Interactions between Heliospheric and Ionospheric Plasma in Geospace

Abstract

The Sun interacts with the Earth through electromagnetic radiation, gravitationally, and through the solar wind. The solar wind carrying the Sun’s magnetic field interacts with the Earth’s magnetosphere and drives the dynamics of near Earth space. In near Earth space, plasma from the Earth’s ionosphere can be accelerated into the Earth’s magnetosphere due to the dynamics induced by the solar wind. The ionospheric plasma can then interact with the solar wind plasma. In this thesis, we explore the concept of a geopause, a surface defined where solar wind and ionospheric plasma quantities are equal. For example, the number density geopause is defined where the solar wind number density equals the ionospheric plasma number density.

We simulated the geospace environment through the Space Weather Modeling Framework by employing its multifluid magnetohydrodynamics model to track the velocities, mass densities, and pressures of the heliospheric and ionospheric plasmas separately. The Earth’s spin axis and magnetic field were aligned for our numerical simulations in order to isolate the effects of the solar wind magnetic field. We performed numerical experiments where the solar wind number density, pressure, and velocities were kept constant but the solar wind magnetic field had two different configurations. The two configurations used were 1) the solar wind magnetic field was kept parallel to the Earth’s spin axis (northward) and then flipped very quickly to be antiparallel to the Earth’s spin axis (southward) 2) the solar wind magnetic field was kept southward and then flipped northward. The ionospheric inner boundary was kept to have no radial velocity and solely driven by magnetohydrodynamics forces with four different ion compositions of the ionosphere. The compositions used were a hydrogen ionosphere, a hydrogen oxygen ionosphere mix with a mass density equal to the pure hydrogen ionosphere case, an ionosphere composed of hydrogen and oxygen equal in number density, and an oxygen only ionosphere. These numerical experiments generated over 12 hours of simulation data.

Over the course of the 12 hours of simulation data, we recorded the location of the mass density, number density, and pressure geopauses. In one study, we analyzed the geopauses throughout the simulation to examine the relation of the geopause size in response to the ionospheric composition and the conditions in the solar wind magnetic field. At the last time step of the simulation, we generated another study by comparing the mass density, number density, and pressure geopauses to the magnetopause in steady state to understand whether one physical process governed the transition in a plasma being dominated by the solar wind to the ionospheric plasma. In the last study, we calculated the source terms in the momentum equations at 12:00 and compared them to the geopauses to develop an understanding in the physical properties of the geopauses.