Morphology of tropical mesospheric echoes observed by VHF radar

This short communication reports on an attempt which has been made to study mesospheric echo occurrence duration for the first time, using a large data base (1998 to 2004) collected from MST radar located at Gadanki (13.5.degrees N, 79.2.degrees E), a tropical station in India. It is well known that VHF radar echoes in the mesosphere are due to both the presence of irregularities with scales of half the radar wavelengths, and also the production and loss rates of electrons in the lower D-region. The main aim of the present paper is to estimate the duration of mesospheric echoes at each altitude and to establish the preference of their occurrence, if any, in altitude and time. In general, mesospheric echoes show tilted layers, a sporadic nature above/below the main layer (70-80 km), and a seasonal shift in the height of occurrence. Based on the time and height of echo occurrence, we broadly divide the periods of mesospheric echo occurrence (duration) into 5-20 minutes (T1), 20-40 minutes (T2), and >= 40 minutes (T3), and the height of occurrence into 65-70 km (R1), 70-80 km (R2), and 80-85 km (R3). It is observed that long duration echoes (T2 and T3) occur mostly in the R2 region and are highly sporadic in the R1 and R3 regions. In addition, the solar zenith angle dependence on the duration of mesospheric echoes is also studied and no significant variation was found in any of the above-mentioned categories. This study will contribute to a better understanding of the dynamical aspects of the mesosphere using VHF radar observations.