Electromagnetic Drift Waves at Substorm Onset: Theory and Observations

Abstract

AbstractThe onset of a modest substorm observed by THEMIS is studied and interpreted using a new model of current sheet instability. The wave‐like structuring of the first observed auroral arc is characterized using auroral imagers. Similarly, the magnetic fluctuations measured by the THEMIS satellites, located in the near‐Earth current sheet (10 RE) and magnetically connected to the arc, are determined. Both sets of measurements show that oscillatory components with periods from 7 to 20 s are detected at the very early phase of the onset. This is compared with the predictions of a recent theoretical model (Tsareva et al., 2019, https://doi.org/10.1017/S002237781900028X) that describes the kinetic instability of electromagnetic drift waves in 2‐D Harris sheet configurations. They are in a reasonable agreement with the observations since the measured magnetic fluctuations are characterized by frequencies, growth rates, and polarization that are consistent with the model. It is also remarkable that the oscillations are observed when the ion perpendicular velocity (drift velocity) exceeds 90–100 km/s. The theory indeed shows that the drift is the main trigger of the instability, with a possible threshold at 80–100 km/s. This suggests that above a threshold of magnetic stress, and thus ion drift, the current sheet equilibrium is affected by the generation of electromagnetic drift waves.