The Magnetopause: a quasi-tangential interface between the magnetosphere and the magnetosheath

This thesis aims to study the Earth's magnetopause, defined as the boundary between the Earth's magnetosphere and the solar wind. Although considered in first approximation as a clear barrier between the two plasmas, the reality is more complex, as the solar wind plasma and the magnetosphere plasma mix with each other in the magnetopause in ways not yet fully understood. In particular, the thesis focuses on regions of the magnetopause where this region is described as a discontinuity. Indeed, current models fail to describe the properties observed from in situ data. In situ data from the Magnetospheric Multiscale Mission (MMS, NASA) are used for this purpose. The first part of the work aims to develop an instrument that estimates the direction of the magnetopause normal more accurately than current instruments. This tool is then tested both on the MMS mission data and through a numerical simulation obtained through the hybrid-PIC code Menura. Then, the in situ data are used to study the magnetopause equilibrium in detail. In particular, by exploiting the normal obtained through the previously developed instrument, the study shows that the divergence of the pressure tensor plays a key role in this equilibrium, unlike what is assumed in classical theory. Finally, one part of the project is focused on Mercury's magnetosphere, deviating slightly from the main objective of this thesis. In this analysis, full-kinetic simulations are used to analyze the generation of whistler waves in the reconnection region in the magnetocode.