Observation of polar cap patches and calculation of gradient drift instability growth times: A Swarm case study

The Swarm mission represents a strong new tool to survey polar cap patches and plasma structuring inside the polar cap. In the early commissioning phase, the three Swarm satellites were operated in a pearls-on-a-string configuration making noon-midnight transpolar passes. This provides an unparalleled opportunity to examine the potential role of the gradient drift instability (GDI) process on polar cap patches by systematically calculating GDI growth times during their transit across the pole from day to night. Steep kilometer-scale gradients appeared in this study as initial structures that persisted during the approximate 90 min it took a patch to cross the polar cap. The GDI growth times were calculated for a selection of the steep density gradients on both the dayside and the nightside. The values ranged from 23 s to 147 s, which is consistent with recent rocket measurements in the cusp auroral region and provides a template for future studies. Growth times of the order of 1 min found both on the dayside and on the nightside support the existing view that the GDI may play a dominant role in the generation of radio wave scintillation irregularities as the patches transit the polar cap from day to night.