Geometry of depleted plasma regions in the equatorial ionosphere

Abstract

An image intensified, all‐sky, 6300‐Å airglow imaging system operating on Ascension Island is early 1981 has been used to assemble a photographic data base of several hundred airglow depletion patterns. By mapping the airglow depletion features along geomagnetic field lines to the equatorial plane, the geometrical characteristics of plasma depleted regions may be described over the 400‐ to 1200‐km altitude region above the equator. Such an analysis revealed that single or linear depletions tilted toward the west with an average skewness of 0.6° longitude west/100‐km altitude, corresponding to a zenith angle tilt of ∼40° at a height of 800 km. The tilts increased with local time during the evening hours, showing that an effective velocity shear of ∼0.05 m/s/km is required to account for the observed evolution. In several cases, the depletion geometry showed a ‘wishbone’ or bifurcated pattern. Such features had a mean height of bifurcation of 740 km and tilt factors very similar to those found for linear depletions observed during the same local time periods.

Airglow observations over the equatorial ionization anomaly zone in Taiwan

“…The westward tilt of depletion with respect to the magnetic meridian has been reported by investigators (Mendillo and Baumgardner, 1982;Mendillo and Tyler, 1983;Rohrbaugh et al, 1989;Abalde et al, 2001;Kelley et al, 2003). It is explained based on shear in zonal plasma drift over the equator due to the altitude variation of integrated conductiv- ities (Zalesak et al, 1982), or the latitude variation of zonal winds (Anderson and Mendillo, 1983).…”

“…One common characteristic of the depletions is bifurcation of their northern ends. Most of the depletions appear bifurcated from the time when they appear in the images, revealing Y-shaped or wishbone patterns (Mendillo and Tyler, 1983). The bifurcation is less structured in the images in 1997 and 1998, but often appears to be very complex in the other years, irrespective of magnetic activity.…”

“…The small bulge that appears around 20:54 LT, on the eastern wall of the depletion, which is near the western edge of the image, gradually evolves to a bifurcation (images in the second and third rows). Mendillo and Tyler (1983) had speculated that a secondary depletion developing in the steep gradients of the eastern wall of a westward tilted depletion could appear as bifurcation. The examples in Figs.…”

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

“…As plasma bubbles buoyantly rise upward, they become highly elongated in the vertical direction (Tsunoda and Towle, 1979), sometimes attaining very high altitudes (>1500 km) at the magnetic equator (Mendillo and Tyler, 1983;Sahai et al, 1994) and the depleted regions extend poleward in the flux tubes (Sales et al, 1996), reaching dip latitudes of over ±15 • (Rohrbaugh et al, 1989).…”

Solar Photodetachment Rates of Monatomic Anions

“…Through the advancement of rocketry, it is now possible to conduct ionospheric modification experiments (Anderson and Mendillo 1983;Mendillo and Tyler 1983;Wand and Mendillo 1984). The purpose of these experiments is to study a variety of ionospheric properties by artificially inducing large-scale long-lived negative ions and creating regions of plasma depletion in the ionosphere..…”