Highly periodic stormtime activations observed by THEMIS prior to substorm onset

Abstract

On March 24, 2007 THEMIS observed near the dusk flank several 10 minute quasi-periodic flow and magnetic field oscillations followed by the onset of a strong substorm (AL ∼ -1000 nT). The substorm occurred during an interval of strongly southward IMF, near the start of the recovery phase of a small storm (SYM-H near -80 nT). Each magnetic oscillation was accompanied by a rapid flow variation, auroral intensification, energetic particle injection, and Pi2 pulsations. For several hours both prior to and following the substorm THEMIS observed highly periodic flow oscillations, with the same 10 minute periodicity. The average of these flow oscillations was non-zero and positive, indicating net sunward transport. We suggest that the long interval of oscillatory flow constituted a periodic convective mode of the magnetosphere, and further suggest that the quasi-periodic activations were associated with reconnection near the THEMIS location. Copyright 2008 by the American Geophysical Union.