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 instability that is excited in the lower ionosphere when a large electric field is applied. This mechanism is intriguing because it is the only low‐frequency plasma process that is capable of generating field‐aligned irregularities (FAI) at altitudes below 90 km. Thus far, the only evidence for its validity has come from a single in situ measurement by rocket [Blix et al., 1996]. In this paper, we show that a southward‐viewing 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 backscatter from southerly azimuths, obtained under appropriate ionospheric conditions, did contain a spectral component that could be associated with this new instability. This low‐Doppler‐shifted spectral component was accompanied by type‐1 echoes that were produced by the well‐known modified two‐stream instability around an altitude of 95 km.