Influence of gravity waves and tides on mesospheric temperature inversion layers: simultaneous Rayleigh lidar and MF radar observations

Three nights of simultaneous Rayleigh lidar temperature measurements over Gadanki (13.5° N, 79.2° E) and medium frequency (MF) radar wind measurements over Tirunelveli (8.7° N, 77.8° E) have been analyzed to illustrate the possible effects due to tidal-gravity wave interactions on upper mesospheric inversion layers. The occurrence of tidal gravity wave interaction is investigated using MF radar wind measurements in the altitude region 86–90 km. Of the three nights, it is found that tidal gravity wave interaction occurred in two nights. In the third night, diurnal tidal amplitude is found to be significantly larger. As suggested in Sica et al. (2007), mesospheric temperature inversion seems to be a signature of wave saturation in the mesosphere, since the temperature inversion occurs at heights, when the lapse rate is less than half the dry adiabatic lapse rate

Seasonal variabilities of low-latitude mesospheric winds

International audienceObservations of mesospheric winds over a period of four years with the partial reflection radar at Tirunelveli (8.7°N, 77.8°E), India, are presented in this study. The emphasis is on describing seasonal variabilities in mean zonal and meridional winds in the altitude region 70?98 km. The meridional winds exhibit overall transequatorial flow associated with differential heating in the Northern and Southern Hemispheres. At lower altitudes (70?80 km) the mean zonal winds reveal easterly flow during summer and westerly flow during winter, as expected from a circulation driven by solar forcing. In the higher altitude regime (80?98 km) and at all altitudes during equinox periods, the mean zonal flow is subjected to the semi-annual oscillation (SAO). The interannual variability detected in the occurrence of SAO over Tirunelveli has also been observed in the data sets obtained from the recent UARS satellite mission. Harmonic analysis results over a period of two years indicate the presence of long-period oscillations in the mean zonal wind at specific harmonic periods near 240, 150 and 120 days. Results presented in this study are discussed in the context of current understanding of equatorial wave propagation

Variabilities of mesospheric tides and equatorial electrojet strength during major stratospheric warming events

The present study demonstrates the relationship between the high latitude northern hemispheric major sudden stratospheric warming (SSW) events and the reversal in the afternoon equatorial electrojet (EEJ), often called the counter-electrojet (CEJ), during the winter months of 1998–1999, 2001–2002, 2003–2004 and 2005–2006. As the EEJ current system is driven by tidal winds, an investigation of tidal variabilities in the MF radar observed zonal winds during the winters of 1998–1999 and 2005–2006 at 88 km over Tirunelveli, a site close to the magnetic equator, shows that there is an enhancement of semi-diurnal tidal amplitude during the days of a major SSW event and a suppression of the same immediately after the event. The significance of the present results lies in demonstrating the latitudinal coupling between the high latitude SSW phenomenon and the equatorial ionospheric current system with clear evidence for major SSW events influencing the day-to-day variability of the CEJ

First results from the CAWSES-India Tidal Campaign

The first CAWSES-India Tidal Campaign was conducted by the Indian scientific community during March–April 2006. The objectives of this campaign were: (1) To determine the characteristics of tides in the troposphere and lower stratosphere (0–20 km) and mesosphere and lower thermosphere (MLT) region (80–100 km), (2) to explore and identify what lower atmospheric processes drive middle atmospheric tides in the Indian continental region and (3) to provide information on those short-term variabilities of MLT tides that are likely to have an impact on the ionospheric variabilities and contribute to the upper atmospheric weather. Data sets from experiments conducted at the three low latitude radar sites, namely, Trivandrum (8.5° N, 76.9° E), Tirunelveli (8.7° N, 77.8° E) and Gadanki (13.5° N, 79.2° E) and fortnightly rocket launches from Thumba were made use of in this study. An important observational finding reported in this work is that the radar observations at Tirunelveli/Trivandrum indicate the presence of 15–20 day modulation of diurnal tide activity at MLT heights during the February–March period. A similar variation in the OLR fields in the western Pacific (120–160° longitude region) suggests a possible link between the observed tidal variabilities and the variations in the deep tropical convection through the nonmigrating tides it generates

Study of equatorial plasma bubble during January to April 2012 over Kolhapur (India)

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