In the present work, we consider four dipolarization front (DF) events detected by MMS spacecraft in the Earth’s magnetotail during a substorm on 23rd of July 2017 between 16:05 and 17:19 UT. From their ion scale properties, we show that these four DF events embedded in fast Earthward plasma flows have classical signatures with increases of Bz, velocity and temperature and a decrease of density across the DF. We compute and compare current densities obtained from magnetic and particle measurements and analyse the Ohm’s law. Then we describe the wave activity related to these DFs. We investigate energy conversion processes via J.E calculations and estimate the importance of the electromagnetic energy flow by computing the divergence of the Poynting vector. Finally we discuss the electromagnetic energy conservation in the context of these DFs.

La reconnexion magnétique est l'un des principaux moteurs des processus d'accélération de particules dans les plasmas spatiaux et astrophysiques. Les particules supra-thermiques de basse énergie, émises par reconnexion magnétique sont une source d'ionisation pour les disques circumstellaires, influençant leur évolution chimique, thermique et dynamique. Ce travail a pour objectif d'étudier comment les particules énergétiques peuvent se propager dans le disque circumstellaire d’une étoile TTauri et comment elles affectent la fraction d'ionisation du plasma. Pour ce faire, nous avons rassemblé les données expérimentales et théoriques sur les sections efficaces pour la production de H+, H2+ et He+ par impact d'électrons et de protons. A partir d’un spectre d’injection de protons et d’électrons émis par reconnexion magnétique, nous avons ensuite calculé les spectres propagés dans le disque circumstellaire en tenant compte des processus  pertinents de perte d'énergie. Nous avons entrepris une comparaison des taux d’ionisation par les particules énergétiques suivant différentes configurations de lignes de champ magnétiques le long desquelles celles-ci se propagent, en prenant en compte les caractéristiques des flares tels qu’ils sont observés par le satellite Chandra pour un échantillon de sources dans la nébuleuse d’Orion (COUP). Nous avons testé les taux d’ionisation obtenus pour un disque dont l’équilibre chimique est calculé par le code PRODIMO. Nous trouvons que les particules énergétiques peuvent contribuer à une forte ionisation locale du disque d’accrétion dans le voisinage de l’étoile.

Magnetic reconnection is one of the major drivers of particle acceleration processes in space and astrophysical plasmas. Low-energy supra-thermal particles emitted by magnetic reconnection are a source of ionization for circumstellar disks, influencing their chemical, thermal and dynamical evolution. The aim of this work is to study how energetic particles can propagate in the circumstellar disk of a TTauri star and how they affect the plasma ionization fraction. To this end, we have collected experimental and theoretical data on the cross sections for the production of H+, H2+ and He+ by electron and proton impact. Starting from the injection spectra of protons and electrons emitted by the magnetic reconnection, we have calculated the propagated spectra in the circumstellar disk taking into account the relevant energy loss processes. We have undertaken a comparison of the ionization rates by energetic particles in different magnetic configurations, along which the particles propagate, considering the physical properties of the flares as observed by the Chandra satellite for a sample of sources in the Orion Nebula (COUP). We have tested the ionization rates obtained for a disk which chemical equilibrium is calculated by the PRODIMO code. We find that energetic particles are a source of a strong local ionization of the accretion disk in the surroundings of the star.

This study presents Magnetospheric Multiscale (MMS) in situ observations of an electron diffusion region (EDR) and adjacent separatrix region in the Earth’s magnetotail. The electron diffusion region is a key constituent of the magnetic reconnection process since it is the region where both electrons and ions are demagnetised and the magnetic topology change is enabled. During the diffusion region crossing, we observe significant magnetic field oscillations near the lower hybrid frequency which propagate perpendicularly to the reconnection plane. We also find that the strong electron-scale gradients close to the EDR exhibit significant oscillations at a similar frequency. Such oscillations are not expected for a crossing of a steady 2D EDR, and can be explained by a complex motion of the reconnection plane induced by current sheet kinking propagating in the out-of-reconnection-plane direction. Thus all three spatial dimensions have to be taken into account to explain the observed perturbed EDR crossing.

