Electron kappa distribution and quasi‐thermal noise

Abstract

Abstract 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.