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FOXSI is a direct-imaging, hard X-ray (HXR) telescope optimized for solar flare observations. It detects hot plasma and energetic electrons in and near energy release sites in the solar corona via bremsstrahlung emission, measuring both spatial structure and particle energy distributions. It provides two orders of magnitude faster imaging spectroscopy than previously available, probing physically relevant timescales (, White paper submitted to the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033; 14 pages, 4 figures, 1 table

It is essential that there be coordinated and co-optimized observations in X-rays, gamma-rays, and EUV during the peak of solar cycle 26 (~2036) to significantly advance our understanding of impulsive energy release in the corona. The open questions include: What are the physical origins of space-weather events? How are particles accelerated at the Sun? How is impulsively released energy transported throughout the solar atmosphere? How is the solar corona heated? Many of the processes involved in triggering, driving, and sustaining solar eruptive events -- including magnetic reconnection, particle acceleration, plasma heating, and energy transport in magnetized plasmas -- also play important roles in phenomena throughout the Universe. This set of observations can be achieved through a single flagship mission or, with foreplanning, through a combination of major missions (e.g., the previously proposed FIERCE mission concept)., White paper submitted to the Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033; 5 pages, 1 figure

Interplanetary solar radio type III bursts provide the means for remotely studying and tracking energetic electrons propagating in the interplanetary medium. Due to the lack of direct radio source imaging, several methods have been developed to determine the source positions from space-based observations. Moreover, none of the methods consider the propagation effects of anisotropic radio-wave scattering, which would strongly distort the trajectory of radio waves, delay their arrival times, and affect their apparent characteristics. We investigate the source positions and directivity of an interplanetary type III burst simultaneously observed by Parker Solar Probe, Solar Orbiter, STEREO, and Wind and compare the results of applying the intensity fit and timing methods with ray-tracing simulations of radio-wave propagation with anisotropic density fluctuations. The simulation calculates the trajectories of the rays, their time profiles at different viewing sites, and the apparent characteristics for various density fluctuation parameters. The results indicate that the observed source positions are displaced away from the locations where emission is produced, and their deduced radial distances are larger than expected from density models. This suggests that the apparent position is affected by anisotropic radio-wave scattering, which leads to an apparent position at a larger heliocentric distance from the Sun. The methods to determine the source positions may underestimate the apparent positions if they do not consider the path of radio-wave propagation and incomplete scattering at a viewing site close to the intrinsic source position., 7 pages, 4 figures

The Spectrometer/Telescope for Imaging X-rays (STIX) is one of the 10 instruments on-board the scientific payload of ESA's Solar Orbiter mission. STIX provides hard X-ray imaging spectroscopy in the 4-150~keV energy range, observing hard X-ray bremsstrahlung emission from the Sun. These observations provide diagnostics of the hottest thermal plasmas ($>$10~MK) and information on the non-thermal energetic electrons accelerated above 10~keV during solar flares. STIX has a spectral resolution of 1~keV, and employs the use of in-direct bi-grid Fourier imaging to spatially locate hard X-ray emission. Given that STIX provides critical information about accelerated electrons at the Sun through hard X-ray diagnostics, it is a powerful contribution to the Solar Orbiter suite and has a significant role to explore the dynamics of solar inputs to the heliosphere. This chapter describes the STIX instrument, its design, objectives, first observations and outlines the new perspectives STIX provides over the mission lifetime of Solar Orbiter., 18 pages, 11 figures, Book Chapter for Handbook of X-ray and Gamma-ray Astrophysics

