Chemical release experiments to induce F region ionospheric plasma irregularities at the magnetic equator

The largest-scale plasma instability that occurs naturally in the Earth's ionosphere is a turbulent upwelling of the equatorial F region known as equatorial spread-F (ESF). During an ESF event, high plasma density magnetic fluxtubes at the bottomside of the F region are thought to change places with lower plasma density flux-tubes from below in a Rayleigh-Taylor type (heavy fluid over light fluid) instability. This interchange creates a large-scale (10's of km) density perturbation locally, which rapidly penetrates through to the topside of the F region, creating a plume of cascading smaller-scale (meter to centimeter scale) irregularities from the sharp density gradients at the edges of the rising plasma "bubble". In a theoretical test of this overall scenario for ESF, a linear instability growth rate is derived following the magnetic fluxtube formalism of Haerendel (1973). Using realistic atmospheric and ionospheric density model inputs, growth rates are calculated for a range of geophysical conditions. Time/altitude domains having positive growth rates are found to coincide with observed time/altitude patterns of ESF occurrence, thus supporting the fluxtube model.; The physics also is tested experimentally by the deliberate creation of plasma bubbles in ambient ionospheres that the fluxtube model predicts are susceptible to the Rayleigh-Taylor instability. Two such artificial seed perturbations were generated during the 1990 NASA/Boston University CRRES-at-Kwajalein campaign, when clouds of sulfur hexafluoride (SF{dollar}sb6{dollar}) were released by sounding rockets to initiate plasma recombinations near the bottomside of the equatorial ionosphere. Multiple diagnostics (incoherent scatter radar, high frequency radar, optics and satellite polarimeters at several sites) were used to monitor the pre-launch status of the ionosphere and the electron depleted regions that resulted from the chemical releases. Small ESF plumes were observed to form in the region of the artificial perturbation during both experiments. The successes in being able to model the natural occurrence of ESF, as well as being able to initiate it artificially, lend support to the physical model adopted for the equatorial ionosphere, and open up new avenues of research into ESF predictability on a night-to-night, and even an hour-to-hour, basis.