Reconstruction of the solar spectral UV irradiance for the characterization of planetary atmospheres

La connaissance du flux UltraViolet (UV) solaire et de sa variabilité dans le temps est un problème clé aussi bien dans le domaine de l’aéronomie qu’en physique solaire. Alors que l’extrême UV, entre 10 et 121 nm, est important pour la caractérisation de l’ionosphère, l’UV entre 121 et 300 nm l’est tout autant pour les modélisations climatiques. La mesure continue de l’irradiance dans l’UV est cependant une tâche ardue. En effet, les instruments spatiaux étant dans un environnement hostile se dégradent rapidement. De nombreux modèles basés sur des indices solaires sont alors utilisées lorsque peu de données sont disponibles. Pourtant, l’utilisation de ces indices ne permet pas d’atteindre aujourd’hui une précision suffisante pour les différentes applications en météorologie de l’espace. Comme alternative, ce travail de thèse met en avant l’utilisation de bandes passantes pour reconstruire l’irradiance solaire dans l’UV. En utilisant des méthodes d’analyse statistique multivariée, ce travail met tout d’abord en évidence la forte cohérence de la variabilité spectrale de l’irradiance dans l’UV, ainsi que ses principales caractéristiques. Une première étape consiste à utiliser des bandes passantes existantes afin de tester la faisabilité de notre approche : le flux UV peut ainsi être reconstruit avec une erreur relative d’environ 20%, une bien meilleure performance qu’avec l’utilisation d’indices solaires. Afin de limiter les problèmes de dégradation liés à l’utilisation des filtres, nous proposons un instrument d’un genre nouveau basé uniquement sur des détecteurs à larges bande interdite permettant de sélectionner une bande spectrale (notamment pour l’UV à partir de 120 nm). Un tel radiomètre permettrait de reconstruire les raies spectrales importantes pour la spécification de la thermosphère terrestre avec une bonne précision. Enfin, une modélisation de l’impact du flux UV solaire sur l’atmosphère de Ganymède est exposée. Les émissions atmosphériques pour quelques espèces sont alors calculées, afin de proposer quelques recommandations pour les futures missions pour Jupiter.The knowledge of the solar spectral irradiance in the UV and its variation in time is a key problem in aeronomy but also in climatology and in solar physics. While the Extreme UV (10-121 nm) range is important for thermosphere/ionosphere specification, the Far UV and Middle UV ranges are essential for climate modelling. However, the continuous monitoring of the UV irradiance is a difficult task. Space instruments are indeed suffering from ageing but also signal contamination of many kinds. Because of the lack of long-term measurements of the whole UV range, most thermosphere/ionosphere and climate models rely today on proxies for the solar irradiance, which may however not reflect very well the variability. As an alternative, we proposed in this work to use a few radiometers with properly chosen passbands in order to reconstruct the solar UV irradiance. Using a multivariate statistical approach, we first characterize the high redundancy as well as the different features of the solar UV irradiance. With four passbands from already existing instrument, we test our concept : the solar UV flux is reconstructed with a relative error of about 20%. This work proposes then to define a new kind of instrument, which may use wide bandgap materials as detectors selecting moreover the spectral range without using filters. Filters are indeed very sensitive to the degradation. This new instrument could reconstruct very well some spectral lines important to the Earth thermosphere specification. This thesis finally proposes to model the impact of the solar UV flux on the atmosphere of Ganymede. We predict some atmospheric emissions in the framework of future space mission to Jupiter

Reconstruction du spectre UV solaire en vue de la caractérisation des environnements planétaires

