Multi-event analysis of oscillatory motion of medium-scale traveling ionospheric disturbances observed by a 630-nm airglow imager over Tromsø

The motion of high-latitude medium-scale traveling ionospheric disturbances (MSTIDs) still remains an elusive aspect because of the limited results on their propagation characteristics. In particular, the MSTIDs, which are the dynamical structure in the ionosphere, have not been well investigated over high-latitudes using all-sky imagers because of the hindrance caused by the bright aurora. In this work, we present a comprehensive investigation on the propagation characteristics of dusk side MSTIDs using 630.0 nm airglow emissions over Tromsø (69.6°N, 19.2°E; magnetic latitude: 66.6°N). The unique points of our observation are (1) duskside MSTIDs primarily exhibit eastward motion under quiet conditions but turned in the westward direction associated with geomagnetic disturbances, (2) the westward-moving MSTIDs again turned in the eastward direction when the geomagnetic disturbance ceased, (3) the turning of MSTIDs in the westward direction was associated with the auroral brightening observed at the poleward edge of the all-sky images and local magnetic field perturbations observed by a ground-based magnetometer at Tromsø, and (4) the SuperDARN (Super Dual Auroral Radar Network) convection maps revealed that the location of Tromsø was inside (outside) the duskside convection cell during the time of appearance of westward (eastward) moving MSTIDs. The average eastward and westward velocities of MSTIDs were ~25-80 m/s and ~40-170 m/s, respectively. The Doppler shift measurement of the 630-nm airglow by a Fabry-Perot interferometer at Tromsø showed that northeastward winds were predominant during the appearance of eastward moving MSTIDs. These experimental evidences suggest that the MSTIDs tend to move eastward in accordance with the background F-region dynamo electric field, but show westward motion under the influence of convection electric field associated with auroral activities in the dusk side of two-cell convection pattern. The generation mechanism of the observed MSTIDs remains in question as they might either be triggered by atmospheric gravity waves or by ionospheric instabilities.