An observational evidence of medium‐scale traveling ionospheric disturbances (MSTIDs) reaching to magnetic latitude as low as ~3.5° over the Indian sector is provided for the first time based on OI 630‐nm airglow imaging observation from a low‐latitude station, Gadanki (13.5°N, 79.2°E; 6.6° magnetic latitude), on 12 January 2016. The horizontal wavelength, horizontal phase velocity, and period of the MSTID are found to be 160 ± 6 km, 138 ± 14 m/s, and 19.5 ± 3 min, respectively. These phase fronts are observed to move toward southwest with a propagation angle of ~235° ± 1° with respect to north. In addition to the MSTID, a strong quasiperiodic southward moving wave (QPSMW) from the evening to midnight interval and a small‐scale southward moving wave structure with wavelength and periodicity different from the QPSMW are also detected on the same night. Horizontal wavelength, horizontal phase velocity, and period of the QPSMW are estimated to be 367 ± 14 km, 131 ± 18 m/s, and 46.7 ± 13 min, respectively, and those of the small‐scale southward moving wave are found to be 157 ± 4 km, 121 ± 17.8 m/s, and 21.7 ± 3.4 min, respectively. Global Positioning System‐total electron content maps suggest that the weak and asymmetric equatorial ionization anomaly helped deep ingression of the MSTID on this night. The descent of the F layer seems to have caused the dissipation of the MSTID and QPSMW closer to the dip equator on this night. Therefore, the present investigation shows that the midlatitude MSTIDs can influence the F region plasma processes even over very low latitudes under favorable background conditions.

We analyze an equatorial plasma bubble (EPB) event observed in optical 630 nm image data simultaneously from Gadanki (13.5°N, 79.2°E), Kolhapur (16.8°N, 74.2°E), India. The total electron content data from Gadanki together with the ionosonde data from an equatorial Indian station, Tirunelveli (8.7°N, 77.8°E) confirmed the association of observed EPB event with equatorial spread F (ESF). The optical 630 nm images from a farther low‐latitude Indian station Ranchi (23.3°N, 85.3°E) show clear signatures of tilted east‐west wave structures propagating toward equator. Further, the upward wave energy noted in mesospheric airglow data was found to be negligible. These data suggest that possibly the off‐equatorial tilted east‐west structures triggered the observed EPB/ESF event.

Well-de ned coherent wave sources associated with the passage of short-period gravity waves were observed in all-sky images of OH emission on a total 21 occasions during January to May 2008 at Allahabad (25.45 • N, 81.85• E, dip lat ∼16.49• N) in India. The wave motions exhibited similar spatial and temporal properties during each month, but the north-east ward motions were distinctly dominant in April and May 2008. It is a wellknown theory that the upward propagating gravity waves may be blocked or absorbed at a critical layer. We have computed the magnitude and direction of atmospheric gravity waves subject to blocking by horizontal winds, i.e., critical layer directional ltering. The HWM-93 model (Hedin et al., 1996) was used to compute the two components of neutral wind velocity at Allahabad for the period of observation of gravity waves during March and April 2008. Data from two components of wind velocity were then used to construct the blocking diagrams, which show the directions and apparent phase velocities of wave propagation blocked at a given altitude. The blocking diagrams were then compared with experimental observations of gravity waves in OH airglow to determine the accuracy of the wind model and explain the critical layer theory.

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Abstract. Coordinated measurements of airglow features from the mesosphere-lower thermosphere (MLT) region were performed at Allahabad (25.5 • N, 81.9 • E) and Gadanki (13.5 • N, 79.2 • E), India to study the propagation of gravity waves in 13-27 • N latitude range during the period June 2009 to May 2010 under CAWSES (Climate And Weather of Sun Earth System) India Phase II Programme. At Allahabad, imaging observations of OH broadband emissions and OI 557.7 nm emission were made using an all-sky imager, while at Gadanki photometric measurements of OH (6, 2) Meinel band and O 2 (0, 1) Atmospheric band emissions were carried out. On many occasions, the nightly observations reveal the presence of similar waves at both locations. Typically, the period of observed similar waves lay in the 2.2-4.5 h range, had large phase speeds (∼ 77-331 m s −1 ) and large wavelengths (∼ 1194-2746 km). The images of outgoing long-wave radiation activity of the National Oceanic and Atmospheric Administration (NOAA) and the high-resolution infrared images of KALPANA-1 satellite suggest that such waves possibly originated from some nearby convective sources. An analysis of their propagation characteristics in conjunction with SABER/TIMED temperature profiles and Horizontal Wind Model (HWM 2007) wind estimates suggest that the waves propagated over long distances (∼ 1200-2000 km) in atmospheric ducts.