University of Maryland
Bow Shock Motion, and a Transient Event at the Magnetopause:THEMIS Observations.
This study focuses on THEMIS observations of a long-duration transient event in the vicinity of the dayside magnetopause at ~ 1534 UT on July 18, 2008 that was characterized by features typical of a magnetospheric flux transfer event (FTE): a bipolar (-,+) 5-7 nT signature in the Bn component, a positive monopolar variation in the Bl and Bm components, a ~5-7 nT enhancement in the total magnetic field strength, and a transient density and flow enhancement. The interplanetary magnetic field (IMF) was mostly radial and disturbed during the intervals studied, i.e., it was favorable for the repeated formation, disappearance and reformation of the foreshock just upstream from the subsolar bow shock. We show that varying IMF directions and solar wind pressures created significant effects that caused the compressions of the magnetosphere and the bow shock and magnetopause motions and triggered the transient event. Global signatures of magnetic impulse events (MIEs) in ground magnetograms during the period suggest a wide-spread pressure pulse instead of a localized FTE as the cause of the event in the magnetosphere. The directions of propagation and the flow patterns associated with the event also suggest an interpretation in terms of pressure pulses.