著者人数: 13名, Accepted: 2018-07-04, 資料番号: SA1180325000

While there have been significant advances in our understanding of impulsive energy release at the Sun through the combination ofRHESSIX-ray observations andSDO/AIA EUV observations, there is a clear science need for significantly improved X-ray and EUV observations. These new observations must capture the full range of emission in flares and CMEs (e.g., faint coronal sources near bright chromospheric sources), connect the intricate evolution of energy release with dynamic changes in the configuration of plasma structures, and identify the signatures of impulsive energy release in even the quiescent Sun. TheFundamentals of Impulsive Energy Release in the Corona Explorer(FIERCE) MIDEX mission concept makes these observations by combining the two instruments previously proposed on theFOXSISMEX mission concept – a focusing hard X-ray spectroscopic imager and a soft X-ray spectrometer – with a high-resolution EUV imager that will not saturate for even intense flares. All instruments observe at high cadence to capture the initiation of solar transient events and the fine time structure within events.FIERCEwould launch in mid-2025, near the peak of the next solar cycle, which is also well timed with perihelions of Parker Solar Probe and Solar Orbiter. The goals ofFIERCEinclude improving our understanding of the genesis of space-weather events, andFIERCEobserations would vitally complement observations by other observatories both near Earth and elsewhere in the heliosphere.

第2回観測ロケットシンポジウム（2019年8月5日-6日. 宇宙航空研究開発機構宇宙科学研究所（JAXA)(ISAS)）, 相模原市, 神奈川県, 2nd Sounding Rocket Symposium (August 5-6, 2019. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)(ISAS)), Sagamihara, Kanagawa Japan, 著者人数: 19名, 資料番号: SA6000142014, レポート番号: Ⅴ-1

第19回宇宙科学シンポジウム (2019年1月9日-10日. 宇宙航空研究開発機構宇宙科学研究所(JAXA)(ISAS)相模原キャンパス), 相模原市, 神奈川県, 19th Space Science Symposium (January 9-10, 2019. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency(JAXA)(ISAS)Sagamihara Campus), Sagamihara, Kanagawa Japan, 著者人数: 20名, 資料番号: SA6000144113, レポート番号: P-078

