Electrostatic plasma waves associated with collisionless magnetic reconnection : Spacecraft observations

Sammanfattning: Magnetic reconnection is a fundamental plasma process where changes in magnetic field topology result in explosive energy conversion, plasma mixing, heating, and energization. In geospace, magnetic reconnection couples the Earth’s magnetosphere to the solar wind plasma, enabling plasma transport across the magnetopause. On the sun, reconnection is responsible for coronal mass ejections and flares, which can affect everyday life on Earth, and it influences the evolution of the solar wind. Although collisionless magnetic reconnection has been studied for a long time, some fundamental aspects of the process remain to be understood. One such aspect is if/how plasma waves affect the process. Simulations and spacecraft observations of magnetic reconnection have shown that plasma waves are ubiquitous during reconnection. Particularly interesting are simulation results which show that electrostatic waves can affect the rate at which reconnection occurs, but this has not yet been experimentally verified. The recently launched Magnetospheric Multiscale (MMS) mission was designed to investigate the smallest scales of collisionless magnetic reconnection, making it an excellent mission to study small-scale waves as well. In this thesis, we use MMS to study electrostatic waves associated with magnetic reconnection in geospace. Our first two studies are devoted to the properties of electron holes (EHs), believed to play an important role in collisionless reconnection. Using MMS, we analyze EHs in unprecedented detail, and compare their properties to theory and previous studies. Importantly, we find evidence of EHs radiating whistler waves in the reconnection separatrices, a process which might modulate the reconnection rate. In our third study, we show that the presence of cold ions at the reconnecting magnetopause can lead to the growth of the ion-acoustic instability. This instability leads to dissipation and cold ion heating. The fourth study compares different techniques for determining the velocity of electrostatic waves. Accurate velocity estimates are important, since they are needed to understand how the wave interacts with the plasma. Finally, in our fifth study, we calibrate the E-field measurements made in the solar wind by the Solar Orbiter spacecraft, to aid future studies of solar wind processes, including magnetic reconnection.