Solar Wind Magnetic Fluctuations and Electron Non-Thermal Temperature Anisotropy: Survey of Wind-SWE-VEIS Observations

Abstract:

The solar wind electron velocity distribution function (eVDF) exhibits a great variety of non-thermal features that deviate from thermal equilibrium. These deviations from thermal equilibrium provide a local source for electromagnetic fluctuation emissions, which among others include the electron whistler-cyclotron and firehose instabilities. We present a systematic analysis of Wind-SWE-VEIS observations of solar wind electron plasma and their associated Wind-MFI observed magnetic fluctuations. We show for the first time clear evidence that the temperature anisotropy threshold of the parallel electron anisotropic instability bounds solar wind electrons — when the full electron distribution and its moments are considered — during slow solar wind periods. Analysis shows 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 T_e,perp / T_e,parallel < 1. When considering collisional effects, our results indicate that collisions are rare in the solar wind, yet appear to play a necessary role in regulating the eVDFs. The results of our analysis are strikingly different from those for solar wind ions suggesting that the slow solar wind electron plasma is only marginally stable with respect to parallel propagating instabilities.