Coordinated observations of nighttime medium‐scale traveling ionospheric disturbances in 630‐nm airglow and HF radar echoes at midlatitudes

We investigate nighttime medium‐scale traveling ionospheric disturbances (MSTIDs) using the SuperDARN HF radar at Hokkaido, Japan (43.5°N, 143.6°E), and an OI 630‐nm airglow imager located at Paratunka, Russia (53.0°N, 158.2°E), within the radar field of view. The imager identified southwestward propagating MSTIDs with a horizontal wavelength of ∼300 km on 8 December 2007. Throughout this event, the radar continuously detected ionospheric echoes originating from decameter‐scale field‐aligned irregularities (FAIs) at the F region heights. The Doppler velocities of these echoes showed systematic polarity changes which were consistent with airglow intensity variations. These polarity changes would be attributed to E × B plasma drifts caused by the polarization electric field embedded in the MSTIDs. The FAI echo powers also varied in agreement with the airglow intensity variations: strong (weak) echoes coincided with the airglow depletion (enhancement) region. Considering the MSTID polarization electric field, it is suggested that the observed FAIs were generated by the gradient drift instability on the bottomside of the F region.