Abstract
Abstract. The Space Plasma Exploration by Active Radar (SPEAR) facility has successfully operated in the high-power heater and low-power radar modes and has returned its first results. The high-power results include observations of SPEAR-induced ion and plasma line spectral enhancements recorded by the EISCAT Svalbard UHF incoherent scatter radar system (ESR), which is collocated with SPEAR. These SPEAR-enhanced spectra possess features that are consistent with excitation of both the purely growing mode and the parametric decay instability. In this paper, we present observations of upper and lower E-region SPEAR-induced ion and plasma line enhancements, together with F-region spectral enhancements, which indicate excitation of both instabilities and which are consistent with previous theoretical treatments of instability excitation in sporadic E-layers. In agreement with previous observations, spectra from the lower E-region have the single-peaked form characteristic of collisional plasma. Our observations of the SPEAR-enhanced E-region spectra suggest the presence of variable drifting regions of patchy overdense plasma, which is a finding also consistent with previous results.
Highlights
Among the most important phenomena associated with overdense RF heating are the stimulation of non-propagating plasma density irregularities at the upper-hybrid height and the excitation of Langmuir and ion-acoustic waves at the Omode reflection height (e.g. Robinson, 1989; Rietveld et al, 1993; Kohl et al, 1993; Mishin et al, 2004)
The incoherent scatter radar data presented below were recorded using the EISCAT Svalbard Radar (ESR) (Wannberg et al, 1997), which is collocated with Space Plasma Exploration by Active Radar (SPEAR)
We have provided ESR observations of the temporal evolution of E- and F-region ion and plasma line spectral enhancements caused by the interaction of the SPEAR high-power pump wave with sporadic E-layers
Summary
Among the most important phenomena associated with overdense RF heating are the stimulation of non-propagating plasma density irregularities at the upper-hybrid height and the excitation of Langmuir and ion-acoustic waves at the Omode reflection height (e.g. Robinson, 1989; Rietveld et al, 1993; Kohl et al, 1993; Mishin et al, 2004). The single central peak in the spectrum is indicative of collisional plasma and there is a clear increase in its amplitude during the SPEAR-on period This is consistent with previous observations of HF-enhanced E-region ion line spectra The times of the F-region spectra, shown above the panels, correspond to the times of the E-region spectra These F-region spectra have a considerable degree of variability, with the forms of the SPEAR-enhanced spectra (which are present at altitudes between 160 and 220 km) changing throughout the SPEAR-on interval. The spectra are all generally asymmetric, with the number and positions of identifiable peaks changing over time Some of these peaks occur in the central part of the ion line spectrum and at the ion-acoustic frequencies and such SPEAR-induced spectral enhancements have been identified with the actions of the purely growing mode and the parametric decay instability, respectively (R2006). An interesting feature is that the most significant F-region spectral enhancements occur concurrently with the highest increases in the E-region spectral amplitude
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