Flux transfer events: Spontaneous or driven?

Abstract

Since flux transfer events (FTEs) occur predominantly under conditions of southward interplanetary magnetic field (IMF), one might expect that FTEs could be associated with the southward turning of the IMF which could lead to a transient increase in the merging rate. Lockwood and Wild [1993] recently suggested, in fact, that FTEs are driven by the IMF variations in which the Bz becomes more southward and rather than arising spontaneously during periods of steady IMF. To test this hypothesis, we have surveyed observations of the IMF during the flux transfer events observed in the dayside magnetopause by ISEE spacecraft. The upstream solar wind data from IMP‐8 or ISEE 3 are carefully compared with the magnetosheath data from ISEE to determine the time lag between them, which enables us to survey the simultaneous solar wind data during the FTEs. We find no evidence that FTEs are directly correlated with southward turning of the IMF. Rather, the FTEs more frequently occur when the IMF Bz is steady. A survey of 10‐minute intervals of simultaneous IMF data preceding each FTE show that the IMF fluctuates between northward and southward for less than 20% events and few of these turnings appear to be consistent with triggering the FTE. For majority of events, the IMF stays southward with fluctuations of clock angle less than 30° over the 10‐minute period. Thus we conclude that FTEs do occur spontaneously for steady southward IMF and are not simply directly driven by the observed fluctuations in the IMF Bz component.