Coordinated airborne and satellite measurements of equatorial plasma depletions

Abstract

A series of experiments was conducted in December 1979 to investigate the structure of plasma depletions in the low latitude, nightime ionosphere. The measurements included all sky imaging photometer (ASIP), ionosonde and amplitude scintillation observations from the AFGL Airborne Ionospheric Observatory (AIO), and in situ ion density measurements from the Atmosphere Explorer (AE‐E) Bennett Ion Mass Spectrometer (BIMS). The AIO performed two flights along the Ascension Island (−18° MLAT) magnetic meridian: one in the southern hemisphere and one near the Ascension conjugate point in the northern hemisphere. During these flights, measurements from the AE‐E satellite at 434 km altitude are compared with simultaneous remote ionospheric measurements from the AIO. Density biteouts of approximately one order of magnitude in the dominant ion O+, were mapped to lower altitudes along magnetic field lines for comparison with 6300‐Å and 7774‐Å O I airglow depletions. Because of the different airglow production mechanisms (dissociative recombination of O2+ for 6300 Å and radiative recombination of O+ for 7774 Å) the 6300‐Å depeletions reflect plasma depletions near the bottomside of the F layer, while those at 7774 Å are located near the peak of the layer. The O+ biteouts map directly into the 7774‐Å airglow depletions in the same hemisphere and also when traced into the opposite hemisphere, which indicates magnetic flux tube alignment over north‐south distances of ∼2220 km. The 6300‐Å (bottomside) depletions are wider in longitude than the 7774‐Å (F‐peak) depletions near the equatorward edge of the Appleton anomaly. This difference in topside and bottomside structure is used to infer large‐scale structure near the anomaly and to relate this to structure, commonly observed near the magnetic equator by the ALTAIR radar.