M Meftah

• D Bolsée

• L Damé

• A Hauchecorne

• N Pereira

• A Irbah

• S Bekki

• G Cessateur

• T Foujols

• R Thiéblemont

Solar Irradiance from 165 to 400 nm in 2008 and UV variations in 3 spectral bands during Solar Cycle 24

Keywords: Solar Irradiance, Solar Cycle, Instrumentation and Data Management, Instrumental Effects

Accurate measurements of solar spectral irradiance (SSI) and its temporal variations are of primary interest to better understand solar mechanisms, and the links between solar variability and Earth's atmosphere and climate. The SOLar SPECtrum (SOLSPEC) instrument of the Solar Monitoring Observatory (SOLAR) payload onboard the International Space Station (ISS) has been built to carry out SSI measurements from 165 to 3088 nm. We focus here on the ultraviolet (UV) part of the measured solar spectrum (wavelengths less than 400 nm) because the UV part is potentially important for understanding the solar forcing of Earth's atmosphere and climate. We present here SOLAR/SOLSPEC UV data obtained since 2008, and their variations in three spectral bands during Solar Cycle 24. They are compared with previously reported UV measurements and model reconstructions, and differences are discussed.

Introduction

The solar spectrum is a key input for different disciplines such as:

• Solar Physics where the solar spectrum characterizes the activity of the Sun's outer layers (photosphere, chromosphere, and corona). By comparison with the theoretical reconstructions, an accurately measured solar spectrum allows to validate temperature, composition, and densities of the solar atmosphere.

• Climate Physics where solar-dependent processes are recognized as major drivers of rapid climate changes. Climate models require time-varying solar spectra as forcing. The available information is often based on solar reconstructions and models. However, it is not clear that they correctly represent the different levels of solar activity, notably ultraviolet (UV) levels. As a reminder, atmospheric models with representation of the middle stratosphere have been developed [START_REF] Hegglin | Multimodel assessment of the upper troposphere and lower stratosphere: Extratropics[END_REF]. They strongly focus on simulating ozone, stratospheric dynamics [START_REF] Strahan | Using transport diagnostics to understand chemistry climate model ozone simulations[END_REF], and testing knowledge of what controls global ozone [START_REF] Oman | Multimodel assessment of the factors driving stratospheric ozone evolution over the 21st century[END_REF], notably the springtime polar ozone depletion [START_REF] Von Hobe | Reconciliation of essential process parameters for[END_REF]. The acknowledgement that the atmosphere at its different altitude ranges needs to be modeled as a coupled entity is relatively recent. Currently weather forecast models are including a representation of the mesosphere as it enhances the long term predictability power of the model. Solar variability forces the atmosphere at different altitudes by different mechanisms. The signal is amplified at the upper layers showing some degree of linearity in the chemistry. However, the signal becomes smaller and more complex at lower altitudes making it difficult to extract from model natural variability. Furthermore, atmosphere numerical models seem sensitive to the choice of solar irradiance variability used to force the model. As example, solar spectral irradiance (SSI) measurements in the Herzberg continuum will help to address several questions concerning the climate response to variable solar forcing [START_REF] Sukhodolov | Evaluation of simulated photolysis rates and their response to solar irradiance variability[END_REF]. About the stratospheric ozone, the total column ozone data records analysis show a clear 11-year signature, compatible with the Solar Cycle. However, models have so far not been able to reproduce completely the profile and magnitude of the signal [START_REF] Austin | Coupled chemistry climate model simulations of the solar cycle in ozone and temperature[END_REF]. In fact, many meteorological quantities are correlated with the 11-year solar signal but what are the mechanisms behind those signals?

Before 1970, very few reliable solar irradiance measurements above the atmosphere are available. There is the Neckel and Labs solar spectrum [START_REF] Labs | The Radiation of the Solar Photosphere from 2000 Å to 100 µm[END_REF][START_REF] Neckel | Improved data of solar spectral irradiance from 0.33 to 1.25 microns[END_REF][START_REF] Neckel | The solar radiation between 3300 and 12500 A[END_REF], which is a reference spectrum obtained from ground-based and high-altitude measurements. From 1978, regular space-based measurements of the solar irradiance started. These space-based measurements began with the Shuttle borne Solar Backscatter UltraViolet spectrometer [START_REF] Cebula | Calibration of the Shuttle borne solar backscatter ultraviolet spectrometer[END_REF][START_REF] Cebula | NOAA 11 solar backscattered ultraviolet, model 2 (SBUV/2) instrument solar spectral irradiance measurements in 1989-1994 1. Observations and long-term calibration[END_REF]. The first regular space-based monitoring of the solar spectrum over a broad range (240-2380 nm) was carried out by the SCanning Imaging Absorption spectroMeter for Atmospheric CHar-tographY (SCIAMACHY) onboard Environment Satellite [START_REF] Bovensmann | SCIAMACHY: Mission Objectives and Measurement Modes[END_REF][START_REF] Pagaran | Solar Variability from 240 to 1750 nm in Terms of Faculae Brightening and Sunspot Darkening from SCIAMACHY[END_REF][START_REF] Pagaran | Intercomparison of SCIAMACHY and SIM vis-IR irradiance over several solar rotational timescales[END_REF]. An overview of the main satellite missions that have made SSI observations at wavelengths higher than 100 nm is given by [START_REF] Ermolli | Recent variability of the solar spectral irradiance and its impact on climate modelling[END_REF].