Solar flares are very efficient particle accelerators on a short timescale. The X-ray and type III radio emission emitted during a flare are direct signatures of the accelerated electrons. Hard X-rays are emitted from the accelerated electrons through bremsstrahlung radiation primarily in the dense atmosphere, while type III emissions are caused by the accelerated electrons propagating through the upper corona where they produce Langmuir waves which then converts to radio emission near the local plasma frequency or its harmonics. The aim of the present study is to understand the link between these two different electron populations. The analysis is based on a list of HXR/ type III radio bursts observed by RHESSI or FERMI/GBM, the Nançay Radioheliograph and the PHOENIX/ORFEES/NDA spectrographs in the 1000 GHz-10 MHz range in the 2002-2015 time period. For the list of almost 200 events, we analysed the relationship between the energetic electrons producing the HXR emissions and the type III radio fluxes at different frequencies. We shall present and discuss here some of the results: the correlation between the number of HXR emitting electrons and the peak flux of the type III emissions decreases with increasing frequency and the correlation is better for electrons above 20 keV than for electrons above 10keV.We also find that the peak radio flux is anti-correlated with the electron power-law index deduced from low electron energies from the HXR spectral analysis. These results will be briefly discussed in the context of the numerical simulations or models describing the production of type III bursts in the corona.

En juillet 2017, MMS évoluait dans la queue géomagnétique avec un apogée de 25 rayons terrestres et une séparation moyenne entre satellites de 10 km (échelle du rayon de giration des électrons). Le 23 juillet vers 16:19 TU, MMS située  sur le bord externe de la couche de plasma  a détecté le déclenchement local d'un sous-orage (AE~ 400 nT). Un écoulement rapide de plasma a été enregistré pendant environ  1 heure débutant avec un régime quasi-stationnaire et suivi par une série de jets de plasma ("bursty bulk flow"). Dans cette étude, nous présenterons des mesures associées à la détection d'une couche de courant d'échelle ionique emportée par l'écoulement rapide. Nous analyserons en détail les autres structures cinétiques présentes au voisinage de cette couche de courant et discuterons la loi d'Ohm et les processus de conversion d'énergie.

With the launches of Solar Orbiter and Parker Solar Probe, we can study
radio burst emission observed from multiple vantage points in the
heliosphere. We present here the first simultaneous observations of
single type III radio bursts by four different spacecraft. These
observations are used to calculate the directivity and the source
position of the radio emission. These measurements are compared to the
predictions of radio-wave propagation simulations in which the
radio-wave scattering on turbulent density fluctuations is anisotropic.
The directivity derived from the observations, compared to the
directivity of radio emission in the simulations, is used to estimate
the ratio of anisotropy in the ambiant plasma: in the five radio bursts
presented, this ratio takes values between 0.3 and 0.6, confirming
previous estimations based on statistical analysis of radio burst
emissions, but also demonstrating that the level of anisotropy can
significantly vary for different events.

Sandpile models are lattice-based statistical/computational frameworks offering a powerful approach to the modelling of complex natural phenomena encompassing a range of scales too wide to be accommodated by more conventional methods. In this talk I will first briefly review the use of sandpile models to capture the statistical behavior of solar flares, and in particular the observed spatiotemporally intermittent and scale-free pattern of energy release, as avalanches of localized magnetic reconnection events. I will then show that despite inherently stochastic components in these models, the presence of long-range temporal correlations implies that prediction of future avalanche behavior is possible given information on past avalanching behavior. This property, in conjunction with data assimilation, can be used to design forecasting schemes that significantly outperform climatological forecasts. I will end by presenting an application of this method to the forecast of large flares, working off a set of GOES X-Ray flux time series.