A statistical study of transient event motion at geosynchronous orbit

Abstract

Abstract. The geosynchronous GOES 5 and GOES 6 satellites frequently observe transient events marked by magnetic field strength increases and bipolar magnetic field signatures lasting several minutes. In this study we report a survey of 87 events observed simultaneously by both GOES spacecraft (for a total of 174 individual observations) from August to December 1984. Events detected in the prenoon sector outnumbered those in the postnoon sector by about a 3 to 1 ratio. The distribution of the events versus local time exhibited a significant prenoon peak like the distribution of magnetic impulse events observed in high-latitude ground magnetometers. A cross-correlation analysis of the two GOES data sets indicated lags that range from 0 to over 2 min, with the majority of the events moving antisunward. The short lags correspond to azimuthal speeds of hundreds of kilometers per second, greater than flow speeds in the magnetosheath, but less than fast mode waves. The short lags may indicate that the events move primarily latitudinally and/or that transient events are seldom localized, but rather occur over extended, if not global, regions. Investigations of event occurrence versus interplanetary magnetic field (IMF) B z, event motion versus IMF By, and correspondence between upstream plasma data and the events all indicate that pressure pulses are the likely source of many of the events. About 27% of the events with simultaneous solar wind data were preceded by sharp reversals in one or more IMF components, and nearly all of this particular group of events occurred in the dawn sector. This suggests that the pressure pulses may be commonly generated in the foreshock/bow shock region, since the prenoon magnetopause lies generally behind the quasi-parallel bow shock where such pulses are thought to be triggered by IMF discontinuities. Finally, several events in the data set were also observed by the AMPTE/CCE. These are presented as case studies.

Cluster observations of “crater” flux transfer events at the dayside high‐latitude magnetopause

“…This then casts some doubt as to the correct interpretation of such signatures. In addition, some statistical studies [e.g., Sanny et al, 1996Sanny et al, , 2001 of FTE-type signatures showed no southward IMF dependence, and thus provide support for the pressure-pulse interpretation. However, detailed studies of particle distributions during ''crater'' FTE observations [e.g., Smith and Owen, 1992] indicate that not all the signatures can be accommodated in the Sibeck [1990Sibeck [ , 1992 picture, and it seems thus appropriate to retain the reconnection model of the formation for at least some events.…”

Dayside magnetopause transients correlated with changes of the magnetosheath magnetic field orientation

“…The authors used the typical properties of these layers to predict the signatures and showed that this scenario could explain the features of the crater-type FTEs. Moreover, some statistical studies (e.g., Kawano et al, 1992;Sanny et al, 1996Sanny et al, , 2001) of FTE-type signatures showed no significant southward IMF dependence, thus, they provide support for the pressure-pulse interpretation. Sibeck et al (2005) considered the properties of FTEs observed by Interball-1 on a statistical basis and they found that events observed on the mid-latitude magnetopause, equatorward of the cusps, exhibit a strong tendency to occur for Published by Copernicus Publications on behalf of the European Geosciences Union.…”

Interplanetary magnetic field control of dayside transient event occurrence and motion in the ionosphere and magnetosphere

“…The results of this study should be compared with those obtained by Sanny et al (2001), who surveyed the occurrence patterns and motion of a particular category of event at geosynchronous orbit, namely flux transfer events. Each of these events was identified on the basis of bipolar signatures in the direction normal to the nominal magnetopause and impulsive increases in the magnetic field strength.…”