were found to be larger for unrounded vowels than for the more constrained rounded vowels in [START_REF] Georgeton | Domain-initial strengthening of French vowels and phonological contrasts: Evidence from lip articulation and spectral variation[END_REF]. Taken together these results suggest that phonetic contrasts between vowels tend to be maximized in IP-initial position in dimensions that are less constrained by articulatory and/or acoustic limitations. Cross-linguistic comparisons between languages with different vowel inventories are now necessary to determine how these limitations comply with the language system of contrasts.

Figure 1 :

 1 Figure 1: Mean tongue contours and F1/F2 values of the four vowels /i/ (orange), /e/ (green), /ɛ/ (blue) and /a/ (red) in IPi (solid line) and IPm (dotted line) for the four speakers (SZ, SL, SC, SA).

Table 1 :

 1 Number of renditions analyzed by speaker, prosodic position and vowels.

		SC		SL		SZ		SA
		IPi IPm	IPi IPm	IPi IPm IPi IPm
	/i/	10	10	10	10	11	9	5	5
	/e/	10	10	10	10	8	9	6	6
	/ɛ/	10	10	10	10	9	9	6	6
	/a/	10	10	8	8	11	9	5	6

Acknowledgements

This work has been supported by the French Investissements d'Avenir -Labex EFL program (ANR-10-LABX-0083).