SOLAR/SOLSPEC is a space-based spectro-radiometer developed by CNRS-LATMOS (France), and by BIRA-IASB (Belgium) with a major collaboration of the Heidelberg Observatory (Germany). It is externally mounted on the Columbus module of the International Space Station (ISS). The instrument measures the solar spectral irradiance above the atmosphere since 2008 and is still operational today. SOLAR/SOLSPEC consists of three separated double-spectrometers that use concave holographic gratings made by Jobin-Yvon to cover the specific 'UV' (165-371 nm), 'VIS' (285-908 nm), and 'IR' (646-3088 nm) spectral ranges. The spectral resolution of the 'UV' spectrometer is ranging from 0.6 nm to 1.6 nm, whereas the 'VIS' spectrometer is ranging from 1.6 nm to 2.1 nm, and the 'IR' spectrometer is ranging from 7 nm to 9.5 nm. Using a mechanical shaft, all the gratings are rotating simultaneously and scanning the three spectral ranges at the same time. The detectors are Electro-Mechanical Research photo-multiplier (PM) tubes in the 'UV' and 'VIS' spectral ranges, and a Hamamatsu P2682 PbS cell cooled at -20 o C in the 'IR' spectral band. SOLAR/SOLSPEC scientific objectives, instrument performances, and absolute calibration using a blackbody as primary standard source are described in more details by [START_REF] Thuillier | SOLAR/SOLSPEC: Scientific Objectives, Instrument Performance and Its Absolute Calibration Using a Blackbody as Primary Standard Source[END_REF] and [START_REF] Bolsée | Métrologie de la spectrophotométrie solaire absolue: principes, mise en oeuvre et résultats ; instrument solspec à bord de la station spatiale internationale[END_REF].

The present article is dedicated to the UV portion (165-400 nm) of the solar spectrum and its temporal variations during Solar Cycle 24. Measurements from two separated double-spectrometers of SOLAR/SOLSPEC ('UV' and 'VIS') are combined. From these data, we obtained the UV solar spectrum above the Earth's atmosphere at a distance of one astronomical unit (initial values for studying the UV solar variability). SOLAR/SOLSPEC measurements are compared with previously reported solar UV measurements and model reconstructions and differences are discussed. SOLAR/SOLSPEC UV spectra are first compared with the ATmospheric Laboratory for Applications and Science (AT-LAS) composite [START_REF] Thuillier | The Solar Spectral Irradiance from 200 to 2400 nm as Measured by the SOLSPEC Spectrometer from the Atlas and Eureca Missions[END_REF] and with the Solar Irradiance Reference Spectra (SIRS) for the 2008 Whole Heliosphere Interval (WHI) corresponding to the solar Carrington rotation 2068 from mid-March to mid-April 2008 [START_REF] Woods | Solar Irradiance Reference Spectra (SIRS) for the 2008 Whole Heliosphere Interval (WHI)[END_REF]. Similarly, SOLAR/SOLSPEC UV spectra are compared with reconstructions issued from the Spectral And Total Irradiance REconstruction for the Satellite era (SATIRE-S) semi-empirical model [START_REF] Yeo | Reconstruction of total and spectral solar irradiance from 1974 to 2013 based on KPVT, SoHO/MDI, and SDO/HMI observations[END_REF]. Degradation of the SOLAR/SOLSPEC instrument is highlighted, providing information for the development of future space-based UV instruments. Finally, we present SOLAR/SOLSPEC UV SSI temporal variations from 2010 to 2015 in three spectral bands (165-180, 180-200, and 200-242 nm).

SOLAR/SOLSPEC Observations and Corrections

SOLAR/SOLSPEC was exposed to sunlight for the first time on April 5, 2008. The duration to record a solar spectrum (165-3088 nm) is less than 17 minutes. Between April 2008 and May 2016, more than 700 solar spectra were acquired. The fundamental issues for solar spectra measurements are:

• High accuracy to determine a reference solar spectrum, • And long-term stability to accurately determine SSI change with time.

A typical day of acquisition consists of one solar spectrum measurement and three calibrations for different purposes (radiometric calibrations, spectral resolutions, and wavelength scales). The deuterium lamp (nominal or spare) allows a calibration of the 'UV' spectral band. Similarly, the tungsten ribbon lamp (nominal or spare) allows a calibration of the 'VIS' spectral band. Two tungsten ribbon lamps (nominal and redundant) are associated with the 'IR' spectral band. Moreover, a helium hollow cathode lamp controls spectral resolutions and wavelength scales for the 'UV' and the 'VIS' spectral ranges. Thus, SO-LAR/SOLSPEC has on-board standards (i.e. a lamps system) for calibration to identify possible SSI long-term changes. Indeed, the lamps are used for checking the instrument stability with time in order to provide reference data for aging corrections. However, these corrections are not straightforward (degradation of the lamps themselves as shown in Figure 1, lamps power supply failure, etc.). For example, the calibration of the 'UV' spectral band (deuterium lamps) was no longer possible since April 2009 due to the power supply failure of the deuterium lamps. Other calibrations allow to quantify the SOLAR/SOLSPEC quartz window transmission decreases. Indeed, the solar spectrum measurements are done with or without the quartz window in front of the entrance slit. The mechanical shutters and the quartz windows are used as a shield for limiting the degradation of the optical components due to the harsh space environment.

