Airborne studies of equatorial F layer ionospheric irregularities

Radio wave and optical experiments were conducted onboard a U.S. Air Force research aircraft in March 1977 and March 1978 at low magnetic latitudes to investigate the effects of F region electron density irregularities on transionospheric communications links. Imaging photometer, ionosonde, and satellite amplitude scintillation measurements were used to monitor the development and motion of F region 6300‐Å O I airglow depletions, spread F, and scintillation producing irregularities that are all associated with low‐density bubbles in the postsunset equatorial ionosphere. The 6300‐Å airglow depletions are the bottomside signature of low plasma density within the bubbles. Examples of multiple airglow depletions and their relation to variations in the F layer virtual height (h′F) and to the occurrence of amplitude scintillations on 250‐MHz satellite signals are described. Estimates of the average bottomside electron density, from simultaneous ionosonde measurements and 6300‐Å airglow intensities, show electron density decreases of ∼66% within the bubbles. These decreases are approximately the same for bubbles observed at the magnetic equator and near Ascension Island (18°S magnetic latitude). The measurements at Ascension Island show that airglow depletions extend away from the magnetic equator into the southern 6300‐Å intertropical arc. Variations in the maximum poleward extent of airglow depletions and of associated ionospheric irregularities that give rise to amplitude scintillations were observed. These latitudinal variations are interpreted, using field line mapping considerations, as variations in the maximum altitude of plasma bubbles over the magnetic equator. A north‐south flight confirms that the overall pattern of airglow depletions and associated ionospheric irregularities extends continuously across the magnetic equator to ±15° magnetic latitude.