Quasi-Thermal Noise Spectroscopy, a powerful tool for understanding the plasma in the Heliosphere.

The Quasi-Thermal Noise (QTN) spectroscopy is an efficient tool to study, in the radio frequency domaine, the electrostatic fluctuations due to the thermal motion of the charged particles in a plasma that surrounds a passive antenna. This noise is ubiquitous, and most of the time, is dominant around the electronic plasma frequency.

The voltage power spectrum of the electrostatic fluctuations depends on the velocity distribution of the electrons f_e(v), in addition to the antenna response function. The shape of the QTN in a weakly magnetized plasma allows one to yield an accurate diagnostic of the electron properties such as the total electron density n_e and core temperature T_c, which allows one to analyze the electronic populations in the solar wind with great precision.
We present a semi-automatic method to determine the density of the electrons.

It has been applied on the Parker Solar Probe (PSP) and on the WIND spacecraft, between late 2018 and early 2025.
Yielding a large-scale structure of the solar wind density, down to 10 Solar Radii, we discuss on its radial and temporal variations with the solar cycle.Finally, based on the above method, we discuss on the implementation of a full fitting to deduce a precise diagnostic of the thermal and non-thermal populations of the electrons, both in the solar wind and in the hermean magnetosphere, when the BepiColombo data will be available in early 2027.