Naturally enhanced ion acoustic fluctuations seen at different wavelengths

Abstract

Abstract. We present European Incoherent Scatter (EISCAT) Svalbard Radar observations of enhanced incoherent scatter ion-acoustic spectra with transmitted frequencies slightly shifted relative to the radar central frequency. This study confirms the initial findings that (1) the nat•xrally enhanced ion acoustic lines sometimes show asymmetric power profiles and (2) the amplitude enhancement of upgoing and downgoing ion-acoustic waves occurs at somewhat different altitudes and different power levels. These observational facts are modeled using the parametric decay of beam driven Langmuir wave in an inhomogeneous medium. It is shown that the density gradient with altitude as well as the collisions contribute to an altitude variation of ion-acoustic spectra and may account for the observed signatures. It is also responsible for the transition from upshifted to downshifted enhanced ion lines. Our model shows that the power radiated by enhanced spectra varies with the wavelength and that the power difference between two wavelengths varies with altitude.

Enhanced ion acoustic lines due to strong ion cyclotron wave fields

“…The radar enhancements occur during intense red aurora and soft electron precipitation; the enhancements are usually 2-3 orders of magnitude larger than the quiescent values (such large enhancements cannot be associated with field-aligned irregularities because the relevant ISR observations were made with the radar beam directed far away from perpendicularity to the magnetic field); the enhancements occur at a range of altitudes above 250 km, mostly occurring near 500 km, and can reach 1500 km; radar interferometric observations show that their transverse extent is several hundred meters; they last less than a few tenths of a second; the upshifted and downshifted spectral lines are not equally enhanced; the enhancements can more frequently be seen at 224 MHz as opposed to at 930 MHz and 1290 MHz (Collis et al, 1991;Wahlund et al, 1993;Rietveld et al, 1996;Cabrit et al, 1996;Forme et al, 2001;Grydeland et al, 2004). For an extensive review of NEALS, including observations and theories, see Sedgemore-Schulthess and St.-Maurice (2001).…”

“…Moreover, CDEIA instability cannot explain simultaneous enhancements of both ion lines -a turbulent excitation process may explain the simultaneous enhancements. Forme et al (2001); Kontar and Pécseli (2005) suggested that simultaneous enhancements of both ion lines can occur through parametric excitation of IA waves by coupling to beamdriven Langmuir waves. The simultaneous radar observation of enhanced Langmuir waves and IA waves by Stromme et al (2005) supports the parametric excitation mechanism, however, it is not a proof because the enhancements could be occurring independently.…”

Nonlinear wave interactions as a model for naturally enhanced ion acoustic lines in the ionosphere

“…The shift of Langmuir waves in k ‐space is sufficient for producing substantial changes in local Langmuir turbulence spectra, see Figures 2 and 3. These results demonstrate that an interpretation of NEIALs in terms of Langmuir decay [ Forme , 1999; Forme et al , 2001] is most convincing in regions with vanishing or negative density gradients, corresponding to altitudes around or below the F maximum.…”

“…Several models can account for this feature: the symmetry of the natural ion‐line is broken, for instance, if a current is flowing through the plasma [ Sedgemore‐Schulthess and St.‐Maurice , 2001], or if an electron beam enhances electron plasma waves (Langmuir waves) significantly above the thermal level, so that ion acoustic waves are excited by parametric decay [ Tsytovich , 1995]. Such models were invoked by [ Forme , 1999] and [ Forme et al , 2001]. Recently, observations of simultaneously enhanced levels of ion and electron plasma waves have been reported [ Strømme et al , 2005], indicating such interactions.…”

Nonlinear beam generated plasma waves as a source for enhanced plasma and ion acoustic lines

“…8,9 An electron beam can enhance electron plasma waves (Langmuir waves) significantly above the thermal level, and then ion acoustic waves can be excited by parametric decay of these waves. This latter model was invoked in other studies 10,11 and has gained confidence by several works. 1,4,12 Consistent with the basic features of these proposed models, observations of simultaneously enhanced levels of ion, and electron plasma waves have been reported.…”