Steady State Characteristics of the Terrestrial Geopauses

The boundary separating solar wind plasma from ionospheric plasma is typically thought to be the magnetopause. A generalization of the magnetopause concept called the geopause was developed by Moore and Delcourt (1995, https://doi.org/10.1029/95RG00872). The geopause is a surface defined where solar wind quantities equal the ionospheric quantities. Geopause studies have helped characterize magnetospheric systems. However, comparative studies between the geopauses to the magnetopause have not been conducted. In this paper, we analyze the influence of inner boundary composition and interplanetary magnetic field (IMF) orientation on the steady state terrestrial geopauses and the magnetopause. This study simulates the Earth's magnetosphere by using the multifluid capabilities of the Block Adaptive Tree Solar wind Roe-type Upwind Scheme magnetohydrodynamics model within the Space Weather Modeling Framework. The simulations show that the dayside magnetopause was not influenced by the presence of oxygen in the outflow for both IMF orientations and was larger than the other geopauses. In contrast, the nightside magnetopause was sensitive to the conditions in the outflow. The nightside magnetopause was smaller than the other geopauses with southward IMF. With northward IMF, the nightside magnetopause was the largest structure in comparison with the plasma-based geopauses. Our results indicate that no single boundary surface dictates the transition from a solar wind dominated plasma to ionosphere dominated plasma.