A Superposed Epoch Analysis of SuperDARN Plasma Convection, THEMIS-ASI Auroral Brightness, and Their Covariance During Substorms

Joule heating, driven by the interaction of electric fields and ionospheric
conductivity, is a primary cause of thermospheric expansion during geomagnetic
activity. This expansion increases atmospheric drag on Low-Earth-Orbit (LEO)
satellites, potentially reducing their lifetimes. Accurate Joule heating estimates
require precise knowledge of the electric field, derived from plasma convection,
and conductivity which is proportional to auroral brightness. Using plasma
convection data from the Super Dual Auroral Radar Network (SuperDARN) and
auroral brightness data from NASA’s Time History of Events and Macroscale
Interactions during Substorms (THEMIS) All-Sky Imagers (ASI), we performed a
superposed epoch analysis of 10 auroral substorm events from solar cycle 24,
the most recent completed solar cycle. This analysis characterizes the typical
temporal and spatial evolution of plasma velocity, auroral brightness, and their
covariance during substorms. The superposed epoch shows that brightening and
expansion of the auroral arc are broader and weaker than in individual events.
Plasma velocities are lower than in individual events, but persist to the west
and equatorward of onset, while negative covariance emerges near substorm
onset, strengthening and spreading with the brightest auroral regions. Although
not yet applied in empirical models, the covariance quantity provides a practical
constraint for predicting electric field and conductivity in the auroral zone,
which could ultimately improve Joule heating calculations, improving forecasts
of thermospheric expansion and helping mitigate the risk of premature satellite
reentry due to increased atmospheric drag.