Effects of nightside O+ outflow on magnetospheric dynamics: Results of multifluid MHD modeling

Abstract

Through spacecraft observations and numerical modeling, it is becoming increasingly well established that ionospheric oxygen is present in the magnetosphere in amounts that can have an effect on magnetospheric dynamics, affecting pressure balance, currents, convection flows, and cross polar cap potential (CPCP). However, an understanding of the various processes through which oxygen may bring about changes in magnetospheric structure and dynamics is still lacking. In this paper, we focus on the role that ionospheric oxygen outflow from the nightside auroral zone plays in determining plasma sheet density, size, and pressure and relate the changes in plasma sheet parameters to changes in CPCP, tail geometry, and magnetospheric convection. We use the Multifluid Lyon‐Fedder‐Mobarry model as a virtual laboratory to analyze the effects of ionospheric oxygen on magnetospheric parameters. Our simulations use idealized solar wind conditions, constant and uniform ionospheric conductance, and constant nightside oxygen outflow in order to isolate the effects of a change in outflow flux. We concentrate the ionospheric outflow in the nightside auroral region, so as to avoid the possibility of directly loading the distant tail X line with oxygen and hence to avoid a direct modification of the reconnection rate in order to focus on effects within the magnetosphere. We show that the presence of O+ outflowing from an isolated patch in the nightside auroral zone increases plasma sheet density and thermal pressure, slows convection, decreases the polar cap potential, and increases the length and width of the nightside magnetosphere. We determine that ionospheric O+ mass loading can play a significant role but that strong effects on magnetospheric dynamics require a nightside flux that significantly exceeds statistical observed levels. A more realistic outflow would include both dayside and nightside sources whose total combined flux would be similar to the total fluxes used here.