Plasma, often described as an ionized gas, is the state of matter that makes up most of the visible universe. In many regions throughout the universe, plasmas are collisionless, meaningthat inter-particle collisions are negligible compared to the influence of electromagnetic fields. Various processes affect the particle dynamics and contribute to energy dissipation and heating. One such process is wave-particle interactions, where electromagnetic waves resonate with charged particles and exchange energy with them. The nature of these phenomena can be investigated through near-Earth in situ spacecraft measurements, for example, by the Magnetospheric Multiscale (MMS) mission. The knowledge from in situ measurements also helps understand the dynamics in other environments where similar fundamental processes are believed to be important, notably in astrophysical and laboratory plasmas.

In this PhD thesis, we address the open problem of plasma heating, focusing on wave-particle interactions between electrons and whistler waves – electromagnetic plasma waves known to be important for collisionless energy transfer. We use MMS measurements in the Earth’s magnetosheath, the region downstream of the bow shock where solar wind plasma is heated and compressed due to interaction with the Earth’s magnetic field. Depending on the upstream solar wind, the magnetosheath is either in a more fluctuating state downstream of a quasi-parallel bow shock or in a more stationary quasi-perpendicular configuration. This allows us to investigate how different background conditions influence wave-particle interactions. In Paper I, we show how whistler waves are generated by electrons in the quasi-parallel magnetosheath. In Paper II, we compare the whistler occurrence in the two magnetosheath configurations and estimate their scattering effect on electron velocity distributions. In Paper III, we investigate a method for classifying the magnetosheath using local measurements. Finally, in Paper IV, we estimate the electron heat flux in the magnetosheath and explore how it is regulated by whistler waves.