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Preliminary results on equatorial spread-F irregularities by VHF and HF radars

Tue, 01 Aug 2000 00:00:00 GMT

Title: Preliminary results on equatorial spread-F irregularities by VHF and HF radars Authors: Sekar, R; Kherani, E A; Viswanathan, K S; Patra, A K; Rao, P B; Devasia, C V; Subbarao, K S V; Tiwari, D; Ramachandran, N Abstract: Preliminary results obtained from a co-ordinated campaign involving simultaneous operation of VHF (53 MHz) and HF (18 MHz) radars, located at Gadanki and Trivandrum, on the structures and dynamics of equatorial spread-F irregularities are reported. Such kind of measurement involving 18 MHz radar is attempted, probably, for the first time. The structures as revealed by both VHF and HF radars are more or less similar. However, the Doppler velocities transverse to earth's magnetic field of the 3m irregularities as observed by VHF radar reveal a maximum velocity of 200 ms^-1 in the central regions of plume structures, while the corresponding velocities of 8.3 m irregularities observed by HF radar reveal a maximum of around 50 ms^-1 in contrast to the earlier expectations. This suggests significant dispersive nature indicating that different physical processes are operational in these wavelength regime. Further the observations by VHF radar revealed the plume structures which are generally found to be vertically erect in the altitude region of 300-450 km without having much tilt indicating that the shears in the zonal plasma drift are not significant in this altitude region during equinoctial period over Indian longitude. Page(s): 262-271

Detection of pre-sunrise echoes at mesospheric heights

Tue, 01 Aug 2000 00:00:00 GMT

Title: Detection of pre-sunrise echoes at mesospheric heights Authors: Ratnam, M Venkat; Rao, T Narayana; Rao, S Vijaya Bhaskara; Rao, D Narayana; Kumar, Y Bhavani; Raghunath, K Abstract: Indian MST radar located at Gadanki has been operated in special mode during equinox, i.e. on 23 and 24 Mar.1999 for detection of pre-sunrise echoes at mesospheric heights. These echoes are observed on both the days, 30 to 60 min before sunrise at ground level. On the first day, the radar is operated only in vertical direction and found that the echoes are highly intermittent in space and time similar to the daytime echoes. These echoes are observed mainly above 85 km with a peak at 90 km height on both the days. Echoes are also observed below these heights but less frequently. The Doppler shift and Doppler width on 23 Mar. 1999 is found to be of the order of +2 Hz and 4 ms^-1, respectively. On 24 Mar. 1999, radar was operated by using three different beams (east, zenith Y and south) in order to see whether these echoes are due to turbulence or due to some other mechanism. Interestingly echoes in all the beam positions are observed indicating that the echoes are due to turbulence. Page(s): 258-261

Middle atmospheric temperature measurements using a ground based lidar from low latitude

Tue, 01 Aug 2000 00:00:00 GMT

Title: Middle atmospheric temperature measurements using a ground based lidar from low latitude Authors: Kumar, Y Bhavani; Kumar, V Siva; Rao, P B; Krishnaiah, M; Mizutani, Kohei; Aoki, Tetsuo; Yasui, Motoaki; Itabe, Toshikazu Abstract: Lidar measurements of atmospheric density and temperature in the altitude range 30-80 km have been performed during the period March-December 1998, using NMRF-IJL, Gadanki (l3.8°N, 79.2°E). Sample results on temperature structures in the periods of equinoctial, summer and winter nights, results on wave propagation and evidence of mesospheric temperature inversion on an occasion are presented. The data analysis, method of temperature determination and its standard error are also described. Page(s): 249-257

Thin aerosol clouds at tropopause level

Tue, 01 Aug 2000 00:00:00 GMT

Title: Thin aerosol clouds at tropopause level Authors: Murthy, B V Krishna; Nayar, S R Prabhakaran; Revathy, K; Satheesan, K; Parameswaran, K; Nair, Prabha R; Murthy, K Krishna; Rao, P B; Bhavanikumar, Y; Raghunath, K; Sivakumar, V; Krishniah, M Abstract: Aerosol clouds about 2 km below the tropopause are studied using the data from lidar and MST radar at Gadanki. The cloud shows fluctuations with time scales as short as ¹/₂ h in the backscatter ratio. Depolarization factor indicates ice crystal formation when the cloud is strong. The MST radar wind data show strong turbulence below the cloud level indicating, probably, turbulent mixing /transport of water vapour from lower heights to cloud heights facilitating cloud formation. Page(s): 245-248