La connaissance du flux UltraViolet (UV) solaire et de sa variabilité dans le temps est un problème clé aussi bien dans le domaine de l aéronomie qu en physique solaire. Alors que l extrême UV, entre 10 et 121 nm, est important pour la caractérisation de l ionosphère, l UV entre 121 et 300 nm l est tout autant pour les modélisations climatiques. La mesure continue de l irradiance dans l UV est cependant une tâche ardue. En effet, les instruments spatiaux étant dans un environnement hostile se dégradent rapidement. De nombreux modèles basés sur des indices solaires sont alors utilisées lorsque peu de données sont disponibles. Pourtant, l utilisation de ces indices ne permet pas d atteindre aujourd hui une précision suffisante pour les différentes applications en météorologie de l espace. Comme alternative, ce travail de thèse met en avant l utilisation de bandes passantes pour reconstruire l irradiance solaire dans l UV. En utilisant des méthodes d analyse statistique multivariée, ce travail met tout d abord en évidence la forte cohérence de la variabilité spectrale de l irradiance dans l UV, ainsi que ses principales caractéristiques. Une première étape consiste à utiliser des bandes passantes existantes afin de tester la faisabilité de notre approche : le flux UV peut ainsi être reconstruit avec une erreur relative d environ 20%, une bien meilleure performance qu avec l utilisation d indices solaires. Afin de limiter les problèmes de dégradation liés à l utilisation des filtres, nous proposons un instrument d un genre nouveau basé uniquement sur des détecteurs à larges bande interdite permettant de sélectionner une bande spectrale (notamment pour l UV à partir de 120 nm). Un tel radiomètre permettrait de reconstruire les raies spectrales importantes pour la spécification de la thermosphère terrestre avec une bonne précision. Enfin, une modélisation de l impact du flux UV solaire sur l atmosphère de Ganymède est exposée. Les émissions atmosphériques pour quelques espèces sont alors calculées, afin de proposer quelques recommandations pour les futures missions pour Jupiter.The knowledge of the solar spectral irradiance in the UV and its variation in time is a key problem in aeronomy but also in climatology and in solar physics. While the Extreme UV (10-121 nm) range is important for thermosphere/ionosphere specification, the Far UV and Middle UV ranges are essential for climate modelling. However, the continuous monitoring of the UV irradiance is a difficult task. Space instruments are indeed suffering from ageing but also signal contamination of many kinds. Because of the lack of long-term measurements of the whole UV range, most thermosphere/ionosphere and climate models rely today on proxies for the solar irradiance, which may however not reflect very well the variability. As an alternative, we proposed in this work to use a few radiometers with properly chosen passbands in order to reconstruct the solar UV irradiance. Using a multivariate statistical approach, we first characterize the high redundancy as well as the different features of the solar UV irradiance. With four passbands from already existing instrument, we test our concept : the solar UV flux is reconstructed with a relative error of about 20%. This work proposes then to define a new kind of instrument, which may use wide bandgap materials as detectors selecting moreover the spectral range without using filters. Filters are indeed very sensitive to the degradation. This new instrument could reconstruct very well some spectral lines important to the Earth thermosphere specification. This thesis finally proposes to model the impact of the solar UV flux on the atmosphere of Ganymede. We predict some atmospheric emissions in the framework of future space mission to Jupiter.ORLEANS-SCD-Bib. electronique (452349901) / SudocSudocFranceF

Analysis of solar eclipses observed by PREMOS/PICARD

We perform the analysis of recent solar eclipses observed by PREcision MOnitor Sensor (PREMOS) onboard PICARD mission in six spectral channels (three UV, one visible and two near infrared). Light curves of the eclipses were used to obtain observational center-to-limb variations of solar brightness (CLV) in the corresponding wavelength ranges. The theoretical CLV were modelled with the radiative transfer COde for Solar Irradiance (COSI). We compare the theoretical results with the PREMOS measurements and discuss the constrains which our analysis of the eclipses imposes on the models of solar atmosphere. A similar investigation of eclipse light curves has been recently performed using measurements with Herzberg channel of LYRA instrument onboard PROBA-2 mission. We make the comparison of the main results and show that the measurements in six spectral channels of PREMOS allow us to significantly complement the analysis of the LYRA data

SOLAR/SOLSPEC: a new solar reference spectrum, SOLAR-ISS 165-3000 nm and 9 years observations of solar spectral irradiance from space from the ISS

International audienceFor 9 years since April 5, 2008 and until February 15, 2017, the SOLSPEC (SOLar SPECtrometer) spectro-radiometer of the SOLAR facility on the International Space Station (ISS) performed accurate measurements of Solar Spectral Irradiance (SSI) from the far ultraviolet to the infrared (165 nm to 3000 nm). These measurements, unique by their large spectral coverage and long time range, are of primary importance for a better understanding of solar physics and of the impact of solar variability on climate (via Earth's atmospheric photochemistry), noticeably through the "top-down" mechanism amplifying ultraviolet solar forcing effects on the climate (UV affects stratospheric dynamics and temperatures, altering interplanetary waves and weather patterns both poleward and downward to the lower stratosphere and troposphere regions). SOLAR/SOLSPEC, with almost 9 years of observations covering the essential of the unusual solar cycle 24, followed UV temporal variability and established a new reference solar spectra from UV to IR (165-3000 nm). A complete reanalysis of data was possible thanks to revised engineering corrections, improved calibrations and advanced procedures to account for thermal influence, aging (degradation) and pointing corrections. These intensive ground and space calibrations allowed a proper evaluation of uncertainties on these measurements. Results, UV variability and absolute reference spectrum (SOLAR-ISS), are displayed and compared with other measurements (WHI, ATLAS-3, SCIAMACHY, SORCE/SOLSTICE, SORCE/SIM) and models (SATIRE-S, NRLSSI2)

Recent variability of the solar spectral irradiance by using SOLAR/SOLSPEC data

Accurate measurements of the solar spectral irradiance (SSI) and its temporal variation are of primary interest to better understand solar mechanisms and the links between solar variability and Earth's atmosphere and climate. We will present recent Ultra Violet (UV) SSI observations performed by the SOLAR/SOLSPEC spectrometer on board the International Space Station. SOLAR/SOLSPEC observations covered the essential of the solar cycle 24 from April 5, 2008 to February 15, 2017. We wish to provide evolution of solar spectral irradiance during Cycle 24 using the SOLAR/SOLSPEC data thanks to revised engineering corrections, improved calibrations, and advanced procedures to account for thermal and aging corrections of the instrument. The SOLAR/SOLSPEC observations will be directly compared with other measurements (SORCE/SOLSTICE, SORCE/SIM) and models (SATIRE-S, NRLSSI). <P /

