Physics
Scientific paper
Feb 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994jatp...56..167g&link_type=abstract
Journal of Atmospheric and Terrestrial Physics (ISSN 0021-9169), vol. 56, no. 2, p. 167-183
Physics
2
Atmospheric Boundary Layer, Electron Density (Concentration), Interplanetary Magnetic Fields, Ion Density (Concentration), Magnetospheres, Particle Precipitation, Plasma Sheaths, Polar Caps, Solar Wind, Daytime, Dmsp Satellites, Field Aligned Currents, Neutral Sheets, Particle Acceleration, Satellite Observation, Wind Velocity
Scientific paper
The morphology of precipitating particles, measured at low altitude in the polar regions, varies systematically with the strength and direction of Interplanetary Magnetic Field (IMF) B(sub z) and with solar wind speed V(sub sw). We use particle data taken onboard the Defense Meteorological Satellite Program (DMSP) satellites to determine these variations. Both individual satellite passes during the storm/quieting period of 26 and 27 August 1990, and statistical maps compiled from a data base over 4.5 yr are presented. We focus attention on those magnetospheric populations that have magnetosheath characteristics, the boundary populations. We show that the precipitating ion boundary population, whose down-coming spectra can be fitted to streaming Maxwellians, expands from a region confined near the dayside cusp for southward IMF, to a thick, annular region, including the dayside cusp, for northward IMF. The expansion in local time is inhibited by increasing solar wind speed. Boundary electrons behave somewhat differently. They have easier access to the polar regions and their variations have shorter spatial/temporal scale lengths than the boundary ions. For strongly northward IMF, intense, agitated boundary electrons can be found over all or part of the polar cap. Broad regions (up to approximately 100 km) of strongly acccelerated electrons (several keV) that produce visible arcs are embedded in this population. Two features of the ion boundary population help identify its source. (1) The spectra of the boundary ions expanding into the polar cap exhibit field-aligned streaming, which, downtail, is toward the Earth. (2) The region into which the boundary ions expand best maps magnetically to a dawn-dusk cut across the neutral sheet, rather than to the low-altitude boundary layer. Therefore, we conclude that the immediate source for boundary ions in the polar regions during northward IMF is the plasma sheet boundary layer. These ions reach tail lobe field lines by convection whose direction when mapped to the ionosphere is sunward. Significant change in the topology of the magnetospheric magnetic field, and, in particular, the closing of high-latitude field lines, is not required to explain the data.
Brautigam D. H.
Gussenhoven Susan M.
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