High Latitude Ionospheric Electrodynamics During STEVE and Non-STEVE
Substorm Events
Abstract
Previous studies have shown that Strong Thermal Emission Velocity
Enhancement (STEVE) events occur at the end of a prolonged substorm
expansion phase. However, the connection between STEVE occurrence and
substorms and the global high-latitude ionospheric electrodynamics
associated with the development of STEVE and non-STEVE substorms are not
yet well understood. The focus of this paper is to identify
electrodynamics features that are unique to STEVE events through a
comprehensive analysis of ionospheric convection patterns estimated from
SuperDARN plasma drift and ground-based magnetometer data using the
Assimilative Mapping of Geospace Observations (AMGeO) procedure. Results
from AMGeO are further analyzed using principal component analysis and
superposed epoch analysis for 32 STEVE and 32 non-STEVE substorm events.
The analysis shows that the magnitude of cross-polar cap potential drop
is generally greater for STEVE events. In contrast to non-STEVE
substorms, the majority of STEVE events investigated accompany with a
pronounced extension of the dawn cell into the pre-midnight subauroral
latitudes, reminiscent of the Harang reversal convection feature where
the eastward electrojet overlaps with the westward electrojet, which
tends to prolong over substorm expansion and recovery phases. This is
consistent with the presence of an enhanced subauroral electric field
confirmed by previous STEVE studies. The global and localized features
of high-latitude ionospheric convection associated with optical STEVE
events characterized in this paper provide important insights into
cross-scale magnetosphere-ionosphere coupling mechanisms that
differentiate STEVE events from non-STEVE substorm events.