Probing the inner heliosphere and corona with electric antennas: quasi-thermal noise spectroscopy on Solar Orbiter and Solar Probe Plus.

Abstract:

Solar wind electrons are expected to play an important role for energy transport in the solar corona and wind. Solar wind electron velocity distributions exhibit three components: a thermal core, a suprathermal halo, and a magnetic field aligned strahl, which is usually moving away from the Sun. The origin of these non-thermal distributions is unknown. Are such distributions already present in the solar corona or are they only a consequence of the solar wind transport in the interplanetary medium? The answer to these questions is of paramount importance to understand the origin of the solar wind. It requires accurate in situ measurements of the electron properties. Traditional electron analysers generally suffer from spacecraft charging and photoelectron perturbations, but the alternative method of Quasi-Thermal Noise Spectroscopy (QTN), which has been successfully used in various space plasma environments, is immune to these limitations. This method is based on the electrostatic fluctuations induced by the thermal motion of the ambient plasma particles, which can be measured with a sensitive radio wave receiver connected to a wire dipole antenna. As this quasi-thermal noise is completely determined by the particle velocity distributions in the frame of the antenna, QTN is a high-accuracy robust method for determining electron moments together with some non-thermal features. After a short review of the QTN method, we describe its recent developments and how it will be implemented on the upcoming missions Solar Orbiter and Solar Probe Plus. New simulations of QTN measurements in the inner heliosphere are presented for typical expected corona, solar wind and ICMEs conditions down to 9.5 solar radii.