SOLAR/SOLSPEC Preflight Calibration and Expected Uncertainties

Pre-flight calibration to identify critical SOLAR/SOLSPEC performance parameters was described by [START_REF] Thuillier | SOLAR/SOLSPEC: Scientific Objectives, Instrument Performance and Its Absolute Calibration Using a Blackbody as Primary Standard Source[END_REF] and in more details in [START_REF] Bolsée | Métrologie de la spectrophotométrie solaire absolue: principes, mise en oeuvre et résultats ; instrument solspec à bord de la station spatiale internationale[END_REF]. Before the SOLAR mission, the first version of the SOLSPEC instrument has flown on SpaceLab I, on the ATLAS missions, and on EURECA. It provided data from 200 to 2400 nm [START_REF] Thuillier | The Solar Spectral Irradiance from 200 to 2400 nm as Measured by the SOLSPEC Spectrometer from the Atlas and Eureca Missions[END_REF]. This heritage has been taken into account in the design of SOLAR/SOLSPEC to increase its accuracy [START_REF] Thuillier | SOLAR/SOLSPEC: Scientific Objectives, Instrument Performance and Its Absolute Calibration Using a Blackbody as Primary Standard Source[END_REF]. Pre-flight, the absolute calibration of SOLAR/SOLSPEC was carried out using the spectral irradiance standards, such as the deuterium lamps and the blackbody of the Physikalisch-Technische Bundesanstalt (PTB, Braunschweig, Germany). During the absolute calibration at the PTB, the temperature of the SOLAR/SOLSPEC instrument was 23.3±1.5 o C.

Expected standard measurements uncertainties (1 σ) are between 2% to 4% in the 165-370 nm range and less than 2% in 370-400 nm (see [START_REF] Bolsée | Métrologie de la spectrophotométrie solaire absolue: principes, mise en oeuvre et résultats ; instrument solspec à bord de la station spatiale internationale[END_REF], which correspond to begin of life (BOL) values for SOLAR/SOLSPEC uniform temperatures (no temperature gradient).

SOLAR/SOLSPEC SSI Equation -Ideal Case

The SOLAR/SOLSPEC instrument has made continuous SSI measurements from April 2008 until today. In the best case (BOL and SOLAR/SOLSPEC temperature close to 23 o C), Equation 1allows to determine the SSI evolution over time.

SSI S (λ, t) = S n (λ, t) - < DC(t) > τ (t) × R(λ, t) × z(t) 1 au 2 , (1)
where SSI S (λ, t) is the SSI as measured by SOLAR/SOLSPEC at a given wavelength (λ) and for a given time (t). S n (λ, t) (in counts s -1 ) is the linearized signal provided by the detector (solar blind photomultiplier). The two spectral ranges ('UV' and 'VIS') are linear within their whole dynamical range (until 20 000 counts s -1 ). < DC(t) > (in counts) is the mean dark current of the detector taken before and after a solar spectrum measurement during the integration time (τ (t) in s). R(λ, t) (in mW m -2 nm -1 counts -1 s) represents the absolute responsivity of the instrument. z(t) (in km) is the distance between SOLAR/SOLSPEC and the Sun. The astronomical unit (1 au) is equal to 149 597 870.700 km.

The first SOLAR/SOLSPEC UV spectral distribution of the Sun's radiation at the mean Earth-Sun distance of one astronomical unit is shown in Figure 2 (red curve). No correction was applied such as photocathode quantum efficiency change with the temperature, which is a relatively weak effect. All solar spectra acquired between April 2008 and August 2015 are shown in blue for the 'UV' spectral band and in green for the 'VIS' spectral band. One can notice than the SOLAR/SOLSPEC SSI variations are much higher than the expected values. Indeed, below 400 nm, the long-term SSI variability increases with decreasing wavelength, reaching ∼15% of SSI variation at 165 nm using SATIRE-S data from 2008 to 2015. Yet, the SOLAR/SOLSPEC SSI variation at 165 nm can reach more than 80% from 2008 to 2015. This is obviously an abnormal variation, which highlights the degradation of the SOLAR/SOLSPEC instrument. Indeed, SOLAR/SOLSPEC such as different solar space-based instruments (Meftah et al., 2014a;[START_REF] Cessateur | Solar irradiance observations with PREMOS filter radiometers on the PICARD mission: In-flight performance and data release[END_REF] is vulnerable to the degradation due to the space environment [START_REF] Benmoussa | On-Orbit Degradation of Solar Instruments[END_REF]. Thus, the SOLAR/SOLSPEC signal (S n (λ, t)) may evolve over time. Moreover, possible mechanical distortions (location of intermediate and exit slits) associated with temperature gradients can change the responsivity of the instrument.

