Abstract
Abstract. Previous studies of the aspect sensitivity of heater-enhanced incoherent radar backscatter in the high-latitude ionosphere have demonstrated the directional dependence of incoherent scatter signatures corresponding to artificially excited electrostatic waves, together with consistent field-aligned signatures that may be related to the presence of artificial field-aligned irregularities. These earlier high-latitude results have provided motivation for repeating the investigation in the different geophysical conditions that obtain in the polar cap ionosphere. The Space Plasma Exploration by Active Radar (SPEAR) facility is located within the polar cap and has provided observations of RF-enhanced ion and plasma line spectra recorded by the EISCAT Svalbard UHF incoherent scatter radar system (ESR), which is collocated with SPEAR. In this paper, we present observations of aspect sensitive E- and F-region SPEAR-induced ion and plasma line enhancements that indicate excitation of both the purely growing mode and the parametric decay instability, together with sporadic E-layer results that may indicate the presence of cavitons. We note consistent enhancements from field-aligned, vertical and also from 5° south of field-aligned. We attribute the prevalence of vertical scatter to the importance of the Spitze region, and of that from field-aligned to possible wave/irregularity coupling.
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 Correspondence to: R
We present observations of aspect sensitive E- and F-region Space Plasma Exploration by Active Radar (SPEAR)-induced ion and plasma line enhancements that indicate excitation of both the purely growing mode and the parametric decay instability, together with sporadic E-layer results that may indicate the presence of cavitons
1992; Kohl et al, 1993; Stubbe, 1996; Honary et al, 1999; Rietveld et al, 2000; Dhillon and Robinson, 2005). These wave modes give rise to enhancements in ion and plasma line incoherent scatter spectra and are thought to be caused by excitation of instabilities (Perkins and Kaw, 1971) that include the purely growing mode (PGM) (Fejer and Leer, 1972), called the oscillating two-stream instability or modulational instability (Rietveld et al, 2002), and the parametric decay instability (PDI) (Fejer, 1979). These two instabilities are thought to be driven by the effects of the ponderomotive force (e.g. Kohl et al, 1993), thereby resulting in RF-induced Langmuir and ion-acoustic waves (PDI), and Langmuir waves associated with a stationary electron density perturbation (PGM)
Summary
Stubbe et al, 1992; Kohl et al, 1993; Stubbe, 1996; Honary et al, 1999; Rietveld et al, 2000; Dhillon and Robinson, 2005). SPEAR-induced spectral enhancements are easier to identify in the plasma line data, as significant increases in the UPL and DPL amplitudes (at ±4.45 MHz from the radar frequency) will most probably be caused by SPEAR-excited instabilities. The SPEAR beam pointed field-aligned and this corresponds to the highest backscatter power, with appreciably lower powers for pointing directions furthest from FA, i.e. enhanced CUTLASS backscatter was seen only during interval 2, which, together with the F-region ion line enhancements and ionograms, is consistent with SPEAR interacting with the plasma in the F-region. The symmetric nature of the spectra presented here is in agreement with the high degree of symmetry seen in previous SPEAR-enhanced F-region ion line spectra (Robinson et al, 2006), which were obtained when no significant E-region was present It contrasts with the asymmetric F-region spectral data, reported by Dhillon et al (2007), which were obtained during the presence of a sporadic E-layer. F-region spectral signatures are present, there is little evidence for SPEAR-induced amplitude increases in these F-region spectra
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