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The optical observations of ionospheric and mesospheric OI 630.0 nm, OI 557.7 nm along with OH emission carried out from low latitude Indian station, Kolhapur (16.8°N, 74.2°E) using CCD based all-sky camera system. The features of night airglow variations observed during the period of a strong geomagnetic storm, which commenced on March 17, 2015, at ∼04:30 UT (10:00 IST (Indian Standard Time = UT + 5.5 h)). Dst of ∼−222 nT was seen in this storm suggest that this is among the strongest. The OI 630.0 nm images on 16, 17 and 18 March show the development of Equatorial Plasma Bubbles (EPBs) and bright intensity regions in OI 630.0 nm emission. Generally, EPBs move from west to east direction but it moved in reverse direction on the strong magnetically disturbed night. The EPBs drift velocity was less by ∼100 m/s than the velocity measured on magnetically quite night 16–17 March 2015. The bright intensity regions are also observed in OI 557.7 nm airglow, but there is no intensity enhancement seen in OH emission. It is also observed that the OI 630.0 nm intensity variation well matches with the GPS VTEC variation for PRN-2. The reversal in EPBs drift velocity and the nightglow intensity variations due to the strong magnetic storm are discussed.

In this work, we have studied the characteristics of EPBs, such as their zonal drift and tilt, from the low latitude and dip equatorial region in the Indian longitude sector during the main phase of the 17 March 2015 storm. All-sky airglow imaging observations from Tirunelveli (8.7oN, 77.8oE geographic, 1.7oN dip latitude) and Kolhapur (16.7oN, 74.3oE geographic, 11.5oN dip latitude) are utilized here. On 17 March 2015, EPBs were observed to drift eastward during 14:30-16:30 UT between 3oS and 15oN dip latitudes. A westward drift presumably under the influence of the disturbance dynamo electric field (DDEF) initially appeared at higher dip latitudes almost 10 hours after the storm onset, and subsequently the same was observed at lower dip latitudes. The EPBs attained a peak westward drift at ~17:00 UT followed by a gradual decrease in their speed till ~18:30 UT. After regaining their westward speed, the EPBs continued to drift westward till 22:00 UT. Moreover, a latitudinal gradient in the drift motion of the EPBs was also observed on this night. Another interesting observation made from the images obtained from Tirunelveli was the presence of a large westward tilt of the EPBs. The most intriguing finding of this study, however, was the asymmetry in the tilt of the EPBs at conjugate points during the pre-midnight hours on 17 March 2015. In this study, the possible mechanisms that can explain these observations are discussed in the light of the current understanding of the equatorial electrodynamics and EPBs.

All-sky imaging observations of OI 630.0 nm airglow were carried out in campaign mode from Panhala (16.8°N, 74.1°E geographic; 11.1°N dip latitude), India, during January to March 2008. On 14 of 37 nights, equatorial plasma bubbles were observed. The drift speeds were observed to decrease with time in concurrence with the previous results. The tilts were mostly westward while on rare occasions the plasma bubbles tilted eastwards. The drifts were found to be relatively lesser on disturbed nights while the tilts appear to be marginally larger. The interdepletion distances (or bubble spacings) also showed a decreasing trend with time till midnight indicating that the bubbles approach each other with the passage of time. Such a behavior is not reported earlier and it seems to have important implications for understanding the time evolution of plasma bubbles. On occasions, the bubbles occurred in groups. An ionosonde operating over Indian dip equatorial site Tirunelveli (1.1°N dip latitude) was used to study the variations in the base height of the ionosphere during the plasma bubble observations. The ionosonde measurements indicate lack of significant pre-reversal enhancement (PRE) during geomagnetic quiet days in which the bubbles were observed.