SOLAR/SOLSPEC SSI Equation -Real Case

With the main correction factors, Equation 2 allows to determine a more realistic SOLAR/SOLSPEC SSI evolution over time.

SSI R (λ, t) = SSI S (λ, t) × 1 1 -α T (λ,t) 100 × (T P -T S (t)) × 1 Deg(λ, t) × 1 I E (λ, t) ,
(2) where α T (λ, t) is the temperature coefficient of the UV PM (% o C -1 ). T P represents the temperature reference of the UV PM during ground calibration (23.3 o C). T S (t) (in o C) represents the temperature of the UV PM in space. Deg(λ, t) characterizes the degradation of the instrument (dimensionless factor where Deg(λ, t) 1). The SOLAR/SOLSPEC internal optics and the quartz plate degradations can be monitored. Correction of the absolute responsivity (R(λ, t)) degradation of the instrument could have been obtained with the deuterium lamps calibration during a period. I E (λ, t) corresponds to instrumental effects that impact the optical quality of the instrument. I E (λ, t) is a dimensionless factor, which is equal to 1 for an optimal calibration of the instrument at a temperature of 23.3 o C such as the absolute calibration at the PTB.

Reversible Instrumental Effects Mainly Depicted by the I E (λ, t)

Dimensionless Factor

Recent tests (March 2016) were realized to better understand the behavior of the SOLAR/SOLSPEC instrument. The scanning mechanism was maintained at a fixed position in order to measure the SSI evolution over time at a given wavelength (210.2 nm). This test highlights that the instrument is unstable during short-time period (∼6% of variation). It was repeated several times (Figure 3), which shows the reversibility of the instrumental effect (I E (λ, t)). SOLAR/SOLSPEC measurements during 20 minutes show SSI variations that are above the expected values during a solar cycle. The thermo-mechanical stability of the instrument is questionable and requires an opto-thermo-mechanical analysis. One can notice that during SOLAR/SOLSPEC switch-on, the signature of the signal is slightly different. Currently, we try to understand the link between the SOLAR/SOLSPEC UV photo-multiplier (UV PM) temperature and the measured signal. Until April 2009, the degradation of the instrument (Deg(λ, t)) has been important (Figure 4) and was wavelength dependent. There is a dramatic aging due to the deuterium lamps use. From April 2009 to mid-2010, the degradation began to decrease. Since mid-2010, the SOLAR/SOLSPEC degradation follows a two-term exponential model where two degradation modes exist. The instrument is composed of several optical elements, which have different degradation time process. This law is demonstrated for different wavelengths of the instrument with different time responses. As a reminder, the instrument contains four lamps (nominal and spare) for checking the responsivity (R(λ, t)) of the 'UV' and 'VIS' spectrometers in order to correct the degradation. As explained in Section 2, the aging correction with the lamps is not trivial. This work is still in progress. Firstly, we can make the assumption that the degradation follows an exponential law to control the ability of SOLAR/SOLSPEC for measuring UV variability. The square markers represent the annual mean measurements for each spectral bands.

2.6. SOLAR/SOLSPEC Main Temperatures Evolutions -α T (λ, t) and I E (λ, t)

Over time, the main shutter temperature of SOLAR/SOLSPEC increases (Figure 5, top), which highlights the degradation of the instrument thermal control. One can notice a temperature increase of ∼5 o C from April 2008 to mid-2015. The instrument thermal heat sink facing the Sun uses a white paint, which degrades over time (increase of solar absorption). That is another level of degradation that impacts the temperature of the instrument and introduces temperature gradients. Figure 5 (bottom) displays the mean SOLAR/SOLSPEC temperatures (UV PM, main shutter, and centre plate) of the time series as a function of wavelength. As a reminder, the duration to record a solar spectrum is less than 17 minutes. Since the beginning of the mission, the measurement process is the same. Indeed, all gratings are rotating and scanning the three 'UV'-'VIS'-'IR' spectral ranges at the same time from short wavelengths toward higher. The mean temperature of the UV photo-multiplier (UV PM) is close to 5 o C during each SSI measurement. During the absolute calibration at the PTB, the UV PM's temperature was close to 23 o C. Thus, the SSI as seen by SO-LAR/SOLSPEC (SSI S (λ, t)) requires a thermal correction (α T (λ, t)) of the UV PM temperature that is wavelength dependent. However, this thermal correction is weak and depends on the PM manufacturer data. One can notice that the temperature between the front face of the instrument (main shutter) and the central part (centre plate) is not the same. Moreover, they change significantly during the SSI measurements. There is ∼7.5 o C of variation for the front face and ∼2.5 o C of variation for the central part of the instrument. There is a thermal gradient on SOLAR/SOLSPEC between the front face and the central part, which is wavelength dependent. Thus, there is mechanical distortions on SOLAR/SOLSPEC due to thermal gradients that cause displacement of inter- mediate and exit slits. The understanding of this phenomenon is still under study and requires a correction to SOLAR/SOLSPEC data (I E (λ, t)).

