On the Preferential Generation of Large Amplitude Alfven Waves in Downward Field-Aligned Current Regions

Abstract:

Global scale upward and downward region 1 and region 2 field-aligned current (FAC) systems are established as a result of magnetosphere-ionosphere coupling (MIC). The magnitude and structure of these FAC systems changes as a result of dynamical changes in the solar wind and/or in the magnetosphere, resulting in a reconfiguration which is developed as a result of the generation, propagation, and reflection of Alfvén waves. Here we use data from the European Space Agency (ESA) Swarm mission to investigate the nature of the electrodynamics associated with large scale FACs at Swarm altitudes. We present a series of case studies which demonstrate a clear indication that the downward FACs can be strongly and preferentially associated with large amplitude Alfvén waves during magnetic storms. In contrast, immediately adjacent upward FACs do not appear to be associated with such large amplitude waves at these altitudes. Examining historical data sets, it is clear that such behavior has been observed even in very early data sets such as from the Triad satellite, but to our knowledge has not been examined or explained in detail. Here we present an explanation in terms of the active feedback of the ionosphere which generates a preferential source of Alfvénic wave disturbances in ambient downward FAC regions. Such a large discrepancy in wave power between the upward and downward FAC regions in low-Earth orbit suggests that wave Joule heating effects might also be preferentially confined to the downward FAC regions. This could have potentially important implications for global MIC models, and for energy deposition in the coupled magnetosphere-ionosphere-atmosphere system.