This paper presents the observations of zonal drift velocities of equatorial ionospheric plasma bubbles and their comparison with model values. These velocities are determined by nightglow OI 630.0 nm images. The nightglow observations have been carried out from the low latitude station Kolhapur (16.8° N, 74.2° E; 10.6° N dip lat.) during clear moonless nights. Herein we have presented the drift velocities of equatorial plasma bubbles for the period of February–April 2011. Out of 80 nights, 39 showed the occurrence of equatorial plasma bubbles (49%). These 39 nights correspond to magnetically quiet days (ΣKp < 26). The average eastward zonal velocities (112 ± 10 m s−1) of equatorial plasma bubbles increased from evening sector to 21:00 IST (Indian Standard Time = Universal Time + 05:30:00 h), reach maximum about 165 ± 30 m s−1 and then decreases with time. The calculated velocities are in good agreement with that of recently reported values obtained with models with occasional differences; possible mechanisms of which are discussed.

In the night time low latitude ionosphere, the equatorial ionization anomaly (EIA) crest move towards the equator as a result of change in the direction of the zonal electric field from eastward to westward. This is referred to as reverse plasma fountain. On some nights, imaging observations of OI 630.0 nm thermospheric nightglow taken from low latitude Indian station Panhala (16.8°N, 74.1°E; 11.1°N dip latitude) during deep solar minimum period revealed southward movement of a broad enhanced airglow intensity region that is aligned along east–west direction. This could be seen in the meridional keograms and is interpreted as the equatorward passage of the EIA crest. The studies that concentrate on the night time evolution of EIA with the help of airglow observations are sparse. In addition to the equatorward motion, the thickness of the crest region decreased as a result of recombination. This reduction in thickness is pronounced in the earlier part of the night and brings about an apparent drift that is added to the true equatorward drift of the EIA crest. In this work, we describe a method to measure the equatorward drift speed of the EIA crest from airglow imaging observations. The method includes the estimation and correction for the apparent drift caused by recombination. The results show that drift speed varies widely between 28 m/s and 89 m/s (~100 km/h to ~315 km/h) with an average speed of 52 m/s (188 km/h). A part of the variability might be due to variations in thermospheric meridional wind. The drifts observed during magnetically disturbed days were found to be relatively smaller than that on quiet days.

Simultaneous observations of the mean zonal winds at 88 km obtained by the medium-frequency (MF) radars at Kolhapur (16.8° N, 74.2° E) and Tirunelveli (8.7° N, 77.8° E) have been used to study the intraseasonal oscillation (ISO) in the MLT region. The influences of the intraseasonal variations in the lower tropospheric convective activity associated with the Madden-Julian oscillations on the latitudinal behavior of intraseasonal oscillations (ISO) of the zonal winds in the equatorial mesosphere and lower thermosphere (MLT) have been studied. The ISO activity in the lower tropospheric convective activity is examined by employing outgoing long wave radiation (OLR) as a proxy for deep convective activity occurring in the tropical lower atmosphere. The ISO activity in the zonal wind over TIR is more correlated with that in the convective activity compared to the ISO over KOL. The latitudinal and temporal variabilities of the ISO in MLT zonal winds are explained in terms of the intraseasonal variabilities in the convective activity.

Over 53 nights of all sky airglow imager data collected during January-April 2012 from the low latitude station Kolhapur (16.68°N, 74.26°E; 10.6°N dip latitude) have been analyzed to study the F-region dynamics through the imaging of OI 630 nm emission line. The observed night airglow data were supported by the ionosonde measurements from Tirunelveli (8.7°N, 77.8°E; 0.51°N dip latitude). Well defined magnetic field aligned depletions were observed during the observation period. Out of 53 nights, 40 nights exhibited the occurrence of north-south aligned equatorial plasma bubbles. These plasma bubbles were found moving towards east with drift speed in range between 70 to 200 m s-1. We have analyzed the zonal drift velocity variation and relation of bubble occurrence with the base height of the ionosphere together with the effects of the geomagnetic Ap and solar flux F10.7 cm index in its first appearance.

[1] In this paper we study the planetary-scale wave features using concurrent observations of mesospheric wind and temperature, ionospherich'F, and tropospheric wind from Tirunelveli, Gadanki, and Kolhapur, all located in the Indian low latitudes, made during February 2009. Our investigations reveal that 3 to 5 day periodicity, characterized as ultrafast Kelvin (UFK) waves, was persistent throughout the atmosphere during this period. These waves show clear signatures of upward wave propagation from troposphere to the upper mesosphere, linking the ionosphere through a clear correlation between mesospheric winds and h'F variations. We also note that the amplitude of this wave decreased as we moved away from the equator. These results are the first of their kind from Indian sector, portraying the vertical as well as latitudinal characteristics of the 3 to 5 day UFK waves simultaneously from the troposphere to the ionosphere.