Wavelengths Shift Effect

As a reminder, SOLAR/SOLSPEC has a mechanism (gratings positioning) for directing the selected color to an exit slit. On SOLAR/SOLSPEC raw data, wavelengths shift were observed over time as shown in Figure 6 (top). This effect requires a correction of raw data, which follows a one-term exponential law (stability of the mechanism and its stepper motor that supports the gratings). After applying this correction, Equation 2 is applicable.

Results and Discussion

SOLAR/SOLSPEC Solar Irradiance from 165 to 400 nm (2008 Minimum)

Accurate determination of the solar irradiance from 165 to 400 nm is challenging. Calibration is an essential part of any space-based instrument. SO-LAR/SOLSPEC response was precisely characterized (radiometric calibration, wavelength calibration, slit-function characterization, stray-light, field of view, etc.) during on-ground calibration phase [START_REF] Bolsée | Métrologie de la spectrophotométrie solaire absolue: principes, mise en oeuvre et résultats ; instrument solspec à bord de la station spatiale internationale[END_REF]. This step will allow us to have the initial values to determine the UV SSI variability. The main challenge is to keep the ground-based characterization of the instrument during in space operations. Sun-observing instruments exposed to the space environment degrade due to the harsh space environment. SOLAR/SOLSPEC does not escape the rule (Figure 4). It is very important to monitor the degradation during the time of the mission and to have in-flight calibration for correcting the raw data. The SOLAR/SOLSPEC responsivity change is derived from comparison of the transmission of the second quartz plate (infrequently used), direct quartz-plate transmission measurements, and deuterium lamps data. The main correction is obtained from deuterium lamps. SOLAR/SOLSPEC used two deuterium lamps, which may have the same behavior in space. However, their rate of use during in-space calibrations are different inducing a different aging and therefore a different correction. Thus, the correction of SOLAR/SOLSPEC raw data by using the deuterium lamps is not obvious. From Figure 1, the degradation of the signal (200-242 nm) seen by the instrument with the D1 lamp is close to -0.11±0.01% per day (2 σ). From Figure 6 (bottom), the degradation of the instrument (200-242 nm) is estimated at -0.10±0.03% per day. These results are consistent and facilitated by the fact that solar activity is close to zero during this period. However, the degradation of the signal (200-242 nm) seen by the instrument with the D2 lamp is more higher (-0.18±0.01% per day). By standardizing lamp operating time (D1t), the degradation of the signal (200- 242 nm) seen by the instrument with the D1 lamp is close to that seen by the D2 lamp. The lamps have deteriorated as the instrument.

λ [nm]
Another approach is to define a UV solar spectrum using the properties of Equation 2. The ideal case corresponds to a configuration where the instrument is not affected by the degradation (Deg(λ, t)=1) and by the instrumental effects (I E (λ, t)=1). We emphasize that SOLAR/SOLSPEC consists of a subtractive three double-spectrometers. Instrumental effects tend to reduce the measured SSI. Thus, the SSI maximum values measured by SOLAR/SOLSPEC (SSI S (λ, t)) from April 2008 to December 2008 correspond to the most probable SSI values of the real UV solar spectrum during the 2008 minimum. The red curve (Figure 2) is close to the most probable UV solar spectrum without temperature coefficient of the UV PM correction. Most of the SSI values used on this UV solar spectrum are derived from SOLAR/SOLSPEC spectra acquired from April 2008 to June 2008 (instrument BOL and quiet Sun). Figure 7 displays the UV solar spectrum using the thermal correction (α T (λ, t)) at the temperature associated with the wavelength measurement (Figure 5, bottom). Two SOLAR/SOLSPEC spectrometers ('UV' spectral band between 165.86 and 356.26 nm and 'VIS' spectral band between 356.50 and 400.42 nm) were used to obtain this UV solar spectrum (resolution and uncertainty optimization).

Definition of the SOLAR/SOLSPEC Analyzed Spectral Bands

To assess the SOLAR/SOLSPEC UV spectrum (Figure 7), comparisons are needed with reference spectra (ATLAS 3, WHI 2008) or model (SATIRE-S). In this article, we limited our study to a comparison with the SATIRE-S semiempirical model. However, other solar models exist and could be compared.

Several spectral bands considered as important absorption bands of solar radiation [START_REF] Bovensmann | SCIAMACHY: Mission Objectives and Measurement Modes[END_REF] were selected in order to compare different data sets (Table 1) and to improve the number of sample data to analyze (for the UV solar variability, see Section 3.5).

