The present study for the first time reports the global gravity wave activity in terms of their potential energy derived from TIMED/SABER observations right from the stratosphere to the mesosphere lower thermosphere (MLT) region. The potential energy profiles obtained from SABER temperature are validated by comparing them with ground based LIDAR observations over a low latitude site, Gadanki (13.5°N, 79.2°E). The stratospheric and mesospheric global maps of gravity wave energy showed pronounced maxima over high and polar latitudes of the winter hemisphere. The interannual variability of the stratospheric gravity wave activity exhibited prominent annual oscillation over mid-latitudes. The equatorial gravity wave activity exhibited quasi-biennial oscillation in the lower stratosphere and semi-annual oscillation in the upper stratosphere. The MLT region maps revealed summer hemispheric maxima over polar latitudes and secondary maxima over the equatorial region. The results are discussed in the light of present understanding of global gravity wave observations. The significance of the present study lies in emphasizing the importance of satellite measurements in elucidating gravity waves, which is envisaged to have profound impact on parameterizing these waves.

[1] In the present communication, initial results from the allSKy interferometric METeor (SKiYMET) radar installed at Thumba (8.5°N, 77°E) are presented. The meteor radar system provides hourly zonal and meridional winds in the mesosphere lower thermosphere (MLT) region. The meteor radar measured zonal and meridional winds are compared with nearby MF radar at Tirunalveli (8.7°N, 77.8°E). The present study provided an opportunity to compare the winds measured by the two different techniques, namely, interferometry and spaced antenna drift methods. Simultaneous wind measurements for a total number of 273 days during September 2004 to May 2005 are compared. The comparison showed a very good agreement between these two techniques in the height region 82-90 km and poor agreement above this height region. In general, the zonal winds compare very well as compared to the meridional winds. The observed discrepancies in the wind comparison above 90 km are discussed in the light of existing limitations of both the radars. The detailed analysis revealed the consistency of the measured winds by both the techniques. However, the discrepancies are observed at higher altitudes and are attributed to the contamination of MF radar neutral wind measurements with Equatorial Electro Jet (EEJ) induced inospheric drifts rather than the limitations of the spaced antenna technique. The comparison of diurnal variation of zonal winds above 90 km measured by both the radars is in reasonably good agreement in the absence of EEJ (during local nighttime). It is also been noted that the difference in the zonal wind measurements by both the radars is directly related to the strength of EEJ, which is a noteworthy result from the present study.Citation: Kumar, K. K., G. Rankumar, and S. T. Shelbi (2007), Initial results from SKiYMET meteor radar at Thumba (8.5°N, 77°E): 1. Comparison of wind measurements with MF spaced antenna radar system, Radio Sci., 42, RS6008,

[1] Recent studies of the equatorial ionosphere have found evidence of forcing by atmospheric Ultra Fast Kelvin (UFK) waves. This study investigates the quasi-3-day UFK wave and its effects on the variations of the ionosphere at low latitudes and midlatitudes using coordinated observations of both the atmosphere and ionosphere during the January 2010 URSI World Day campaign. The global maps of TEC from the IGS ground-based GPS product demonstrate a 3-day periodic variation during January 15-25. This variation has the largest amplitude at 15 magnetic latitude and extends into lower latitudes. Simultaneously, a 3-day wave is observed in the mesosphere in the zonal wind measurements by a meteor radar at the magnetic equator. The latitudinal range of the TEC variation (20 S-20 N) is also consistent with that of the 3-day wave. The Incoherent Scatter Radar (ISR) observations show a 3-day signature in vertical ion drifts over Jicamarca (11.9 S, 76 W) and in the electron densities in the top side of ionosphere measured from Millstone Hill (42.6 N, 71.5 W). This signature is consistent with the fountain effect in the equatorial region, and shows the impact on the topside ionosphere at midlatitudes. The UFK wave is trapped within AE30 geographic latitude, but this study shows that the effects of the wave could reach the ionosphere at the higher latitude even as high as 40 N (50 N magnetic latitude).

are used to estimate the gravity wave momentum fluxes in the Mesosphere Lower Thermosphere (MLT) region over Trivandrum (8.5°N, 76.9°E), a low-latitude station in India. The radial velocity variances in the 82-98 km height region, which are mainly caused by gravity waves, measured by the meteor radar are used to determine the gravity wave momentum fluxes. Using a novel method proposed by Hocking (2005), altitude profiles of momentum fluxes of short period (less than 2-3 h) gravity waves are estimated for three continuous years. Seasonal variation in the gravity wave momentum fluxes showed semi annual variation with equinoctial maximum and solstitial minimum. By using estimated gravity wave momentum fluxes, an attempt is made to quantify their contribution in driving the Mesospheric Semi Annual Oscillation (MSAO). The mean flow acceleration estimated from the divergence of gravity wave momentum fluxes is compared with the observed mean flow acceleration computed from monthly mean zonal winds during six MSAO cycles over three years. This comparison reveled that the gravity wave contribution toward the westerly phase of MSAO varies from $20-60% while that toward the easterly phase varies from $30-70%. Variations are observed from cycle-to-cycle in the gravity wave forcing toward both phases of MSAO. The significance of the present study lies in estimating the gravity wave momentum fluxes in the MLT region and quantifying their contribution toward the generation of MSAO over the low-latitude for the first time.

[1] Ultra-fast Kelvin waves with periods of 3-5 days are important in the coupling of the lower atmosphere to the thermosphere and ionosphere. Here we focus on the observations and effects of a 3-day wave during January 2010. As this time period coincides with a stratospheric warming event, a coordinated set of observations with incoherent scatter radars are available. While there is no evidence that the occurrence of this 3-day wave is connected with this event, these observations offer an unprecedented glimpse of the thermospheric conditions during this period, including the first-ever detection of a 3-day wave with an incoherent scatter radar. Using a combination of ground-and space-based observations, we identify an eastward moving zonal wave number-one 3-day equatorial wave that is comprised of a Kelvin wave at the lowest latitudes and a Rossby-gravity wave at higher latitudes. In the equatorial region, the vertical wavelength is $40 km and the wave peaks in amplitude around 95-100 km altitude. The wave observed here is only seen to propagate to around 105 km altitude. Evidence of an interaction between this wave and the diurnal tide is seen between 82-88 km. The resultant 3-day periodicity in the diurnal tide is seen to propagate up to altitudes of $150 km. This could have a significant impact on the ionosphere via modulation of the E-region dynamo, thus carrying the 3-day periodicity to higher altitudes.