Multiscale/multifluid simulations of flux ropes at the magnetopause within a global magnetospheric model

Abstract

The magnetopause current sheet is known to have a thickness comparable to an ion gyroradius/skin depth where the magnetic and electric field can differ markedly from that assumed in MHD treatments. Multifluid/multiscale simulations are used to provide the first investigation of these processes in a global simulation that includes high‐resolution (200 km) gridding around the magnetopause. It is shown that the model is able to capture the quadrupole core magnetic field and the fast (tens of ion cyclotron periods) reconnection seen in idealized studies reconnection for a Harris current sheet. Within a global magnetosphere, multiple X‐line reconnection occurs for southward IMF due to localized pinching of the magnetopause current sheet via the convection of the magnetosheath plasma against a three‐dimensional magnetopause. Localized flux ropes with a thickness of a few hundred to a few thousand kilometers develop and can expand laterally due to current sheet acceleration of ions that have a gyroradius comparable to the current sheet thickness. These flux ropes are shown to have essentially the same properties as flux transfer events (FTEs), including being quasi‐periodic with a curvature greater on the magnetosheath side than on the magnetospheric side, a strong core magnetic field, and a mixture of magnetospheric and magnetosheath plasma. The speeds of the plasma flows associated with flux ropes are also similar to those observed with FTEs. The presence of multiple X‐line reconnection is shown to produce the rippling of the magnetopause and gives a nature explanation to the multiple magnetopause encounters that typically occur for slow moving spacecraft. These small‐scale processes are shown to have global effects with a reduction of the cross‐polar cap by as much as 20% seen between simulations with and without high resolution about the magnetopause.