First In-situ Measurements of Electron Density and Temperature from Quasi-Thermal Noise Spectroscopy with Parker Solar Probe/FIELDS

International audienceWe present in situ measurements of electron density and temperature in the inner heliosphere obtained during the two first solar encounters at 35 solar radii of the Parker Solar Probe mission. These preliminary results are obtained from the analysis of the plasma quasi-thermal noise (QTN) spectrum measured by the RFS/FIELDS instrument along two trajectories extending between 0.4 and 0.17 UA from the sun, revealing different states of the emerging solar wind, five months apart. The local electron density is deduced from a new specific tracking of the plasma line, which enables accurate density measurements, independent of calibrations and spacecraft perturbations, whereas the temperatures of the thermal and supra-thermal parts of the electron velocity distribution, are deduced only from the calibrated min and max levels of the plasma line. The temperature of the weakly collisional core population varies radially with a power law index of about -0.8, much slower than adiabatic, whereas the temperature of the supra-thermal population exhibits a much flatter radial variation, as expected from its nearly collisionless state. These measured temperatures are close to extrapolations towards the sun of Helios measurements. This is the first time that the technique of QTN spectroscopy is implemented with so short electric antennas, of length not exceeding the plasma Debye length. As PSP will approach the sun, the decrease in Debye length is expected to considerably improve the accuracy of the temperature measurements