On the role of steepened structures and drift waves in equatorial spread F

Abstract

A time domain analysis, performed upon electron density data obtained on the bottomside of the F peak during equatorial spread F conditions, indicates that the observed k -2 wave number spectrum is due to sharp gradients in the medium and not to plasma turbulence. The gradients are then shown to be unstable to drift waves with a peak growth rate of about 1 s -• which occurs at perpendicular wavelengths near the ion gyroradius. These waves may be responsible for some of the intense backscatter detected by VHF radar during equatorial spread F conditions. The linear theory for collisionless drift waves with wavelengths near the ion gyroradius is discussed in a companion paper. INTRODUCTION Progress toward a better understanding of equatorial spread F has increased greatly in the past several years as a result of both experimental and theoretical research. The purpose of this paper is to discuss a unified model of equatorial spread F which organizes much of the observational data and theoretical results in a self-consistent way. Some of the ideas in this model have evolved from work by other investigators which is not readily available in the literature. G. Haerendel (unpublished manuscript, 1974), for example, in a famous preprint, discussed equatorial spread F in terms of a 'hierarchy of instabilities' in which the Rayleigh-Taylor mode was primary and resulted in the growth of shorter-wavelength modes. Hudson et al. [1973] also discussed a 'two-step' theory for equatorial spread F in which drift waves accounted for the full spectrum of irregularities. In concept, both of these developments are similar to the ideas presented here. Recent work by Woodman and Basu [1978], as well as numerous discussions with R. Woodman, has influenced this investigation. In brief, we suggest that an initial bottomside instability nonlinearly evolves to the point that very steep gradients develop and that these steepened structures are responsible for the k -• power spectra observed by probe experiments. This primary process is very likely the Rayleigh-Taylor instability •'Dungey, 1956; G. Haerendel, unpublished manuscript, 1974; Hudson and Kennel, 1975a, b], perhaps modified or enhanced by the E x B instability [Martyn, 1959; Kelley et al., 1978]. Rocket [Kelley et al., 1976], radar [Woodman and La Hoz, 1976], and satellite [McClure et al., 1977] observations indicate that some of these bottomside structures rise into the topside.We show that the observed gradients on the edges of these rising bubbles should be unstable to drift waves with a linear growth rate of about 1 s -• and a perpendicular wavelength, for maximum growth, near the ion gyroradius. We suggest that these waves are responsible for some of the intense backscatter which occurs at VHF radar frequencies.These ideas are developed in the next section and are followed with a summary and an expanded comparison of the model with theoretical and experimental work. EVIDENCE FOR AND CONSEQUENCES OF STEEPENED STRUCTURES IN EQUATORIAL SPREAD FObserved density and electric field sp...

Late time barium cloud striations and their possible relationship to equatorial spread F

“…(To convert from seconds to kilometers, multiply by 2.) The rocket probes detected intense irregularities on the bottomside of the equatorial F region [Costa and Kelley, 1978] which had structure very similar in appearance to the barium cloud data. In addition, both sets of' data have nearly identical power spectra not only in spectral form but absolute value.…”

On 11‐cm irregularities during equatorial spread F

“…During the rocket flight, which occurred subsequent to the 11 cm radar backscatter measurements, in situ probes detected sharp density gradients (L n P 45m; Kelley, private communication, 1980) and large density depletions (n e ~ several x 10 4 cm-3 ; E. Szuszczewicz, private communication, 1980) at high altitude (-500 km). We note that previous in situ rocket measurements have found density gradient scale lengths as small as 30m (Costa and Kelley, 1978a).…”

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.…”

“…Similar k )2 power law scaling was observed by Dyson et al (1974) using AE satellite data, by Morse et al (1977) in the Equion experiment and also by Kelley and Mozer (1972) using the electric ®eld data. Costa and Kelley (1978) found that sharp density gradients (gradient scale length, L < 100 m) could de-stabilize collisional drift waves. Rino et al (1981) discussed the ®xed-bias Langmuir probe derived electron density power spectra, obtained from Plumex-I rocket, launched from Kwajalein Atoll (9.4°N, 167.45°E).…”

“…This partly explained the high altitude spread F, observed by Farley et al (1970), in terms of primary instability operating below the peak in F region electron density (F peak ). Costa and Kelley (1978) analyzed the bottom side electron density data, obtained from the same¯ight, and found that the steepened coherent structures exhibited shallow spectra, with k )2 power law behavior in intermediate (10 km > k > 100 m) and transitional (100 m > k > 10 m) scales instead of a turbulent cascade of structures. Similar k )2 power law scaling was observed by Dyson et al (1974) using AE satellite data, by Morse et al (1977) in the Equion experiment and also by Kelley and Mozer (1972) using the electric ®eld data.…”