A statistical study of transient events in the outer dayside magnetosphere

Abstract

The AMPTE CCE satellite frequently observed transient (1 < r < 8 min) events marked by magnetic field strength increases and bipolar magnetic field signatures (peak-topeak amplitudes > 4 nT) while in the outer dayside magnetosphere. We report a survey of 59 prominant events observed from August to November 1984. The bipolar signatures and minimum variance analysis reveal that most events move poleward and antisunward, except in the immediate vicinity of local noon. Here the motion of the events appears to be better governed by the spiral/orthospiral interplanetary magnetic field (IMF) orientation than magnetic curvature forces associated with IMF By. The IMF orientation appears to have little or no influence on event occurrence or orientation. We interpret the events in terms of solar wind/foreshock pressure pulse induced ripples on the magnetopause surface. Our results can be reconciled with those obtained in previous studies which made use of ISEE 1/2, AMPTE IRM, and AMPTE UKS observations if pressure pulses produce large-amplitude events and bursty merging (or reconnection) produces small-amplitude events.Paper number 95JA03063. 0148-0227/96/95JA-03063505.00 vidual events and their statistical occurrence patterns (see review by Sibeck [1994]). Distinguishing between these two mechanisms is important. Under the merging interpretation, the prevalence of transient events implies that bursty merging is a major contributor to the solar wind-magnetosphere interaction [e.g., Cowley, 1982]. Under the pressure pulse interpretation, the prevalence of transient events implies that the magnetosphere is constantly being buffeted by highly variable solar wind parameters, perhaps with preconditioning in the foreshock [Sibeck, 1990].Since the bursty merging and pressure pulse models predict differing patterns for event occurrence, orientation, and direction of motion as a function of IMF orientation, local time, and latitude, statistical studies should help determine the relative significance of each model in the production of transient events. We begin by outlining the contrasting predictions of the two models concerning event occurrence patterns as a function of IMF orientation. We then present a case study and a statistical survey of transient events observed by the AMPTE CCE satellite in the outer dayside magnetosphere, and conclude with an attempt to reconcile the new results with those obtained in previous studies. Predictions of the Bursty Merging ModelMerging occurs when any one of several current layer instabilities [e.g., Schindler, 1974;Huba et al., 1977;Lui et al., 1991] enhances resistivities, disrupts currents, and allows magnetosheath and magnetospheric magnetic field lines to interconnect across the magnetopause. Since instability growth rates increase with increasing current density and there is no clear dependence of the current layer thickness upon the magnetosheath magnetic field orientation [Berchem and Russell, 1982], we expect growth rates to increase as the shear between the magnetosheath and magnetosp...

Motion of flux transfer events: a test of the Cooling model

“…In the context of this debate, Kawano et al (1992) introduced a "characteristic time" (t char , defined as the time between the positive and negative peaks in the B N signature) to distinguish between longer events with bipolar B N signatures (t char >90 s) which were found to occur over a wide range of McIlwain L-shells and were not correlated to periods of reconnection as evidenced by AE index or southward interplanetary magnetic field (IMF), and shorter events (t char <90 s) which occurred nearer the magnetopause during periods of high AE index and southward IMF. Sanny et al (1996) investigated similar signatures, over a wider range of radial distances from Earth and also concluded that the shorter events were FTEs, whilst the longer events were more likely to be due to magnetopause waves.…”

“…Sibeck et al, 1989;Lanzerotti, 1989;Sibeck, 1990;Elphic, 1990;Sckopke, 1991;Lockwood, 1991;Sibeck, 1992;Smith and Owen, 1992;Kawano et al, 1992;Elphic et al, 1994;Song et al, 1994Song et al, , 1996Newell, 1995, 1996;Sanny et al, 1996). In the context of this debate, Kawano et al (1992) introduced a "characteristic time" (t char , defined as the time between the positive and negative peaks in the B N signature) to distinguish between longer events with bipolar B N signatures (t char >90 s) which were found to occur over a wide range of McIlwain L-shells and were not correlated to periods of reconnection as evidenced by AE index or southward interplanetary magnetic field (IMF), and shorter events (t char <90 s) which occurred nearer the magnetopause during periods of high AE index and southward IMF.…”

Cluster observations of bounday layer structure and a flux transfer event near the cusp

“…This model has been the cause of much debate (e.g. Lanzerotti, 1989;Elphic, 1990;Lockwood, 1991;Smith and Owen, 1992;Kawano et al, 1992;Elphic et al, 1994;Song et al, 1994Song et al, , 1996Newell, 1995, 1996;Sanny et al, 1996). In particular, Kawano et al (1992) studied bipolar signature events near the low-latitude magnetopause, and concluded that long (B N peak-to-peak time-scale >90 s) events showed no correlation with the interplanetary magnetic field (IMF) or AE index and occurred over a wide range of McIlwain L-shells.…”

“…However short (time-scale <90 s) events tended to be observed near the magnetopause during periods of southward IMF and high AE index, and were ascribed to reconnection. Sanny et al (1996) used similar criteria to identify bipolar "transient events" observed by the AMPTE CCE satellite across a wider range of radial distances from the Earth. The occurrence of these transient events was not correlated with the IMF orientation, and they were interpreted as signatures of solar wind/foreshock pressure pulses.…”

Characteristics of merging at the magnetopause inferred from dayside 557.7-nm all-sky images: IMF drivers of poleward moving auroral forms

“…However, Sibeck et al (2003) showed that for a spiral IMF the postnoon first impact created activity that propagated past noon toward dawn. Sanny et al (1996) studied dayside magnetospheric responses to observed increases in the IMF strength and bi-polar fluctuations in B X exceeding 4 nT. They argue that pressure pulses are the dominant cause of events observed in the closed field line part of the outer magnetosphere.…”