The Sun is an active star and one manifestation of its activity is the production of solar flares. It is currently admitted that solar flares are caused by the release of magnetic energy during the process of magnetic reconnection in the solar upper atmosphere, the solar corona. During these flares, a large fraction of the magnetic energy is transferred to the acceleration of particles (electrons and ions). However, the details of particle acceleration during flares are still not completely understood. Several scenarios and models have been developed to explain particle acceleration. In some of them, electric fields, produced at the location of current sheets, which can be fragmented or collapsing, and which are preferentially located on quasi-separatrix layers (QSLs), are accelerating particles. To investigate a possible link between energetic particles and direct electric fields produced at current sheet locations, we looked for a correlation between X-ray emission from energetic electrons and electric currents which can be measured at the photospheric level. We used the Reuven Ramaty High Energy Solar Spectrometric Imager (RHESSI) data to produce spectra and images of the X-ray emissions during GOES X-class flares, and spectropolarimetric data from the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) to calculate the vertical current densities from the reconstructed 3D vector magnetic field. A correlation between the coronal X-ray emissions (tracing the energetic electrons near the acceleration site) and the strong current ribbons at the photospheric level (tracing the coronal current sheet) was found in the five studied X-class flares. Moreover, thanks to the 12-minute time cadence of SDO/HMI, we could study for the first time the time evolution of electric currents: in several flares, a change in the current intensity, occurring during the flare peak, was found to be spatially correlated with X-ray emission sites. These observations enlighten a common evolution of both electric currents and X-ray emissions during the flare, and both evolutions are interpreted as consequences of the magnetic reconnection process.X-ray observations provide the most direct diagnostic of energetic electrons produced during solar flares. However, it is important to understand the propagation of energetic electrons between the acceleration site and the X-ray emission sites. In several events, energetic electrons seem to be trapped in the coronal source on the top of the loop (somehow close to the acceleration region). One way to explain this trapping is to assume that turbulent magnetic fields are responsible for a diffusive transport of the electrons. In the case of one flare, we used imaging spectroscopy to study the electron spectra in the coronal X-ray sources and in each of the two footpoints. We showed that there is a significant excess of energetic electrons leaving the coronal source, compared to the number of energetic electrons precipitating in the footpoint sources. This suggests that a large amount of energetic electrons was trapped in the coronal source; in the case of a diffusive transport model, the estimated mean free path in this event would be in the range 10$^{8}$ - 10$^{9}$ m, which is smaller than the loop itself and thus implies an efficient turbulent trapping. These observations are in agreement with observations of the same event in the microwave range (radio gyrosynchrotron emissions).Finally, I participated in the software development for the STIX instrument which will be launched on the Solar Orbiter mission. During my PhD, the software team focused on the simulation of the onboard software. I wrote the calibration module and the detector failure detection module. I also tested the rate control regime module and the flare location module. This work was carried out to simulate the real onboard software which is written by a private company, to verify the proper functioning of the software; and on the other hand, the testing of the flare location module determined which values of the threshold parameters should be used to have a good chance to locate properly a flaring event. I also participated in the test of a first version of the analysis software producing photon spectra. I used data from several flares observed by RHESSI to simulate an input photon flux and spectrum, to then visualize what would be the count and photon fluxes derived from STIX observations., Le soleil est une étoile active, et les éruptions solaires sont une des manifestations de cette activité. Il est admis que l'énergie disponible pour les éruptions solaires a une origine magnétique, et est transmise au milieu lors de phénomènes de reconnexion magnétique dans la couronne. Une partie de cette énergie permet d'accélérer les particules du milieu (électrons et ions). Cependant, les détails concernant les conditions dans lesquelles les particules sont accélérées et se propagent des régions d'accélération aux sites d'interaction lors des éruptions solaires ne sont pas encore tous compris.Plusieurs modèles d'accélération de particules ont été développés dans le cadre de l'étude des éruptions solaires. Dans certains modèles, les particules sont accélérées par un champ électrique généré au niveau de couches de courants électriques, qui peuvent être fragmentées, et qui sont préférentiellement localisées au niveau de surfaces quasi-séparatrices. Afin d'étudier le lien entre l'accélération de particules et le champ électrique direct produit au niveau de couches de courants, nous avons recherché s'il y avait des corrélations entre les sites d'émission des particules énergétiques et les courants électriques mesurés au niveau de la photosphère. Les observations X (dur) représentent les diagnostics les plus directs des électrons énergétiques produits pendant les éruptions solaires (rayonnement de freinage des électrons dans l'atmosphère solaire) et nous avons donc utilisé les observations X du satellite RHESSI (Reuven Ramaty High Energy Solar Spectrometric Imager) afin de produire des images et des spectres du rayonnement X dur des électrons énergétiques. Afin de caractériser les courants électriques dans la région éruptive, nous avons utilisé les données spectropolarimétriques de l'instrument HMI (Helioseismic and Magnetic Imager) du satellite SDO (Solar Dynamic Observatory) et nous avons calculé les densités de courants verticales photosphériques à partir du champ magnétique vectoriel reconstruit. Une corrélation entre les émissions X coronales (dues aux particules énergétiques proches du site d'accélération) et les rubans de forte densité de courants photosphériques (traces des couches de courants coronales) a été mise en évidence pour les cinq éruptions de classe X étudiées. De plus, grâce à la cadence temporelle de 12 minutes de SDO/HMI, nous avons pu étudier pour la première fois l'évolution des courants électriques au cours des éruptions: pour plusieurs éruptions, un changement localisé d'intensité des courants électriques a eu lieu au cours de l'éruption, et est co-spatial avec l'apparition de nouvelles sources d'émissions X. Ces observations montrent qu'il y a un lien entre évolution des courants électriques et des émissions de particules énergétiques au cours de l'éruption ; ces deux évolutions sont interprétées comme deux conséquences du phénomène de reconnexion magnétique.Les sites d'émissions X ne sont pas nécessairement co-spatiaux avec les régions d'accélération des particules et il est donc crucial de comprendre également les mécanismes de transport de ces particules entre le site d'accélération et le site d'émissions X. Les observations récentes de RHESSI, en particulier les observations de spectro-imagerie, ont montré dans certains cas, qu'un pourcentage important des électrons énergétiques semblait piégé dans la couronne, dans la partie haute des boucles magnétiques, et donc proche des sites présumés d'accélération. Un mécanisme possible pour expliquer ces phénomènes de piégeage intense serait lié à la diffusion des particules, due à des fluctuations turbulentes du champ magnétique, et qui entraînerait un transport non-collisionnel diffusif des particules du haut de l'arche (au voisinage des sites d'accélération) à leurs pieds. Nous avons étudié dans ce contexte le cas de l'éruption du 21 mai 2004. L'utilisation de la technique de spectro-imagerie X nous a permis de comparer les spectres et les flux d'électrons entre la source coronale (haut de la boucle) et les sources situées aux pieds de la boucle. Nous avons montré qu'une partie des électrons énergétiques est effectivement piégée dans la couronne, et expliqué ce piégeage par un transport diffusif avec un libre parcours moyen des électrons de $10^8$-$10^9$ centimètres, ce qui est plus petit que la taille typique des boucles coronales. Le résultat de l'analyse des observations X est en accord avec ceux obtenus (et publiés) d'observations radio du rayonnement gyrosynchrotron des électrons de la même éruption, dans le domaine du GHz ; en comparant les résultats des observations dans ces deux domaines de longueur d'onde, nous avons montré que le piégeage des particules était plus important pour les particules de plus haute énergie responsables du rayonnement radio, ce qui semble indiquer que le processus de diffusion par les ondes serait dépendant de l'énergie des particules. Dans le proche futur, l'instrument STIX (Spectrometer/Telescope for Imaging X-rays) à bord de la mission Solar Orbiter fournira des données de spectro-imagerie X. Une partie de ma thèse a été consacrée à la préparation de l'expérience STIX. Certaines caractéristiques de cette expérience (en particulier celles liées aux développements de logiciels auxquels j'ai contribué) sont décrites dans le manuscrit. Lors de mon travail de thèse, l'équipe de développement logiciel avait pour premier objectif la simulation du logiciel de bord de STIX ; au sein de cette équipe, j'ai écrit les modules d'étalonnage en énergie et de détection des défaillances des détecteurs. J'ai également testé les modules responsables de l'activation des régimes de contrôle et de la localisation grossière des éruptions solaires. Ce travail avait pour objectif la vérification du bon fonctionnement du logiciel de bord de l'instrument, écrit par ailleurs dans le cadre d'un contrat avec une entreprise privée. D'autre part, les différents tests exercés sur ce simulateur du logiciel de bord ont permis de déterminer les valeurs les plus idéales des différents paramètres dans ce logiciel. J'ai également participé aux tests d'une première version du logiciel d'analyse de spectres. J'ai utilisé les données d'éruptions observées par RHESSI, afin de visualiser la réponse de l'instrument STIX et les spectres résultant.

