A superposed epoch of superDARN plasma convection, THEMIS all-sky auroral brightness, and their covariance during substorms

Joule heating, driven by the interaction of electric fields and ionospheric conductivity, is a major source of thermospheric expansion during geomagnetic activity. Enhanced thermospheric density increases atmospheric drag on Low-Earth-Orbit (LEO) satellites, potentially shortening satellite lifetimes. Accurate estimates of Joule heating require reliable characterization of both plasma convection, which determines the electric field, and ionospheric conductivity, which is related to auroral brightness. In this study, we investigate the typical temporal and spatial evolution of plasma velocity, auroral brightness, and their covariance during auroral substorms. We performed a superposed epoch analysis of 10 auroral substorm events from solar cycle 24 using plasma convection measurements from the Super Dual Auroral Radar Network (SuperDARN) and auroral brightness observations from NASA’s Time History of Events and Macroscale Interactions during Substorms (THEMIS) All-Sky Imagers (ASI). The analysis examined the evolution of plasma velocity, auroral brightness, and their covariance relative to substorm onset. The superposed epoch results show that auroral brightening and expansion are broader and weaker than in individual events. Plasma velocities are reduced relative to individual-event magnitudes but persist westward and equatorward of onset. Negative covariance emerges near substorm onset and strengthens and expands with the regions of brightest auroral emission. These results capture the general behavior of plasma velocity, auroral brightness, and their covariance during substorms. Although not yet incorporated into empirical models, this covariance quantity may provide a practical constraint for predicting electric fields and conductivity in the auroral zone. Improved representation of these quantities could enhance Joule heating calculations, leading to better forecasts of thermospheric expansion and atmospheric drag on LEO satellites.