Dual Maxwellian-Kappa modeling of the solar wind electrons: new clues on the temperature of Kappa populations

Context. Recent studies on Kappa distribution functions invoked in space plasma applications have emphasized two alternative approaches that may assume the temperature parameter either dependent or independent of the power-index κ. Each of them can obtain justification in different scenarios involving Kappa-distributed plasmas, but direct evidence supporting either of these two alternatives with measurements from laboratory or natural plasmas is not available yet. Aims. This paper aims to provide more facts on this intriguing issue from direct fitting measurements of suprathermal electron populations present in the solar wind, as well as from their destabilizing effects predicted by these two alternative approaches. Methods. Two fitting models are contrasted, namely, the global Kappa and the dual Maxwellian-Kappa models, which are currently invoked in theory and observations. The destabilizing effects of suprathermal electrons are characterized on the basis of a kinetic approach that accounts for the microscopic details of the velocity distribution. Results. In order to be relevant, the model is chosen to accurately reproduce the observed distributions and this is achieved by a dual Maxwellian-Kappa distribution function. A statistical survey indicates a κ-dependent temperature of the suprathermal (halo) electrons for any heliocentric distance. Only for this approach are the instabilities driven by the temperature anisotropy found to be systematically stimulated by the abundance of suprathermal populations, thus lowering the values of κ-index.