David Berube

A statistical study of transient event motion at geosynchronous orbit

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) Bz, 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.

Plasmaspheric drainage plume observed by the Polar satellite in the prenoon sector and the IMAGE satellite during the magnetic storm of 11 April 2001

[1] During the early main phase of a geomagnetic storm on 11 April 2001, the Polar satellite was inside the magnetosphere in the prenoon sector (∼1000-1100 magnetic local times) and experienced a magnetopause crossing at L ≈ 6 because of the high solar wind dynamic pressure and strong southward interplanetary magnetic field (IMF). Just before the magnetopause crossing, Polar observed cold, dense plasma. That is, the cold, dense plasma was immediately adjacent to the compressed magnetopause. Using simultaneous observations by the IMAGE extreme ultraviolet (EUV) imager, we confirm that the cold, dense plasma observed by Polar is a plasmaspheric drainage plume extending outward from the plasmasphere to the magnetopause during the interval of high geomagnetic activity and strong southward IMF. We compare plasmaspheric mass densities determined from ground magnetometer data at L = 2.3 and 2.9 for a magnetically quiet time interval to mass densities determined during the magnetic storm time interval. We find no significant differences in the mass density between both intervals. These observations suggest that the sunward-convecting plasmaspheric plasma observed at Polar is due to erosion of the outer layers of the plasmasphere beyond L = 2.9.

Beyond the Point Charge: Equipotential Surfaces and Electric Fields of Various Charge Configurations

A laboratory experiment often performed in an introductory electricity and magnetism course involves the mapping of equipotential lines on a conductive sheet between two objects at different potentials. In this article, we describe how we have expanded this experiment so that it can be used to illustrate the electrostatic properties of conductors. Different configurations of electrodes can be used to show that the electric field is zero inside a conductor as well as within a cavity, the electric field is perpendicular to conducting surfaces, and the charge distribution on conducting surfaces can vary.