FAST Observations of Enhanced Alfvénic Activity and Ion Upflow During Large Geomagnetic Storms

Abstract:

We present four years of FAST observations of Alfvénic field-aligned currents (FACs) coincident with 37 moderate to very strong ( Dst ≤ 50 nT) geomagnetic storms. Superposed epoch analysis of Alfvénic activity of storm periods demonstrates a sharp increase in the number of observed Alfvén waves, peaking near sudden commencement and then monotonically decreasing (in a statistical sense) through the main and recovery phases, returning within two or three days of storm commencement to background levels of Alfvénic activity. Stormtime Alfvén events are statistically more likely to be observed at lower latitudes relative to geomagnetically quiet periods, and are also more likely to occur at pre-midnight longitudes.

For the same 37 storms, superposed epoch analysis of ion upflow measurements associated with each Alfvénic FAC not only shows an increase overall number of upflowing ions increases due to the increase in Alfvén activity, but also an increase in the average intensity of ion upflow; this suggests a nonlinear relationship between occurrence rates of Alfvén waves and upflow intensity. We explore one such possibility in terms of the magnitude of the current density and the intensity of upflowing ions related to Alfvénic FACs.