SuperDARN observations of ionospheric convection during magnetospheric substorms

The coupled nature of the magnetosphere-ionosphere system makes measurements of ionospheric convection, such as those provided by the SuperDARN HF radars, extremely useful in diagnosing magnetospheric dynamics.Flux Transfer Events (FTEs) at the dayside magnetopause, for example, are well-resolved in ionospheric flow data as Pulsed Ionospheric Flows (PIFs).Similarly, Bursty Bulk Flows (BBFs) associated with the earthward transport of flux in the tail have a discernable flow signature in the nightside ionosphere.The large-scale convection associated with magnetospheric substorms is also readily identifiable in ionosphere flow data.During the growth phase, for example, the expansion of the polar cap due to enhanced open flux production is evidenced in the equatorward motion of radar backscatter.On the nightside, fast equatorward flows emanating from the polar cap after substorm onset, followed by a poleward contraction of the flow reversal boundary, provide evidence for tail reconnection and the closure of open flux.The complex electrodynamics associated with substorms, however, ensures immense variety in the nature of the flow signatures which are observed.Some studies, for example, have reported a reduction in the nightside flows at the time of substorm onset, possibly resulting from enhancements in auroral conductivity associated with substorm energetic particle precipitation which imposes a limit on the size of the local electric field.Enhanced electric field phenomena such as Substorm-Associated Radar Auroral Surges (SARAS) and Auroral Westward Flow Channels (AWFC) provide additional constraints on the global substorm picture.This paper provides an overview of these and other important convection signatures associated with substorms and briefly discuss how future developments of SuperDARN can further enhance our understanding of substorm physics.