Coincident observations of ionospheric troughs and the equatorial plasmapause |
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Authors: | J.M. Grebowsky N.C. Maynard Y.K. Tulunay L.J. Lanzerotti |
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Affiliation: | NASA/Goddard Space Flight Center, Greenbelt, MD 20771,U.S.A.;Middle East Technical University, Ankara ,Turkey;Bell Laboratories, Murray Hill, NJ 07974,U.S.A. |
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Abstract: | Explorer 45 traversed the plasmapause (determined approximately via the saturation of the d.c. electric field experiment) at near-equatorial latitudes on field lines which were crossed by Ariel 4 (~600km altitude) near dusk in May 1972 and on field lines which were crossed by Isis II (~1400km altitude) near midnight in December 1971 and January 1972. Many examples were found in which the field line through the near-equatorial plasmapause was traversed by Explorer 45 within one hour local time and one hour universal time of Ariel and Isis crossings of the same L coordinate. For the coincident passes near dusk, the RF electron density probe on Ariel detected electron density depletions near the plasmapause L coordinates when Ariel was in darkness. When the Ariel passes were in sunlight, however, electron depletions were not discernable near the plasmapause field line. On the selected near-midnight passes of Isis II, electron density depressions were typically detected (via the topside sounder) near the plasmapause L coordinate. The dusk Ariel electron density profiles are observed to reflect O+ density variations. Even at the high altitude of Isis near midnight, O+ is found to be the dominant ion in the trough region whereas H+ is dominant at lower latitudes as is evident from the measured electron density scale heights. In neither local time sector was it possible to single out a distinctive topside ionosphere feature as an indicator of the plasmapause field line as identified near the equator. At both local times the equator-determined plasmapause L coordinate showed a tendency to lay equatorward of the trough minimum. |
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