Transient events in the outer magnetosphere: Boundary waves or flux transfer events?

Abstract

We present simultaneous solar wind and magnetospheric observations for a series of flux transfer events (FTEs) observed by the UKS and IRM spacecraft in the outer prenoon magnetosphere on October 28, 1984 (day 302). The CCE satellite, located near local noon, observed an enhancement in the magnetospheric magnetic field strength 4 min prior to each IRM/UKS event, suggesting the antisunward propagation of a compression in the magnetopause surface. We suggest that these compressions resulted from wavy magnetopause motion driven by variations in the solar wind dynamic pressure applied to the magnetosphere, rather than the motion of FTEs formed by magnetic merging. In support of this hypothesis we note that the events previously identified as FTEs occurred during a sequence of quasi‐periodic (5–6 min) oscillations in the magnetospheric plasma velocity and magnetic field. The oscillations exhibited all the characteristics expected for antisunward moving magnetopause boundary motion and those cycles identified as FTEs simply exhibited larger amplitudes. The magnetospheric compressions observed by the CCE and IRM/UKS corresponded to enhancements in the interplanetary magnetic field strength (IMF) observed by the ISEE 2 satellite, located just upstream of the subsolar bow shock. Some of the magnetospheric compressions occurred for a strongly northward IMF. There was no evidence for the type of accelerated flows commonly associated with magnetic merging at the magnetopause on this day, and the orientation of the magnetic field inside the UKS/IRM events was not consistent with that expected for FTEs.