DR. T.K. RAMKUMAR

Inertial subrange or Kolmogorov scales of refractive index structure of the atmosphere put constrains on the preference of frequencies of atmospheric probing radars. Commonly, while ultra high frequency (UHF, say 1 GHz) radars are employed to study the planetary boundary layer (up to few kms from the surface of the Earth) characteristics, very high frequency (say 50 MHz) radars are used for the rest of the atmosphere up to about 70 km and medium frequency (say 2 MHz) radars are utilized for the height region of 70-100 km. These frequencies are selected in order to get the consistent and persistent echoes in the respective height regions so that one can get a meaningful observation of the atmospheric dynamics. Even though the range of scales of ionospheric plasma irregularities (fluctuations in the refractive index) is very large (few cms to thousands of kms), depending on the various plasma instability mechanisms, normally ~50 MHz radars are employed to study the ionospheric electrodynamics up to ~800 km, as the radars operating at this frequency can also get meaningful middle atmospheric dynamics. In India, the VHF mesosphere stratosphere troposphere (MST) radar (53 MHz) operating at the Indian tropical station of National Atmospheric Research Laboratory, Gadanki, Tirupati and the medium frequency radar (2 MHz) at the Indian geomagnetic dip equatorial station of Equatorial Geophysical Research Laboratory (Indian Institute of Geomagnetism), Tirunelveli have been providing a wealth of invaluable information on the physical (dynamics & electrodynamics) characteristics of our earth’s atmosphere and ionosphere for the last more than two decades. The physical characteristics include the generation, propagation and dissipation mechanism of various scales of atmospheric and ionospheric waves and turbulences. The knowledge of which are very important not only for the meteorological prediction purposes but also for the clear communication through electromagnetic waves between the surface of the earth and the outer space. The atmospheric “seeing” and ionospheric “scintillation” effects due to fluctuations in the refractive index structure are often very annoying and make it useless at certain frequencies for astronomical observations and GPS related communications. The present talk will focus on these observational issues using the data obtained mostly with the MF and VHF radars located at Tirunelveli (EGRL, IIG) and Gadanki, Tirupati (NARL, ISRO, DOS).