Ahmad Lalti: Electrostatic turbulence and electron heating in collisionless shocks

Abstract

When the supersonic solar wind interacts with Earth’s magnetosphere it forms a shock wave. However, due to the low densities in space, inter-particle collisions play an insignificant role in its dynamics. Earth's bow shock is an example of a collisionless shock, ubiquitous throughout the universe. Their dynamics are complex and their physics remains an active field of research. In this thesis, we use high-resolution measurements from NASA's Magnetospheric Multiscale (MMS) spacecraft to study the plasma wave activity across Earth’s bow shock and its effects on electron heating. In Paper I we train a convolutional neural network (CNN) to identify the different plasma regions that MMS crosses. In Paper II we use the results of this CNN to compile a database of time intervals in which MMS crosses Earth’s bow shock, which we use to find suitable events to tackle the science questions of interest. In Paper III we use multispacecraft methods to properly characterize obliquely propagating whistler waves running upstream of the shock. By analyzing the ion and electron distribution functions we find that their likely source is the instability between the incoming electrons and reflected ions. Shifting our focus to Debye scale electrostatic waves, in Paper IV we develop a method to measure their 3D wave vector based on single-spacecraft interferometry. We are in the process of using this method to study the evolution of Debye scale electrostatic waves across quasi-perpendicular shocks (see Chapter 7). Finally, in Paper V we investigate the electron heating mechanism across quasi-perpendicular shocks. We find the heating mechanism to depend on the Alfvénic Mach number in the deHoffman-Teller frame