VHF and UHF radar observations of equatorial F region ionospheric irregularities and background densities

Abstract

A series of measurements of the properties of equatorial ionospheric irregularities were made at Kwajalein, Marshall Islands (M.I.) in August 1977 and July–August 1978. These measurements, sponsored by the Defense Nuclear Agency (DNA), involved coordinated ground‐based and in situ sensors. The ARPA Long‐Range Tracking and Instrumentation Radar (ALTAIR), operated by Lincoln Laboratory, obtained backscatter and transmission data during five nights in August 1977 and eight nights in July–August 1978. This report describes the ALTAIR data from the night of August 11, 1978, which yield direct quantitative measurements of 1‐m and ⅜‐m irregularities and of plasma depleted regions. These plasma depleted regions, previously predicted on the basis of theoretical analysis and in situ data, were observed during the decay phase and not the generative phase of the field‐aligned irregularities.

Magnetic-field-aligned characteristics of plasma bubbles in the nighttime equatorial ionosphere

“…As might be expected from previous results [Towle, 1979;, the enhanced backscatter regions are related to features in the electron density profiles. Backscatter from bottomside FAI is clearly associated with the base of the F layer.…”

“…These bubbles then buoyantly rise upward into the topside of the F layer. This theoretical model is strongly supported by qualitative features that have been observed in the nighttime equrtorial ionosphere by in-situ measurements [Hanson and Sanatani, 1973;Kelley et al, 1976;McClure et al, 1977;Morse et al, 1977] and by incoherent-scatter (IS) radar [Towle, 1979;Tsunoda, 19791. These measurements have shown that the observed bubbles have characteristics that imply their origin at lower altitudes and subsequent upwarl movement.…”

Backscatter measurements of 11‐cm equatorial spread‐F irregularities

“…That is, the requirement for low electron densities that the theory calls for may preclude the existence of 11-cm FAI elsewhere in the F-region ionosphere, except in equatorial plasma bubbles. Tsunoda et al, 1979;Towle, 1980;Tsunoda, 1980c]. TRADEX was employed for the first time in this kind of experiment to extend radar observations of equatorial FAI to 1320 MHz, the highest frequency ever used.…”

Equatorial plasma bubbles: Vertically elongated wedges from the bottomside F layer

“…Plasma bubbles were first discovered by in-situ satellite measurements [Hanson and Sanatani, 1973;McClure et al, 1977], and have since been detected by rocket [Kelley et al, 1976;Morse et al, 1977;Szuszczewicz et al, 1980] and by incoherent-scatter radar [Towle, 1980;Tsunoda, 1980a,b]. The plasma density within bubbles was found to be as much as three orders of magnitude less than the peak plasma density in the F layer (i.e., the depletion can approach 99.9 percent).…”

“…Moreover, plumes have been found to be generally collocated with plasma-depleted regions Towle, 1980;Tsunoda, 1980a,b;Szuszczewicz et al, 1980].…”