Electron instability thresholds of solar wind magnetic fluctuations in non-thermal anisotropic kappa distribution plasmas: Survey of Wind-SWE-VEIS observations

Abstract:

The solar wind electron velocity distribution function (eVDF) displays a great variety of non-thermal features (e.g., core, halo and strahl electron populations; with superposition of different temperatures, thermal anisotropies, suprathermal tails, beam-like features, etc.) that deviate from thermal equilibrium. These electron nonthermal deviations provide a local source for whistler-cyclotron and firehose instabilities electromagnetic fluctuations that are commonly observed. We present clear observational evidence that the temperature anisotropy whistler instability threshold, of a nonthermal kappa distribution plasma, marginally bounds solar wind magnetic fluctuations — when the full electron distribution is considered, without regard of separation of the various electron components during slow solar wind periods. Analysis seems to suggest that during slow solar wind periods, collisional effects are dominant. During fast solar wind periods, magnetic fluctuations and solar wind anisotropies are enhanced above the parallel whistler anisotropic threshold boundary and collisional effects are drastically reduced. Preliminary calculations further show that the oblique electron whistler mirror anisotropic instability bounds both the slow and fast solar wind. Regardless of solar wind speed, the solar wind electron thermal anisotropy appears globally bounded by the parallel electron firehose instability for anisotropies Te⊥ / Te|| < 1 indicative of a firehose-stable electron plasma. Preliminary analysis suggests that skew-kappa nonthermal distributions also shows marginally stable threshold boundaries when considering electron heat flux instability thresholds. The results of our analysis suggests that the slow solar wind electron plasma, when considered globally as a single eVDF, is only marginally stable with respect to nonthermal skew kappa distributions and parallel propagating instabilities.