Investigation of Nighttime MSTIDS Observed by Optical Thermosphere Imagers at Low Latitudes: Morphology, Propagation Direction, and Wind Filtering

Different types of medium-scale traveling ionospheric disturbances (MSTIDs) have been observed at Cachoeira Paulista (22.4 degrees S; 45.0 degrees W), Brazil, from June 2013 to December 2015, using airglow OI 630.0-nm images. During the period, 58 MSTIDs were identified and classified as follows: dark band MSTIDs (around 10 events) and periodic MSTIDs (48 events). Dark band MSTIDs present phase velocity between 50 and 200m/s and propagation direction to northwestward. On the other hand, periodic MSTIDs have phase velocity of 50 to 200 m/s, horizontal wavelengths from 80 to 160 km, periods ranging between 5 and 45 min, and propagation directions are mainly north-northeastward. The wave parameters indicate that periodic MSTIDs have different characteristics when compared to dark band MSTIDs, suggesting that periodic MSTIDs are not generated through the well-known Perkins and E-F coupling instability. In addition to it, the present study indicates that the spectral characteristics found in Brazil are different from other regions such as Japan and Indonesia. Therefore, we intend to do the statistics of the wave parameters (wavelength, phase velocity, period, propagation direction, and time occurrence) and investigate the generation mechanisms of periodic MSTIDs at low to middle latitude for the first time. Furthermore, the anisotropy observed in periodic MSTID propagation direction can be explained by different mechanisms.