Observation of Polar Cap Patches and Calculation of Gradient Drift Instability Growth Times: A Swarm Case Study

Friday, 19 December 2014
Andres Spicher1, Taylor Grant Cameron2, Eric M. Grono2, Kateryna Yakymenko3, Stephan C Buchert4, Lasse Clausen1, David J Knudsen2, Kathryn A McWilliams3 and Joran Idar Moen1, (1)University of Oslo, Physics Department, Oslo, Norway, (2)University of Calgary, Calgary, AB, Canada, (3)University of Saskatchewan, Saskatoon, SK, Canada, (4)IRF Swedish Institute of Space Physics Uppsala, Uppsala, Sweden
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 role of the gradient drift instability (GDI) on polar cap patches as they convect from day to night. The most rapid GDI growth times are caused by the kilometer-scale gradients, and these gradients appear as initial structuring that persists during the approximate 90 minutes it takes a patch to cross the polar cap. According to linear theory, the calculated GDI growth times range from 23 s to 147 s in this case study. The GDI growth rates estimated in this study are consistent with recent rocket measurements in the cusp auroral region where the structuring is likely to take place. These results will help constrain future modelling of polar cap patches.