Traveling Ionospheric Disturbance Events with Special Propagation Direction Based on Double-layer Airglow Observation

The all-sky airglow imager at Xinglong Station (40.4°N, 117.6°E, and 30.5°MLAT), has captured 611 Medium-scale Traveling Ionospheric Disturbance (MSTID) events from 2011 to 2021 in the 630nm wavelength band. Among all events, a notable majority of 589 demonstrate typical southwestward propagation characteristics, while the remaining 22 events propagate in other directions. To delve deeper into the understanding of these atypical MSTID events, a meticulous examination was undertaken utilizing ground-based coordinated observations of OI 630 nm and OH near-infrared airglow images. From each non southwest direction—northeast, northwest, and southeast—one representative event was selected for detailed discussion. The analysis includes comparison of various parameters associated with MSTID and Atmospheric Gravity Waves (AGW), ray tracing algorithm, and wind field data. Based on the analysis and previous references, the possible sources of the MSTIDs were concluded as follow. MSTID events exhibiting congruent propagation with AGW may be attributed to the Perkins instability induced by upstream AGW propagation. This explanation is plausible for events propagating towards the northeast and some northwestward events, since AGWs propagating to the directions similar with the simultaneous above MSTIDs were spotted in these events. The fluctuation from broken AGWs is filtered by the eastward neutral winds during twilight hours. Normally, only the westward components pass through and continue propagating upwards, which leads to the westward MSTID event discussed in this paper. Comprised with the northwestward event, the relative intensity of the westward event is small, also indicating the westward event may be triggered by the broken AGWs. Solitary wave MSTID events, characterized by their absence of periodic structural features, are posited to originate from the coupling between the E and F layers of the ionosphere, rather than being directly influenced by AGW phenomena. This comprehensive analysis not only advances our understanding of the generation of MSTIDs but also underscores the intricate interplay between various atmospheric processes in shaping ionospheric disturbances.