RADAR EVIDENCE FOR A NEW LOW-FREQUENCY CROSSED-FIELD PLASMA INSTABILITY IN THE POLAR MESOPAUSE REGION - A CASE-STUDY

Dimant and Sudan [1995] have recently described a new plasma instabili ty that is excited in the lower ionosphere when a large electric field is applied. This mechanism is intriguing because it is the only low-f requency plasma process that is capable of generating field-aligned ir regularities (FAI) at altitudes below 90 km. Thus far, the only eviden ce for its validity has come from a single in situ measurement by rock et [Blix et al., 1996]. In this paper, we show that a southward-viewin g 12.3-MHz radar can be used to probe the D region for the presence of FAI. We show, from a case study, that the Doppler spectra of radar ba ckscatter from southerly azimuths, obtained under appropriate ionosphe ric conditions, did contain a spectral component that could be associa ted with this new instability. This low-Doppler-shifted spectral compo nent was accompanied by type-1 echoes that were produced by the well-k nown modified two-stream instability around an altitude of 95 km.