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

Heat transport in the solar corona and wind is still a major unsolved astrophysical problem. Because of the key role played by electrons, the electron density and temperature(s) are important prerequisites for understanding these plasmas. We present such in situ measurements along the two first solar encounters of Parker Solar Probe (PSP), between 0.5 and 0.17 AU from the Sun, revealing different states of the emerging solar wind near solar activity minimum. These preliminary results are obtained from a simplified analysis of the plasma quasi-thermal noise (QTN) spectrum measured by the Radio Frequency Spectrometer (RFS/FIELDS). The local electron density is deduced from the tracking of the plasma line, which enables accurate measurements, independent of calibrations and spacecraft perturbations, whereas the temperatures of the thermal and supra-thermal components of the velocity distribution, as well as the average kinetic temperature are deduced from the shape of the plasma line. The temperature of the weakly collisional thermal population, similar for both encounters, decreases with distance as $R^{-0.74}$, much slower than adiabatic. In contrast, the temperature of the nearly collisionless suprathermal population exhibits a virtually flat radial variation. The 7-second resolution of the density measurements enables us to deduce the low-frequency spectrum of compressive fluctuations around perihelion, varying as $f^{-1.4}$. This is the first time that QTN spectroscopy is implemented with an electric antenna 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.