The Ionospheric Leg of the Substorm Current Wedge: Combining Iridium and
Ground Magnetometers
Abstract
Utilising magnetic field measurements made by the Iridium satellites and
by ground magnetometers in North America we calculate the full
ionospheric current system and investigate the substorm current wedge.
The current estimates are independent of ionospheric conductance, and
are based on estimates of the divergence-free (DF) ionospheric current
from ground magnetometers and curl-free (CF) ionospheric currents from
Iridium. The DF and CF currents are represented using spherical
elementary current systems (SECS), derived using a new inversion scheme
that ensures the current systems’ spatial scales are consistent. We
present 18 substorm events and find a typical substorm current wedge
(SCW) in 12 events. Our investigation of these substorms shows that
during substorm expansion, equivalent field-aligned currents (EFACs)
derived with ground magnetometers are a poor proxy of the actual FAC. We
also find that the intensification of the westward electrojet can occur
without an intensification of the FACs. We present theoretical
investigations that show that the observed deviation between FACs
estimated with satellite measurements and ground-based EFACs are
consistent with the presence of a strong local enhancement of the
ionospheric conductance, similar to the substorm bulge. Such
enhancements of the auroral conductance can also change the ionospheric
closure of pre-existing FACs such that the ground magnetic field, and in
particular the westward electrojet, changes significantly. These results
demonstrate that attributing intensification of the westward electrojet
to SCW current closure can yield false understanding of the ionospheric
and magnetospheric state.