Coincident radar and rocket observations of equatorial spread‐F

Abstract

Coordinated measurements of equatorial spread‐F conducted during July 1979 at the Kwajalein Atoll have yielded the first definitive space‐ and time coincident radar and rocket observations of small scale irregularities and large scale plasma depletions. The results have shown that: (a) During conditions of well‐developed equatorial spread‐F the most intense "in situ" irregularities occurred on the bottomside F‐layer gradient. (b) Within a large scale topside F‐layer depletion radar backscatter and "in situ" irregularity strengths maximized near the depletion’s upper wall. (c) Ion composition within a topside depletion provided signatures of its bottomside source domain and estimates of average maximum vertical drift velocity. For long‐lived depletions, it was found that molecular‐ion signatures (NO+ and O2+) can be lost while bottomside levels of N+ can be maintained when [O+] ≈ Ne ≫ [NO+] + [O2+]; and finally, (d) Large scale fluctuations of O+ accompanied by a near‐constant level of NO+ and O2+on the bottomside F‐layer gradient suggests that neutral atmospheric turbulence is not a major source for bottomside ionospheric plasma irregularities and the associated triggering of equatorial spread‐F.

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

“…The general collocation of plumes and bubbles, shown by results presented here and elsewhere Tsunoda, 1980a, b;Towle, 1980;Szuszczewicz et al, 1980], implies that backscatterproducing irregularities are primarily generated in regions of low plasma density. This preference for low plasma densities is consistent with the production of these irregularities by the lower-hybrid-drift (LHD) instability [Huba et al, 1978;S §Terlinjnd Goldman, 1980;Huba and Ossakow, 1981].…”

“…Bottomside backscatter, which occurs on both sides of the plume pair, tilt; upward in the direction of the plumes. Because bottomside backscatter occurs along the bottomside -radient of the F layer [e.g., Szuszczewicz et al, 1980;Tsunoda, 1930b], the shape of the bottomside backscatter implies that the plume pair is associated with a large-scale upwelling that has an east-west dimension of approximately 400 km, or more. An association of plumes and upwellings has heen suggested by Tsunoda [19811 and verified by Tsunoda and White [1981].…”

“…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].…”

Some new features of electron density irregularities over SHAR during strong spread F

“…Woodman and LaHoz (1976) ®rst identi®ed valley region irregularities below the main plumes in the RTI radar maps obtained from Jicamarca. Vickrey et al (1984) studied electron density irregularities observed in the F region valley, which were obtained from ®xed bias LP¯own on two rockets launched from Natal, Brazil (Kelley et al, 1976(Kelley et al, , 1979Costa and Kelley, 1978) and from Kwajalein Atoll (Szuszczewicz et al, 1980Rino et al, 1981;Kelley et al, 1982b). During these twō ights quasi-sinusoidal¯uctuations in the F region valley (170 to 200 km) were observed where the local zero order vertical plasma density gradient nearly vanishes.…”