The Condor Equatorial Spread F Campaign: Overview and results of the large‐scale measurements

Abstract

During the Condor campaign a number of instruments were set up in Peru to support the rocket experiments. In this series of papers we report on the results of the experiments designed to study the equatorial F region. In this overview paper we summarize the main results as well as report upon the macroscopic developments of spread F as evidenced by data from backscatter rada•s, from scintillation observations, and from digital ionosonde meaurements. In this latter regard, we argue here that at least two factors other than the classical gravitational Rayleigh-Taylor plasma instability process must operate to yield the longest-scale horizontal organization of spread F structures. The horizontal scale typical of plume separation distances can be explained by invoking the effect of a shear in the plasma flow, although detailed comparison with theory seems to require shear frequencies a bit higher than observations indicate. On the other hand, the largest-scale organization or modulation of the scattering layer cannot be explained by the shear theory and must be due to local time variations in the ionospheric drift or to gravity wave induced vertical motions. Using simultaneous rocket and radar data, we were also able to confirm the oft quoted hypothesis that rapid overhead height variations in the scattering region over

Presunrise spread F at Fortaleza

“…(This is the mechavJsm that we are proposing, so we discuss it further in the following paragraphs.) This instability on the sides of the bulges is essentially the same as the instability on a tilted ionosphere during postsunset spread F development that has been discussed in several papers [Kelley et al, 1981[Kelley et al, , 1986Kelley, 1985].…”

Study of spatial and temporal characteristics of L-band scintillations over the Indian low-latitude region and their possible effects on GPS navigation

“…With the decreased ionization density in the E-region after sunset, vertical plasma density gradients form in the bottom side of the F-layer, resulting in the upward density gradients opposite in direction to the gravitational force. This configuration is RayleighTaylor (RT) unstable and allows plasma density irregularities to generate (Kelley et al, 1981(Kelley et al, , 1986Huang and Kelley 1996;Hysell, 2000). The eastward post-sunset electric fields enhance the R-T instability, while westward fields quench it.…”

On the combined effects of vertical shear and zonal electric field patterns on nonlinear equatorial spread F evolution

“…Other plume events observed during the period of westward electric field are generally thought to originate at a location west of Jicamarca and then move into the radar's field of view due to the background eastward plasma drift (the VHF radar acts as a slit camera). Further, on a suificient number of occasions, the plume events are found to be absent [Kelley et al, 1986] (a) Simulation output at 1500 s using zonal plasma drift (as in Figure 3a) and vertical plasma drift (as in case 2 of Figure 3b). (b) Simulation output at 1500 s using zonal plasma drift (as in Figure 3a) and vertical plasma drift (as in case 3 of Figure 3b).…”

“…In this paper we ask a slightly different question. Given that the bottomside echoing region over Jicamarca is often modulated by hundreds-of-kilometer-long features without erupting into plumes [Kelley et al, 1986] and that shear alone cannot suppress the long wavelength structures, what prevents the plume formation on some nights?…”