Waves and instabilities at the magnetopause and its boundary layers: An overview

Abstract:

The magnetopause and its boundary layers are perhaps the most inhomogeneous region in the near-Earth space. Plasmas on the two sides of the boundary can have different origins, and those constituting the regions are often highly nonuniform in both configuration and phase space. There can exist magnetic shear, velocity shear, density jump, and/or ions and electrons with anisotropic or more complex velocity distributions, sometimes all at the same time and location. By using either or a combination of these free energies, there can locally grow waves and instabilities, including kinetic Alfvén waves, lower hybrid waves, whistler waves, tearing mode, Kelvin-Helmholtz mode, and mirror mode. On the other hand, large-scale surface waves on the magnetopause can be driven externally by structures or inhomogeneity already present in the solar wind or generated around the bow shock. They can be important agents for energy conversion, transferring energy from one region to another (often from the solar wind into the magnetosphere) or from one scale to another (e.g., MHD- to kinetic-scale), transporting mass and momentum in real space, and transport of particles in energy space (plasma heating and particle acceleration), and thus are key elements for better understanding of the solar wind-magnetosphere interaction. In this talk, we give an overview of boundary layer waves and instabilities, mostly based on recent in-situ observations at and around the terrestrial magnetopause and related theoretical works.