Abstract
We investigate the causes of large $dB/dt$ events observed by
SuperMAG, by comparing with the time-series of different types of
geomagnetic activity, or “convection state”, for the duration of 2010.
Spikes are found to occur predominantly in the pre-midnight and dawn
sectors. We find that pre-midnight spikes are associated with substorm
onsets. Dawn sector spikes are not directly associated with substorms,
but with auroral activity occurring within the westward electrojet
region. Azimuthally-spaced auroral features drift sunwards, producing
Ps6 (10-20 min period) magnetic perturbations on the ground. The
magnitude of $dB/dt$ is determined by the flow speed in the convection
return flow region, which in turn is related to the strength of solar
wind-magnetospheric coupling. Pre-midnight and dawn sector spikes can
occur at the same time, as strong coupling favours both substorms and
westward electrojet activity; however, the mechanisms that create them
seem somewhat independent. The dawn auroral features share some
characteristics with omega bands, but can also appear as north-south
aligned auroral streamers. We suggest that these two phenomena share a
single underlying cause.