Differences between SOLAR/SOLSPEC and different Reference Spectra

Figure 8 displays the relative differences between SOLAR/SOLSPEC and different reference spectra (ATLAS 3, WHI 2008) or model (SATIRE-S). WHI 2008 solar spectrum was convolved to the SOLAR/SOLSPEC resolution. The SSI variability calculated from the SATIRE-S semi-empirical model is set on the WHI 2008 reference spectra [START_REF] Yeo | Reconstruction of total and spectral solar irradiance from 1974 to 2013 based on KPVT, SoHO/MDI, and SDO/HMI observations[END_REF] for some wavelength bands. Figures 9 and 10 display SSI from 165 to 400 nm for SOLAR/SOLSPEC, AT-LAS 3, WHI 2008, and SATIRE-S. Between 180 and 400 nm, difference between SOLAR/SOLSPEC and WHI 2008 or SATIRE-S is less than 5% while the difference between SOLAR/SOLSPEC and ATLAS 3 is greater than 10%. Data used in the ATLAS 3 spectrum suffered a 1.4% reduction [START_REF] Thuillier | The Solar Spectral Irradiance from 200 to 2400 nm as Measured by the SOLSPEC Spectrometer from the Atlas and Eureca Missions[END_REF]. ATLAS 3 was normalized with a standard value of the total solar irradiance (TSI) of 1367.7 W m -2 . Between 200 and 2400 nm, ATLAS 3 solar illumination is equal to 1315.7 W m -2 in agreement with a TSI of 1367.7 W m -2 . Today, the consensus absolute value of the TSI is close to 1361-1362 Wm -2 (Kopp and [START_REF] Kopp | A new, lower value of total solar irradiance: Evidence and climate significance[END_REF][START_REF] Schmutz | Total solar irradiance measurements with PREMOS/PICARD[END_REF]Meftah et al., 2014b). It seems that the normalization of the ATLAS 3 spectrum is insufficient and could explain the greatest SSI values. Thus, the uncertainty analysis of UV and visible data obtained during the ATLAS 3 mission is questionable. Conversely, SOLAR/SOLSPEC with its uncertainty budget, the WHI 2008 solar spectrum, and the SATIRE-S solar spectrum were found in good agreement in the 180-400 nm spectral band. Between 165 and 180 nm, the differences between SOLAR/SOLSPEC and WHI 2008 or SATIRE-S are of ∼20%. In this spectral band, the agreement is not good. One can notice that the SOLAR/SOLSPEC SSI from 165 to 180 nm is greater than the WHI 2008 SSI or the SATIRE-S SSI in the same band. This may be due to a SOLAR/SOLSPEC ground-based calibration error and or the linearization of the UV signal at low wavelengths. However, we do not exclude that the WHI 2008 SSI is smaller in this spectral band and requires a correction. As a reminder, one can notice than SATIRE-S is wedged on the measurements made by WHI 2008.

Interest of the UV SSI Variability and Difficulty to Perform Measurements

Precise UV SSI variability measurements are required to quantify the topdown mechanism amplifying UV solar forcing on the climate despite the fact that the UV range (115.5 -400 nm) represents only ∼7.7% of the TSI. The main uncertainty in the solar models concerns the wavelength range between 220 and 400 nm, where the magnitude of the variations differs by as much as a factor of three between models [START_REF] Ermolli | Recent variability of the solar spectral irradiance and its impact on climate modelling[END_REF]. UV wavelengths can vary significantly during solar cycles. SOLar STelar Irradiance Comparison Experiment (SOLSTICE, 115-320 nm) and Spectral Irradiance Monitor (SIM, 300-2400 nm) [START_REF] Harder | The Spectral Irradiance Monitor: Scientific Requirements, Instrument Design, and Operation Modes[END_REF] measurements onboard the Solar Radiation and Climate Experiment (SORCE) space-based mission exhibit an unexpected behavior [START_REF] Harder | Trends in solar spectral irradiance variability in the visible and infrared[END_REF]. Currently there is insufficient observational evidence to validate the spectral variations observed by SOLSTICE and SIM. [START_REF] Haigh | An influence of solar spectral variations on radiative forcing of climate[END_REF] have showed that such a spectral variability would lead to a significant change in the understanding of ozone evolution and atmospheric dynamics. The UV variability yielded by SIM and SOLSTICE indicates that the Sun can have substantially contributed to the observed trends of stratospheric ozone from 2004 to 2007. There is an ongoing discussion whether SOLSTICE and SIM did reveal a real solar behavior, or if its measurements were affected by uncorrected sensitivity drifts [START_REF] Lean | How Does the Sun's Spectrum Vary[END_REF]. Overall, the comparison shows that the atmospheric changes simulated with the 3D chemistry-climate SOlar Climate Ozone Links (SOCOL) model driven by the SIM and SOLSTICE SSI are closest to the atmospheric measurements [START_REF] Shapiro | The role of the solar irradiance variability in the evolution of the middle atmosphere during 2004-2009[END_REF]. UV SSI variations measurements are questionable and new time series obtained by space-based instruments are of great interest.

3.5. SOLAR/SOLSPEC SSI Variations in Three Spectral Bands SOLAR/SOLSPEC provides important UV SSI variations contribution monitoring during Solar Cycle 24. We limit this analysis at three spectral bands (165-180 nm, 180-200 nm, and 200-242 nm) due to SOLAR/SOLSPEC instrument effects (Section 2). Equation 3 allows to determine the SOLAR/SOLSPEC SSI evolution over time in three spectral bands.

