Airborne studies of equatorial F layer ionospheric irregularities

Abstract

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-/• 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-/• airglow depletions are the bottomside signatur e 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-A 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 (18øS magnetic latitude). The measurements at Ascension Island show that airglow depletions extend away from the magnetic equator into the southern 6300-A 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 o magnetic latitude.

Equatorial F-region plasma depletion drifts: latitudinal and seasonal variations

“…It should be mentioned that the OI 630 nm imaging observations provide a two-dimensional bottomside view (dark structures in Fig. 1) of the ionospheric bubbles that extend into the topside ionosphere and across the entire equatorial region (Weber et al, 1980). From the spatial displacements of the dark structures, during successive images, we can infer the irregularity zonal drift velocities, which are closely related to the ionospheric zonal plasma drift velocities.…”

Equatorial ionospheric disturbance observed through a transequatorial HF propagation experiment

“…Airglow depletion appears as a band with an eastwest dimension ranging from 50 to 200 km, and is elongated in the direction of the magnetic meridian over 3000 km or more (e.g. Weber et al, 1980;Mendillo and Baumgardner, 1982). A recent nonlinear simulation of R-T instability revealed a three-dimensional structure of a plasma depleted region, which is associated with bottomside uplifts along the magnetic meridian (Keskinen et al, 2003).…”

“…Weber et al (1982) related the topside density biteouts measured by the AE-E satellite to bottomside signatures through airglow and ionosonde measurements. Local uplifts of bottomside isodensity surfaces cause a 630-nm airglow depletion and oblique ionosonde echoes from local electron density gradients (Weber et al, 1980). Airglow depletion appears as a band with an eastwest dimension ranging from 50 to 200 km, and is elongated in the direction of the magnetic meridian over 3000 km or more (e.g.…”

Generation of large-scale equatorial F-region plasma depletions during lowrange spread-F season

“…Several investigators have used wideangle imaging of the F-region emissions to study the onset, evolution and dynamics of plasma bubbles (e.g. Weber et al, 1980;Mendillo and Baumgardner, 1982;Rohrbaugh et al, 1989;Sahai et al, 1994;Tinsley et al, 1997;Taylor et al, 1997;Mukherjee et al, 1998;Fagundes et al, 1999;Sinha et al, 2001;Abalde et al, 2001). …”