vendredi 2 décembre 2022
La soutenance de thèse de Mingzhe LIU aura lieu le mardi 6 décembre 2022 à 15h00, dans l’amphithéâtre Evry Schatzman à Meudon.
Elle sera diffusée en direct sur la chaîne YouTube du LESIA :
La thèse sera soutenue en anglais.
"Large-scale solar wind properties by Parker Solar Probe and shock-foreshock interaction near the Earth"
Heat transport in the solar corona and wind, which is not completely understood, plays a key role in corona heating and wind acceleration. Due to their small mass compared to ions, electrons dominate the thermally driven solar wind expansion. To derive their properties, the Quasi-thermal noise (QTN) technique is a reliable tool : it yields accurate measurements of the electron parameters in the solar wind especially the total electron density without any calibration. The QTN technique thus provides routine cross-checking for traditional particle detectors. The ongoing pioneering Parker Solar Probe (PSP), whose heliocentric distances of orbit perihelia decrease from 35.7 solar radii (Rs) to 9.86 Rs within five years, offers an opportunity to examine the solar wind properties closer to the Sun than previously detected.
First, based on electron parameters obtained from the simplified QTN technique and the bulk proton parameters by Faraday Cups, we investigate the solar wind energy flux as close to the Sun as 27.8 Rs. We obtain that the averaged energy flux value is similar to the previous results based on long-term observations at greater distances and various latitudes, which confirms that this quantity appears as a global solar constant. Furthermore, the normalized energy flux distributions are nearly symmetrical and well fitted by Gaussians, implying the limited interactions between the solar wind and transient plasma structures in the inner heliosphere.
Then, we examine the radial evolution of the total electron temperature (Te), derived from the QTN technique using the high-frequency part of the radio spectrum, with the heliocentric distance varying from about 13 to 60 Rs. We obtain that Te decreases with the distance as R^-0.66, which is much slower than an adiabatic behavior. The extrapolated Te is consistent with the exospheric solar wind model prediction at around 10 Rs, Helios observations at 0.3 AU, and Wind observations at 1 AU, respectively. Furthermore, when the solar wind is slower (or in flux tube with larger mass flux), the radial Te profiles are steeper. More pronounced anticorrelated (Vp, Te) is observed when the solar wind is slower and closer to the Sun. As a byproduct, we derive a database of spectra affected by bursty Langmuir waves and/or electromagnetic emissions, which will be useful for further analysis and routine full fit on the QTN spectra. In addition to the solar wind properties, we study a supercritical quasi-perpendicular interplanetary (IP) shock interacting with the terrestrial foreshock via Wind observations. Some new features of wave activities and particle dynamics, resulting from the shock-foreshock interaction, are identified : (1) Intensive bursty Langmuir waves are detected downstream of the IP shock, coinciding with that the penetrating terrestrial foreshock electron beams are accelerated parallel to the magnetic field toward downstream. (2) The IP shock is interacting with the upstream Alfvén waves/fluctuations, and associated with atypically intensive beam-like gyrating-reflected ions compared to other events with similar shock parameters. These findings raise questions and trigger further investigations regarding particle acceleration (i.e. through precursor whistlers) and shock-Alfvén-wave interaction.
