Solar wind electron distribution functions inside magnetic clouds

This paper presents a study of the kinetic properties of the electron velocity distribution functions within interplanetary magnetic clouds, since they are the dominant thermal component and can contribute as much as 50% of the total electron pressure within the clouds. The study is based on high time resolution data from the Goddard Space Flight Center Wind Solar Wind Experiment vector electron and ion spectrometer. Studies on interplanetary magnetic clouds have shown observational evidence of anticorrelation between the total electron density and temperature, which suggests a polytrope law P e = αn e γ for electrons with the constant γ ≈ 0.5 < 1. This anticorrelation and small γ values are interpreted in the context of the presence of highly non‐Maxwellian electron distributions (i.e., nonthermal tails) within magnetic clouds. We have revisited some of the magnetic cloud events previously studied to quantify the nature of the nonthermal electrons by modeling the electron velocity distribution function using Maxwellian and kappa‐like distribution functions to characterize the kinetic nonthermal effects. The results show that the electron density‐temperature anticorrelation is not a unique feature of magnetic clouds. Within magnetic clouds, κ values are generally small, in the range of 1.6–5.4; however, such small values are also typical of regions outside the clouds. We have shown that the density‐temperature anticorrelation of the electron moments is persistently consistent with similar density‐temperature anticorrelation in the electron halo component of the velocity distribution function and essentially little or no correlation was obtained for the core component. This result clearly shows that the temperature and density of the suprathermal components play a significant role in the temperature‐density anticorrelation because of a relative enhancement of the halo component abundance to the total density.