How impulsive magnetic energy release leads to solar eruptions and how those eruptions are energized and evolve are vital unsolved problems in Heliophysics. The standard model for solar eruptions summarizes our current understanding of these events. Magnetic energy in the corona is released through drastic restructuring of the magnetic field via reconnection. Electrons and ions are then accelerated by poorly understood processes. Theories include contracting loops, merging magnetic islands, stochastic acceleration, and turbulence at shocks, among others. Although this basic model is well established, the fundamental physics is poorly understood. HXR observations using grazing-incidence focusing optics can now probe all of the key regions of the standard model. These include two above-the-looptop (ALT) sources which bookend the reconnection region and are likely the sites of particle acceleration and direct heating. The science achievable by a direct HXR imaging instrument can be summarized by the following science questions and objectives which are some of the most outstanding issues in solar physics (1) How are particles accelerated at the Sun? (1a) Where are electrons accelerated and on what time scales? (1b) What fraction of electrons is accelerated out of the ambient medium? (2) How does magnetic energy release on the Sun lead to flares and eruptions? A Focusing Optics X-ray Solar Imager (FOXSI) instrument, which can be built now using proven technology and at modest cost, would enable revolutionary advancements in our understanding of impulsive magnetic energy release and particle acceleration, a process which is known to occur at the Sun but also throughout the Universe., Next Generation Solar Physics Mission white paper, 2 figures

