THE ALTITUDE OF TYPE-3 AURORAL IRREGULARITIES - RADAR INTERFEROMETER OBSERVATIONS AND IMPLICATIONS

VHF coherent scatter radars at auroral latitudes have observed scatterers with narrow power spectra at sub-ion acoustic mean Doppler shifts. These spectra have been designated "type 3." The mean Doppler shift of these waves is often near the atomic (O+) or, less frequently, the molecular (O2+ and NO+) gyro frequencies. These type 3 echoes have been attributed to an electrostatic ion-cyclotron (EIC) instability in the upper E region (h > 140 km), where the ion collision frequency becomes low enough to permit ion gyromotion. Interferometric analysis of recent coherent radar observations with the CUPRI (Cornell University portable radar interferometer) shows that type 3 events occur at typical electrojet altitudes (100-120 km), however. At these altitudes the ion collision frequency is greater than the ion gyrofrequency, and there can be no cyclotron motion. The cause of the observed type 3 echoes hence remains a mystery.