SSI ∆λ (t d ) = 1 ∆λ λ2 λ1 SSI R (λ, t d ) dλ , (3)
where SSI ∆λ (t d ) is the mean SOLAR/SOLSPEC SSI value in the desired spectral band (∆λ) over time, and t d corresponds to a daily mean time.

With this method, we have collected quantitative data in a spectral band to carry out statistical analysis of the evolution of the mean value (SSI ∆λ (t d )). Figure 11 displays the UV SSI variations as seen by the SOLAR/SOLSPEC instrument during Solar Cycle 24. Similarly, we plotted the SATIRE-S SSI variations in three spectral bands (165-180 nm, 180-200 nm, and 200-242 nm). Figure 12 displays the direct comparison between the ratio SOLAR/SOLSPEC over SATIRE-S for the period between April 2010 and August 2015. The ratio indicates that both data sets agree within ±5% for the peak-to-peak values. The absolute differences between SOLAR/SOLSPEC and SATIRE-S time series depend on the initial SOLAR/SOLSPEC values (see solar spectrum defined in Figure 7). Absolute deviations correspond to those given in Table 9. In the different spectral bands, the Pearson correlation coefficients (R 2 ) between SOLAR/SOLSPEC and SATIRE-S time series are higher than 0.8 that might indicate very strong relationship during same time operation. In the 180-200 nm spectral band, R 2 between SOLAR/SOLSPEC and SATIRE-S time series is close to 0.88. In this spectral band, the similarities are the best between SO-LAR/SOLSPEC and SATIRE-S (UV variability and absolute value). It is also in this band than the instrument is the most stable (temperature gradient between the front part (main shutter) and the centre plate is close to zero as seen in Figure 5). These comparisons were not made with the SORCE instruments because there is a data gap in 2013. Indeed, all SORCE instruments ceased operations from July 30, 2013 to December 22, 2013 due to an anomaly of the satellite (battery degradation). For each data set, we compute the difference between the minimum and the maximum of the UV SSI variations from 2008 to 2015 (Table 2). Each data set was filtered with a 51.9-days moving mean. In the 165-180 nm spectral band, the results are close for each data set. In the 180-200 nm spectral band, SOLAR/SOLSPEC maximum UV SSI variation is between SATIRE-S and SORCE/SOLSTICE. In the 200-242 nm spectral band, SOLAR/SOLSPEC maximum UV SSI is slightly higher than SORCE/SOLSTICE maximum variation. However, there is a significant difference with SATIRE-S maximum SSI variation from 2008 to 2015. This difference can be explained by an instrumental effect (see Sections 2.4 to 2.7) and by the use of a wider spectral band of analysis (∆λ=42 nm). For a better comparison of the results, a spectral analysis for each data set was carried out. We took the same time of observations as those made by SOLAR/SOLSPEC from 2008 to 2015. Figure 13 displays the Lomb-Scargle periodograms of two time-series (daily mean), which show the link between SOLAR/SOLSPEC and the SATIRE-S semi-empirical model. Similar solar periodicities were highlighted (∼27, ∼52, and ∼130 days). One can notice a SO-LAR/SOLSPEC solar periodicity ∼60 days (not significant) in the 165-180 nm spectral band. This period was also observed with the Sun Ecartometry Sensor (SES) of the Picard mission at 782 nm, which is also observed in Picard solar radius data and sunspot number data.

Conclusions

SOLAR/SOLSPEC provides SSI data since April 2008 without discontinuity. Operations were very well managed since the beginning of the mission, where there is a repeatability of the observation sequences. The instrument degradation process corresponds to a two-term exponential model. A method was developed to determine the SOLAR/SOLSPEC SSI evolution during Solar Cycle 24. SO-LAR/SOLSPEC provides preliminary new time series of the UV solar spectral irradiance from 2008 to 2015, which are consistent with the SATIRE-S semiempirical model. During this period, the peak-to-peak UV variation (data with 51.9-days moving mean) is close to ∼7% in the 165-200 nm spectral band from 2008 to 2015. In the 200-242 nm spectral band, the peak-to-peak UV variation is close to ∼3.5%. This last result has to be consolidated with a more in-depth analysis. Indeed, this result is higher than that obtained from the SATIRE-S semi-empirical model and close to the SORCE/SOLSTICE observations. SOLAR/SOLSPEC can be assumed valid for short-term variations. On longer timescales, SOLAR/SOLSPEC stability is lower and requires a better characterization of instrument effects and degradation. Finally, we plan to use the helium hollow cathode lamp to correct the data. However, we need to do a ground test to characterize the aging of the lamp.

Figure 1 .

 1 Figure 1. Temporal variations of SOLAR/SOLSPEC signal (200-242 nm) during calibrations with deuterium lamps (D1 and D2). The blue curve (D1t) represents the normalized temporal variation during a calibration with the D1 lamp for the same operation time as the D2 lamp.

Figure 2 .