Impulsive particle acceleration and plasma heating at the Sun, from the largest solar eruptive events to the smallest flares, are related to fundamental processes throughout the Universe. While there have been significant advances in our understanding of impulsive energy release since the advent of RHESSI observations, there is a clear need for new X-ray observations that can capture the full range of emission in flares (e.g., faint coronal sources near bright chromospheric sources), follow the intricate evolution of energy release and changes in morphology, and search for the signatures of impulsive energy release in even the quiescent Sun. The FOXSI Small Explorer (SMEX) mission concept combines state-of-the-art grazing-incidence focusing optics with pixelated solid-state detectors to provide direct imaging of hard X-rays for the first time on a solar observatory. We present the science objectives of FOXSI and how its capabilities will address and resolve open questions regarding impulsive energy release at the Sun. These questions include: What are the time scales of the processes that accelerate electrons? How do flare-accelerated electrons escape into the heliosphere? What is the energy input of accelerated electrons into the chromosphere, and how is super-heated coronal plasma produced?

International audience; Solar flares are among the most energetic events in the solar system. Magnetic energy previously stored in the coronal magnetic field is transferred to particle acceleration, plasma motion and plasma heating. Magnetic energy release is likely to occur in coronal currents sheets associated with regions of strong gradient of magnetic connectivity. Coronal current sheets can be traced by their footprints at the surface on the Sun, in e.g. photospheric current ribbons. We aim to study the relationship between the current ribbons observed at the photospheric level which trace coronal current sheets, and the flare energetic electrons traced by their X-ray emissions. The photospheric magnetic field and vertical current density are calculated from SDO/HMI spectropolarimetric data using the UNNOFIT inversion and Metcalf disambiguation codes, while the X-ray images and spectra are reconstructed from RHESSI data. In a first case (the GOES X2.2 flare of February 15, 2011), a spatial correlation is observed between the photospheric current ribbons and the coronal X-ray emissions from energetic electrons (Musset et al., 2015). Moreover, a conjoint evolution of both the photospheric currents and the X-ray emission is observed during the course of the flare. Both results are interpreted as consequences of the magnetic reconnection in coronal current sheets. Propagation of the reconnection sites to new structures during the flare results in new X-ray emission sites and local increase of the photospheric currents We will examine in this contribution whether similar results are obtained for other X-class flares.

International audience

An accurate forecast of flare and coronal mass ejection (CME) initiation requires precise measurements of the magnetic energy buildup and release in the active regions of the solar atmosphere. We designed a new space weather mission that performs such measurements using new optical instruments based on the Hanle and Zeeman effects. The mission consists of two satellites, one orbiting the L1 Lagrangian point (Spacecraft Earth, SCE) and the second in heliocentric orbit at 1AU trailing the Earth by 80 degrees (Spacecraft 80, SC80). Optical instruments measure the vector magnetic field in multiple layers of the solar atmosphere. The orbits of the spacecraft allow for a continuous imaging of nearly 73% of the total solar surface. In-situ plasma instruments detect solar wind conditions at 1AU and ahead of our planet. Earth-directed CMEs can be tracked using the stereoscopic view of the spacecraft and the strategic placement of the SC80 satellite. Forecasting of geoeffective space weather events is possible thanks to an accurate surveillance of the magnetic energy buildup in the Sun, an optical tracking through the interplanetary space, and in-situ measurements of the near-Earth environment.

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International audience