Spacecraft measurements of charged particles in the solar wind show that their velocity distributions deviate from thermal Maxwellian model, having energetic components with quasi‐scale‐free power law velocity dependence, f ∼ v^−α, in the high‐velocity range. It is customary to model such a feature by means of the kappa distribution, first introduced in the 1960s to empirically fit the data, but attempts have been made to understand the kappa distribution in terms of nonextensive statistical mechanics. A recent series of papers explore an alternative model, which may be interrelated with the nonextensive thermostatistical concept. According to the model, the electron kappa distribution is understood as a self‐consistent steady state solution of the weak turbulence equation in which the electrons and electrostatic Langmuir turbulence are in dynamic equilibrium state. These papers also compared the theoretical kappa parameter with observations of quiet time solar wind electrons near 1 AU. However, the electrostatic Langmuir turbulence, also known as the quasi‐thermal noise, requires a more in‐depth analysis. The present paper extends the recent theory by including a discussion of electrostatic fluctuations in greater detail, thus bringing the theory to a full closure.

Using particle‐in‐cell simulations and kinetic theory, we explore the current‐driven turbulence and associated electron heating in the dissipation region during 3D magnetic reconnection with a guide field. At late time the turbulence is dominated by the Buneman and lower hybrid instabilities. Both produce electron holes that co‐exist but have very different propagation speeds. The associated scattering of electrons by the holes enhances electron heating in the dissipation region.

Lion roars are low‐frequency (∼100 Hz) whistler waves frequently observed in the Earth's magnetosheath. By analyzing both wave and electron data from the Cluster spacecraft, and comparing with linear Vlasov kinetic theory, Masood et al. (2006) investigated the underlying cause of the lion roar generation. However, the analysis based upon the bi‐Maxwellian distribution function did not adequately explain the observations qualitatively as well as quantitatively. This outstanding problem is revisited in the present paper, and a resolution is put forth in which, the flat‐top non‐Maxwellian distribution function with a velocity power law energetic tail, known as the ( r , q ) distribution, or the generalized kappa distribution is employed. Upon carrying out the linear stability analysis of the ( r , q ) distribution against the whistler wave perturbation, and upon comparison with the Cluster data, good qualitative and quantitative agreements are found between theory and data.

Charged particle acceleration takes place ubiquitously in the Universe including the near-Earth heliospheric environment. Typical in situ spacecraft measurements made in the solar wind show that the charged particle velocity distribution contains energetic components with quasi scale-free power-law velocity dependence, f∼v ^−α , for high velocity range. In this Review a theory of quiet-time solar-wind electrons that contain a suprathermal component is discussed, in which these electrons are taken to be in dynamical equilibrium with Langmuir turbulence. This Review includes an overview of the Langmuir turbulence theory, as well as a discussion on asymptotic equilibrium solution of Langmuir turbulence/suprathermal electron system. Theoretical predictions of high-energy electron velocity power-law distribution index is then compared against the recent observations of the superhalo electron velocity distribution made by instruments onboard WIND and STEREO spacecraft. It is shown that the theoretical prediction of velocity power-law index is intermediate to the observed range.

The present Letter puts forth a possible explanation for the outstanding problem of measured proton temperature anisotropy in the solar wind at 1 AU apparently being regulated by the mirror and oblique fire-hose instabilities. Making use of the fact that the local magnetic field intensity near 1 AU undergoes intermediate-scale temporal variations, the present Letter carries out the quasilinear analysis of the temperature anisotropy-driven instabilities with a time-varying local $B$ field, assuming arbitrary initial temperature ratios and parallel betas. It is found that the saturated states in (${\beta }_{\parallel }$, $T_{\perp }/T_{\parallel }$) space are bounded by the mirror and oblique fire-hose instabilities, which is superficially similar to the observation.