Azimuthal Asymmetry of Quasi-Zonally Propagating Medium-Scale Traveling Ionospheric Disturbances Revealed by the Low Latitude Long-Range Ionospheric Radar

Using the Low-lAtitude long-Range Ionospheric raDar (LARID) at Hainan Island, we present a statistical survey of medium-scale traveling ionospheric disturbances (MSTIDs) at low latitudes in the East and Southeast Asia sector. MSTID signatures manifested as structured enhancements in the backscatter echo power maps, with dominant periods of 30–40 min and wavelengths of 150–250 km. The MSTIDs were observed to propagate quasi-zonally, with a higher occurrence rate in the quasi-eastward direction (74%) than in the quasi-westward direction (26%), a feature corroborated by nearby total electron content observations. The possible causes of the east-west asymmetry are discussed in terms of the geomagnetic activity, the atmospheric gravity wave (AGW) source and the neutral wind effects on the vertical propagation of AGW. We propose that the filtering effect of daytime westward thermospheric wind could contribute to the observed quasi-eastward propagation preference of MSTIDs. The propagation direction of quasi-zonally propagating MSTIDs in LARID and GNSS observations was slightly shifted poleward from the due zonal direction. The poleward shift could be contributed by the enhanced convection activities in the lower latitudes close to the equator, which could serve as an important source of AGWs.