Measuring suprathermal electron parameters in space plasmas: Implementation of the quasi‐thermal noise spectroscopy with kappa distributions using in situ Ulysses/URAP radio measurements in the solar wind

Abstract

We discuss the implementation of the quasi‐thermal noise spectroscopy using kappa velocity distribution functions for the electrons. The quasi‐thermal noise spectroscopy is a powerful tool for in situ space plasma diagnostics based on the analysis of the electrostatic field spectrum produced by the quasi‐thermal fluctuations of electrons. The electron parameters are obtained by fitting the voltage power spectrum calculated with a model of the particle velocity distribution to the voltage measured at the terminals of an electric antenna. In the present work, the model of spectrum due to the electrons is depending on only three parameters and computed by assuming that the electron velocity distribution is a generalized Lorentzian or “kappa” distribution. The three fitted parameters are thus the electron density, total temperature and kappa index of the distribution. We show for the first time that this model produces good quality fittings for solar wind measurements obtained by the radio receiver on Ulysses, which should enable to improve the accuracy of the measurement of the total electron temperature and give better information on the suprathermal electrons.