"Study of the non-thermal character of electron distribution functions in the solar wind"
Lundi 11 mai 2009 à 10h30, dans la salle de conférence du Château de l’Observatoire de Meudon
The solar wind plasma is found to be quiet tenuous while relatively hot at once. In such a media the effect of Coulomb collisions, which maintain the thermodynamic equilibrium, is quite limited and particle velocity distribution functions (VDFs) are expected to exhibit non-thermal properties. Indeed, electron VDFs observed in the solar wind are highly non-Maxwellian and exhibit strong non-thermal tails. Electrons play a signiﬁcant role in the solar wind dynamics. They produce an am-bipolar electric ﬁeld through their thermal pressure gradient and above all carry a substantial amount of the solar wind heat ﬂux. Therefore, a complete description of eVDFs non-thermal features is required to better understand the process of the solar wind expansion. We performed a statistical study of a substantial amount of eVDFs acquired in the low ecliptic latitudes covering the heliocentric distance from 0.3 up to 4 AU. For our study, a new model was proposed which, for the ﬁrst time, describes all observed eVDF components analytically. Main electron characteristics, i.e., the density, temperature and heat ﬂux, are examined as a function of the heliocentric distance and we provide a radial evolution of the model eVDF. Furthermore, we study possible constraints imposed by particle-particle and wave-particle interactions on the electron temperature anisotropy and the electron heat flux. We show that both, kinetic plasma instabilities and Coulomb collisions represent effective control mechanisms of overall eVDF properties.