[1] We herein report the first observations of an unusual phenomenon recorded by an all-sky airglow imager from the low-latitude site Panhala (16.8°N, 74.1°E, geographic; 8.2°N geomagnetic), on the night of 1 February 2008, during the main phase of a moderate geomagnetic storm. The observations of OI 557.7 nm emission reveal discrete, transient, filamentary structures referred to as “streaks.” No such features were seen in the OI 630.0 nm emission and the mesospheric sodium and hydroxyl emissions. Here we speculate on possible mechanisms for generation of such structures, though we cannot conclude firmly that any one of them was responsible for the observed features. This is a puzzling observation made from very low geomagnetic latitude during the main phase of a moderate recurrent geomagnetic storm in the declining phase of solar cycle. In spite of the limitations in identifying the mechanism or mechanisms responsible for this striking observation, it is felt that understanding of processes driving such unusual and rare events will substantiate our knowledge on the mysterious coupling processes occurring in the equatorial upper atmosphere.

An advanced meteor radar, viz, Sri Venkateswara University (SVU) meteor radar (SVU MR) operating at 35.25 MHz, was installed at Sri Venkateswara University (SVU), Tirupati (13.63°N, 79.4°E), India, in August 2013 for continuous observations of horizontal winds in the mesosphere and lower thermosphere (MLT). This manuscript describes the purpose of the meteor radar, system configuration, measurement techniques, its data products, and operating parameters, as well as a comparison of measured mean winds in the MLT with contemporary radars over the Indian region. It is installed close to the Gadanki (13.5°N, 79.2°E) mesosphere-stratosphere-troposphere (MST) radar to fill the region between 85 and 100 km where this radar does not measure winds. The present radar provides additional information due to its high meteor detection rate, which results in accurate wind information from 70 to 110 km. As a first step, we made a comparison of SVU MR-derived horizontal winds in the MLT region with those measured by similar and different (MST and MF radars) techniques over the Indian region, as well as model (horizontal wind model 2007) data sets. The comparison showed an exquisite agreement between the overlapping altitudes (82–98 km) of different radars. Zonal winds compared very well, as did the meridional winds. The observed discrepancies and limitations in the wind measurement are discussed in the light of different measuring techniques and the effects of small-scale processes like gravity waves. This new radar is expected to play an important role in our understanding of the vertical and lateral coupling of different regions of the atmosphere that will be possible when measurements from nearby locations are combined.

Well-defined 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 well-known 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 filtering. 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.

A CCD based all-sky (180° field view) airglow imaging system was operated from a low latitude station, Kolhapur (16.8° N, 74.2° E; 10.6° N dip lat.) in India to map the thermospheric motion and to study the low latitude ionospheric irregularities by measuring the nightglow signatures at 630 nm. In general, all-sky imaging techniques offer broad, instantaneous coverage (2.5 million sq km area at 150° field of view at 300 km height) of the spatial and temporal characteristics of the airglow features from the selected layers of the thermosphere and ionosphere system. In addition to the all-sky camera, photometer and radio scintillation observations together with ionospheric soundings were carried out from Kolhapur to verify the signature of small scale as well as large scale size F-region irregularities. Initial results from these measurements display good examples of north-south motion of the thermosphere with apparent drift speeds of 143–200 m/s during geomagnetically quiet nights. The north-south aligned (>1000 km) bands of plasma depletions or bubbles with east-west dimension of 50–250 km moving with drift speed of 140 m/s towards east have been observed. The zonal component of plasma drift speed matches well with the eastward component of the neutral speed computed using Hedin’s recent model. However, the meridional components of plasma drift speed are generally higher than the components of the neutral wind speed computed using the model. The results confirm the earlier quantitative investigations of equatorial depletion characteristics from Brazil and elsewhere.