The generation of kilometer scale irregularities in equatorial spread F

Abstract

Two rockets launched into spread F during March 1983 show that the intermediate-scale spectrum of density irregularities sometimes exhibits a knee near 1 km scale length. This result confirms previous reports which are reviewed. Two different nonlinear theories of spread F turbulence are studied here and shown to be consistent with the experimental spectra when the knee is observed. Also, the existence or nonexistence of the knee may be effected by two linear mechanisms: the injection of kilometer scale turbulence at the steepened walls of spread F bubbles and the nonlocal role of the compressible E region in the development of spread F. This spectral feature, which seems to be associated with the most turbulent state of equatorial spread F, has not yet been identified in computer simulations. centrates on the characteristics of the medium and intermediate wavelength ranges: wavelengths between 100 m and 10 km. Companion papers treat some of the global aspects [Argo and Kelley, this issue], the long wavelengths [Kelley et al., this issue] (hereinafter referred to as paper 1), and the transitional and short-wavelength regime [LaBelle et al., this issue]. Also among the companion papers is a study of the scintillations associated with the medium-and intermediate-scale irregularities, including a discussion concerning the relation between the scintillation observations and the rocket observations [Basu et al., this issue].Equatorial spread F irregularities are highly field aligned, and therefore the spread F turbulence is effectively twodimensional. At most wavelengths the spectrum is probably isotropic in that plane, but at some scale length the spectrum must become anisotropic, reflecting the fact that the ionosphere is much more narrowly bounded vertically than horizontally. Therefore one must take extra caution when comparing spectra o. btained by different experimental techniques, especially at scale sizes approaching the dimensions of the F region.Observations of the spectrum of spread F irregularities in the range from 100 m to 10 km basically fall into two categories. First, satellite-borne probes and ground-based observations of phase and amplitude scintillations allow measurement of the horizontal wave number spectrum. Very frequently, the intermediate-scale spectrum observed by these techniques takes the form of a power law; in other words, the power spectral density is proportional to some power of the wave number: lk oc k n. The quantity n is called the spectral index. Satellite experiments consistently observe a power law spectrum of density irregularities, with the spectral index varying from -1 to -3 [Dyson et al., 1974; Valladares et al., 1983; Livingston et al., 1981; Kelley and McClure, 1981]. There is also evidence from the satellite data that in this range of wave numbers the spectrum of electric field irregularities is also characterized by a spectral index of roughly -2 [Kelley and Mozer, 1972]. Scintillation observations imply a spectral index consistent with the in situ results [Basu et al.,...

Electric field observations of equatorial bubbles

“…The physical origin of the loss of updrafting required for the death of a type I bubble requires speculation, which can only be inferred from the one-dimensional satellite trajectories through these two-dimensional structures. This loss of updrafting on pinching off can be seen in Ossakow and Chaturvedi' s [ 1978] Kelley, 1986] have speculated on east wall-west wall asymmetries in ERMS and Edc. A perusal of the 12 events in Figure 9 shows no large asymmetry in the opposing walls as might be expected if the collisional drift instability was a major force in the dynamics of these structures.…”

Nonlinear Rayleigh‐Taylor instabilities, atmospheric gravity waves and equatorial spread F

“…The presence of F region irregularities can produce image structure in the E region or F1 region [Vickrey et al, 1984;Heelis and Vickrey, 1990]. The effects of the E region on the F region plasma dynamics have been studied by many authors [Francis and Perkins, 1975;Vickrey and Kelley, 1982;Zalesak et al, 1982;LaBelle and Kelley, 1986]. We use a model similar to those used by Vickrey and Kelley [1982] and Zalesak et al [1982].…”

Remote sensing of possible plasma density bubbles in the inner Jovian dayside magnetosphere

“…Large‐scale ionosphere density bubbles develop due to the Raleigh‐Taylor fluid instability, ranging from hundreds of kilometers in height and tens of kilometers in width. However, observation [ LaBelle and Kelley , 1986] and simulation [ Zalesak et al , 1982] indicates that secondary small‐scale modes about the bubble perimeter are also excited, these with scale sizes from 1 km to a few meters. The small‐scale modes are the actual reflection sites for the HF radar signals.…”

“…In essence, where the boundary becomes too sharp, natural processes occur to smooth it out. Figure 5 (adapted from LaBelle and Kelley [1986]) shows a spread‐F bubble observed during the upleg portion of a rocket flight. The intermediate scale turbulence at the bubble top is associated with a topside boundary instability, giving rise to secondary fluctuations superimposed on the larger fluid instability.…”

“…Ionospheric density from the Condor rocket flight showing a 30 km spread‐F plasma bubble and associated intermediate‐scale structure on the high‐altitude bubble top (Region I activity). Figure adapted from Labelle and Kelley [1986].…”