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

Abstract

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 2 Moncuquet et al.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.

The Electron Structure of the Solar Wind

“…It is generally accepted that the core electrons of the solar wind are a population that is "trapped" by (a) an interplanetaryelectric-field potential barrier as the core electrons move away from the Sun and (b) the magnetic mirror force as the core electrons move toward the Sun (e.g., Lie-Svendsen and Leer, 2000;Marsch, 2006). This being the case, the temperature T core of a measured distribution of core electrons is related to the electrical potential difference between the measurement location and the distant-from-the-Sun heliosphere (Feldman et al, 1975;Boldyrev et al, 2020;Moncuquet et al, 2020) [For a different interpretation of T core , see Scudder (2019)]. On average, the core electron temperature of the solar wind decreases with distance from the Sun (Pilipp et al, 1990;McComas et al, 1992;Halekas et al, 2020;Moncuquet et al, 2020), in general agreement with exosphere models of the interplanetary electrical potential ϕ, with ϕ(r) decreasing in magnitude (with respect to infinity) with distance r from the Sun (Lemaire and Scherer, 1971;Meyer-Vernet and Issautier, 1998;Meyer-Vernet et al, 2003).…”

Solar Wind Electron Parameters Determination on Wind Spacecraft Using Quasi‐Thermal Noise Spectroscopy

“…The method of constraining τ h using signal ratio between the plasma peak and the low‐frequency plateau was applied successfully by Moncuquet et al. (2020) to Parker Solar Probe (PSP) Fields measurements where electron halo was very low, but for the case of Wind TNR, data do not bring an improvement to our method due to non‐negligible values of a h at 1 au.…”

Localized magnetic field structures and their boundaries in the near-Sun solar wind from Parker Solar Probe measurements

“…The electron density is determined from the Quasi-Thermal Noise technique (QTN) (Moncuquet et al 2020), which uses the location of the plasma line in electric field spectra to infer the electron density. This technique offers the advantage of providing a density estimate that is independent of calibrations and spacecraft perturbations.…”

“…The two techniques of estimate of the plasma density, QTN and averaging over ion distribution function are complementary. QTN determines with the high precision a local electron density making use of the position of the peak in the spectra corresponding to the zero of the dielectric permittivity (Meyer-Vernet et al 2017;Moncuquet et al 2020). When the electron gyrofrequency is much smaller than the plasma frequency (it is our case) this peak position as a function of frequency is very close to the plasma frequency that allows one to evaluate the plasma density with high precision.…”

“…There are many spikes of high intensity in the frequency range from 5.7 Hz to several tens of Hz. The panel d shows an electron density profile obtained from electric field measurements making use of the QTN technique(Moncuquet et al 2020). The profile presented is smoothed and can not correctly represent the sharp boundaries.…”