Electron Acceleration Signatures During Substorm Onset: Linking DMSP Energy–Time Spectrograms to Ionospheric Plasma Velocity Dropouts and Auroral Structures

Substorm onset and expansion phases involve rapid magnetosphere–ionosphere coupling, yet the mechanisms linking magnetotail energy release to ionospheric plasma flow remain uncertain. We investigate whether observed drops in high-latitude plasma velocity during substorms are preferentially associated with specific electron acceleration processes. Using particle energy–time spectrograms from the DMSP satellites, plasma drift measurements from SuperDARN, and auroral brightness from THEMIS all-sky imagers, we examine electron spectral signatures coincident with velocity dropouts and auroral structures.

We analyze whether these flow reductions coincide with inverted-V electron spectra or with broader, more dispersed spectra consistent with Alfvén wave–driven acceleration. We also analyze average electron energies to determine whether characteristic energy ranges correspond to particular auroral structures and flow responses.

Our results constrain which acceleration modes dominate during velocity dropouts and clarify how different electron precipitation regimes modulate ionospheric plasma dynamics during substorm onset and expansion.