 2 Figure 2. Measured spectral distribution of UV SSI (165-400 nm) from April 2008 to August 2015. Measurements of the 'UV' spectral band are represented with the blue dots. Measurements of the 'VIS' spectral band are represented with the green dots. The red curve represents the first SOLAR/SOLSPEC UV spectrum. The black curve represents the last UV solar spectrum obtained in August 2015.

Figure 4 .

 4 Figure 4. For different spectral bands, the evolution of the degradation during time is plotted. The square markers represent the annual mean measurements for each spectral bands.

Figure 5 .

 5 Figure 5. (top) Temperature evolution of SOLAR/SOLSPEC main shutter from April 2008 to mid-2015. (bottom) The Y-axis represents the mean temperatures (UV PM, main shutter, and SOLAR/SOLSPEC centre plate) from April 2008 to mid-2015 during SSI measurement with the 'UV' spectrometer at a given wavelength (X-axis).

Figure 6 .

 6 Figure 6. (top) SOLAR/SOLSPEC wavelengths shift. Marker 1 corresponds to a transition period. (bottom) Estimation of the degradation during time for the 200-242 nm band. A first linear fit corresponds to the degradation of SOLAR/SOLSPEC during the use of deuterium lamps (April 2008 to April 2009). A second fit corresponds to the transition period (transient outgassing period, wavelengths shift, etc.). A third exponential fit (Levenberg-Marquardt method) corresponds to SOLAR/SOLSPEC degradation from September 2009 to now. This fit is based on all points in the period 2009-2010 where solar activity is close to zero and on annual mean data. Uncertainty of the third exponential fit is ±0.0065 (1σ) on Deg(λ, t) values.

Figure 7 .

 7 Figure 7. Solar irradiance from 165 to 400 nm as measured by SOLAR/SOLSPEC with the proposed method.

Figure 8 .

 8 Figure 8. Relative differences between SOLAR/SOLSPEC and different reference spectra (ATLAS 3, WHI 2008) or model (SATIRE-S).

Figure 9 .Figure 10 .

 910 Figure 9. Solar irradiance for three spectral bands (165-180 nm, 180-200 nm and 200-242 nm) as seen by SOLAR/SOLSPEC, ATLAS 3, WHI 2008, and SATIRE-S.

Figure 11 .

 11 Figure11. UV SSI variability in three spectral bands(165-180, 180-200, and 200-242 nm) during Solar Cycle 24. The blue curves with dots correspond to the measurement made by SOLAR/SOLSPEC. The red curves represent the values obtained with the SATIRE-S model. R 2 is the Pearson's linear correlation coefficient between SOLAR/SOLSPEC and SATIRE-S data.

Figure 12 .

 12 Figure12. Data comparison between SOLAR/SOLSPEC and SATIRE-S in three spectral bands(165-180, 180-200, and 200-242 nm).

Figure 13 .

 13 Figure13. Lomb-Scargle periodogram for unevenly sampled time series (SOLAR/SOLSPEC) in three spectral bands(165-180, 180-200, and 200-242 nm) from 2008 to 2015. The blue curves correspond to the SOLAR/SOLSPEC periodogram. The red curves correspond to the SATIRE-S periodogram. α correspond to the statistical significance levels (0.01 corresponds to 99% significance level).

Table 1 .

 1 Important absorption bands of solar radiation in the mesosphere, stratosphere, and troposphere.

	∆λ [nm] Absorber	Principal location
	165-200	O 2 Schumann-Runge bands	Thermosphere / Mesosphere
	200-242	O 2 Herzberg continuum / O 3 Hartley band Stratosphere
	242-310	O 3 Hartley band	Stratosphere
	310-400	O 3 Huggins band	Stratosphere / Troposphere

Table 2 .

 2 Maximum UV SSI variations from 2008 to 2015 for SOLAR/SOLSPEC (with statistical uncertainty), SATIRE-S, and SORCE/SOLSTICE.

	∆λ [nm]	SOLAR/SOLSPEC SATIRE-S SORCE/SOLSTICE
	Lyman-α (comparison) -	-	∼28%
	165-180	∼7.0±2.0% (1σ)	∼6.7%	∼8.3%
	180-200	∼6.9±2.5% (1σ)	∼5.4%	∼8.2%
	200-242	∼3.5±1.7% (1σ)	∼2.1%	∼3.1%

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Acknowledgements The SOLAR/SOLSPEC investigation is supported by the Royal Belgian Institute for Space Aeronomy, by the Centre National d'Etudes Spatiales (CNES, France), by the Bundesministerium für Forschung und Technologie (Germany), and by the Centre National de la Recherche Scientifique (CNRS, France). The authors would like to thank the Laboratory for Atmospheric and Space Physics (LASP, United States) for providing the SORCE data and the Max Planck Institute for Solar System Research (Germany) for providing the SATIRE-S data. The authors gratefully acknowledge the anonymous reviewer for carefully reading the manuscript and providing constructive comments that have led to an improved paper.

Disclosure of Potential Conflicts of Interest:

The authors declare that they have no conflicts of interest.

Appendix

A. Spectral Irradiance of the Sun (mW m -2 nm -1 ) from 165 to 400 nm