Spatial relationship of nighttime medium‐scale traveling ionospheric disturbances and F region field‐aligned irregularities observed with two spaced all‐sky airglow imagers and the middle and upper atmosphere radar

We report simultaneous observations of medium‐scale traveling ionospheric disturbances (MSTIDs) and field‐aligned irregularities (FAIs) in the F region using two all‐sky airglow imagers and the middle and upper atmosphere (MU) radar. MSTIDs propagating southwestward were observed simultaneously in 630‐nm airglow images over Sakata (39.0°N, 139.9°E) and Shigaraki (34.9°N, 136.1°E), Japan, on the night of 16 June 2004. By using all‐sky images over both sites, we estimated the altitude of the airglow layer to be 260 km by the triangulation method. During the MSTID event, FAIs in the F region were observed by making multibeam measurements with the MU radar at Shigaraki. In order to investigate the spatial relationship between the MSTIDs and FAIs, the FAIs were mapped onto the 630‐nm airglow layer (altitude, 260 km) along the geomagnetic field lines. We found that FAIs with an intense (weak) signal‐to‐noise ratio coincided with the airglow depletion (enhancement) caused by the MSTIDs. FAI velocity obtained from a combination of the Doppler velocities on the three radar beams oscillated in the northwest‐southeast direction, with an amplitude of approximately 82 m/s. The FAI velocity was northwestward (southeastward) at the airglow depletion (enhancement). The directions of the FAI velocity were consistent with those of the E × B drifts caused by the polarized electric fields associated with the MSTIDs. The northeastward polarized electric field at the airglow depletion region strengthened the background eastward effective electric field and drove the gradient drift instability generating FAIs. This might be the reason why the FAIs preferred to occur at the airglow depletion region.