Accélération et propagation des particules énergétiques dans la couronne solaire : de l'analyse des données de l'instrument RHESSI à la préparation de l'exploitation de l'instrument STIX sur Solar Orbiter

Abstract : 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.
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Submitted on : Wednesday, January 18, 2017 - 7:32:32 PM
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  • HAL Id : tel-01439907, version 1

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Sophie Musset. Accélération et propagation des particules énergétiques dans la couronne solaire : de l'analyse des données de l'instrument RHESSI à la préparation de l'exploitation de l'instrument STIX sur Solar Orbiter. Physique [physics]. PSL Research University; Observatoire de Paris, 2016. Français. ⟨tel-01439907⟩

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