Observation of the modified two‐stream plasma instability in the midlatitude E region ionosphere

We present the first clear evidence of the occurrence of the modified two stream (Farley‐Buneman) instability and excitation of pure type 1 irregularities in the midlatitude ionospheric E region. The observations are made with a bistatic 50‐MHz Doppler radio experiment set up recently in Crete, Greece. The system can perform high‐frequency resolution coherent backscatter measurements along a fixed direction, from 3‐m magnetic aspect sensitive irregularities inside a limited ionospheric volume in the E layer at the invariant geomagnetic latitude of 30.8° (L = 1.35). The observations presented here are from an event of backscatter characterized by large Doppler motions caused, presumably, by an impulsive electric field reaching a magnitude at least 14 mV/m. Apparently, the unusually high electron drifts along the radar viewing direction were sufficient in this case to excite pure Farley‐Buneman waves. This had been manifested convincingly by the measured power Doppler spectra, which are reminiscent of the typical spectral signature of type 1 echoes observed regularly in the equatorial 50‐MHz backscatter. Further, the spectral data confirmed the anticipation that in the midlatitude E region plasma exist two irregularity types corresponding to those of type 1 and type 2 echoes in the equatorial electrojet. The important difference with the equatorial results, however, is that the threshold conditions for the two‐stream instability are seldom met at moderate latitudes, thus the medium is highly suitable for studying secondarily generated short‐scale plasma turbulence.