Reconstruction of electron precipitation spectra at the top of the upper atmosphere using 427.8 nm auroral images

International audienceWe present an innovative method to reconstruct the characteristics of precipitated electrons in auroral regions from optical measurements. This method is based on an optimization implemented between numerical simulations of the Transsolo code and tomographic maps made from the Auroral Large Imaging System (ALIS) network. We focus on the Volume Emission Rate (VER) of the blue line ${\mathrm{N}}_2^{+}$ 1NG 427.8 nm, which is the most representative line of the energy deposition by electrons. The optimization is tested with the ALIS measurements carried out on March 05, 2008, at 18:41:30 UT and 18:42:40 UT. The reconstruction is performed by extracting the energy flux and the mean energy of the precipitating particles. Both Maxwellian and quasi-monoenergetic energy distributions are considered. Calculations performed with a Maxwellian energy distribution yielded a mean energy ranging from 1.8 to 5.2 keV with energy flux from 0.1 to 44.3 erg·cm−2·s−1 for 18:41:30 UT, and a mean energy from 2.2 to 9.5 keV with energy flux from 2.1 to 136.7 erg·cm−2·s−1 for 18:42:40 UT. Assuming a quasi-monoenergetic energy distribution, we find a mean energy ranging from 4.2 to 11.8 keV with energy flux ranging from 0.1 to 45 erg·cm−2·s−1 for 18:41:30 UT, and 8 to 17.1 keV with energy flux ranging from 2.2 to 110.1 erg·cm−2·s−1 for 18:42:40 UT. Moreover, we show this method allows us to reconstruct the energy characteristic of the precipitating electrons on a large region covering approximately 150 km × 150 km. This study also shows that some VER profiles of the maps are better fitted by quasi mono-energetic distributions while some others correspond to broadband distributions. It appears clearly that the energy flux is linked to the column integrated intensity, the mean energy is linked with the peak altitude of the emission, and the width of the energy distribution with the altitude thickness of the emissions

SOLAR/SOLSPEC mission on ISS: In-flight performances for SSI measurements in the UV

International audienceThe SOLar SPECtrum (SOLSPEC) experiment is part of the Solar Monitoring Observatory (SOLAR) payload, and has been externally mounted on the Columbus module of the International Space Station (ISS) since 2008. SOLAR/SOLSPEC combines three absolutely calibrated double monochromators with concave gratings for measuring the solar spectral irradiance (SSI) from 166 nm to 3088 nm. This physical quantity is a key input for studies of climatology, planetary atmospheres, and solar physics.Aims. A general description of the instrument is given, including in-flight operations and performance of the ultraviolet (UV) channel from 175 nm to 340 nm.Methods. We developed a range of processing and correction methods, which are described in detail. For example, methods for correcting thermal behavior effects, instrument linearity, and especially the accuracy of the wavelength and absolute radiometric scales have been validated by modeling the standard uncertainties.Results. The deliverable is a quiet Sun UV reference solar spectrum as measured by SOLAR/SOLSPEC during the minimum of solar activity prior to cycle 241. Comparisons with other instruments measuring SSI are also presented

Quo vadis, European Space Weather community?

This paper was written by a group of European researchers believing that now is the right time to frame the Space Weather and Space Climate discipline in Europe for future years. It is devoted to openly discussing the organisation and sustainability of the European Space Weather community and its assets in the (near) future. More specifically, we suggest that the European Space Weather community lacks a uniting organisation to help the community to sustain and develop the successful efforts made thus far. Our aim is not to draw a complete and exhaustive panorama of Space Weather throughout the world, nor even throughout Europe. It is not a new white paper on the science and applications: there exist many (e.g. Tsurutani et al., 2020 Nonlinear Processes Geophys 27(1): 75-119); nor another roadmap: several important have been published recently (e.g. Schrijver et al., 2015. Adv Space Res 55(12): 2745-2807; Opgenoorth et al., 2019. J Space Weather Space Clim 9: A37). Our aim is to question our practices and organisation in front of several changes that have occurred in the recent years and to set the ground to provide coordinated answers to these questions being posed in Europe, and to make these answers discussed throughout the world. This group was assembled first through a series of sessions devoted to the sustainability of Space Weather research during the European Space Weather Week (ESWW) series of meetings, specifically: ESWW 14 (2017), ESWW 15 (2018), and ESWW 16 (2019). It then grew from discussions and personal contacts. The authors do not pretend to identify the full range of opinions in Europe, although they do come from 13 different European countries with a large span of ages (around half are below the age of 40 years old at the time of writing) with a good gender balance ending with a diverse mix of young and motivated scientists and senior people who have played a role in shaping the Space Weather community in Europe. The questions and the propositions to organise Space Weather in Europe in the future result from their discussions through these meetings and through remote meetings during the pandemic. We wish to share them with all those who consider themselves as members of the European Space Weather community and/or are interested in its future and to propose actions. We do this, bearing in mind that Europe plays a key international role in Space Weather which extends beyond the ESA and EU/EC geographic area.Peer reviewe