Abstract
Abstract. The polar rain electrons near the open–closed field line boundary on the nightside often exhibit energy-latitude dispersion, in which the energy decreases with decreasing latitude. The solar wind electrons from the last open-field line would E × B drift equatorward as they move toward the ionosphere, resulting in the observed dispersion. This process is modeled successfully by an open-field line particle precipitation model. The existing method for determining the magnetotail X line distance from the electron dispersion underestimates the electron path length from the X line to the ionosphere by at least 33%. The best estimate of the path length comes from using the two highest energy electrons in the dispersion region. The magnetic field line open–closed boundary is located poleward of the highest energy electrons in the dispersion region, which in turn is located poleward of Defense Meteorological Satellite Program (DMSP) b6, b5e, and b5i boundaries. In the four events examined, b6 is located at least 0.7–1.5° equatorward of the magnetic field line open–closed boundary. The energy-latitude dispersion seen in the electron overhang may result from the plasma sheet electron curvature and gradient drifts into the newly closed field line.
Highlights
Magnetic reconnection is an important process in space and plasma physics
Larger X line distance would result in a larger displacement
APL-OPM successfully models the polar rain electron energy-latitude dispersion and demonstrates quantitatively that this dispersion results from the solar wind suprathermal electron entries in the last open-field line and E ×B equatorward convection
Summary
Magnetic reconnection is an important process in space and plasma physics. On the dayside, magnetic reconnection between the interplanetary magnetic field (IMF) and the magnetospheric magnetic field lines causes the closed magnetospheric field lines to become open. Shirai et al (1997) attributed the energy-latitude dispersion to being a signature of the last injected solar wind suprathermal electrons before the open field-lines reconnect in the magnetotail and become closed. Because the nightside open–closed boundary (magnetotail X line) is known and can even be arbitrarily set in APL-OPM, the model can be a useful tool to evaluate how well the polar rain dispersion can be used to estimate the X line location. We model the polar rain electron energy dispersion using a modified APL-OPM (Wing et al, 1996, 2001) Based on this model, we examine the accuracy of the existing method for using the dispersion to estimate electron path length and X line distance. We examine the issue of where the magnetic field line open–closed boundary is located based on our modeling and observational work
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