A comparison study of zonal drift velocities measurements as seen by MF spaced antenna and HF Doppler radar in the Indian dip equatorial mesospheric and lower thermospheric (80–100 km) region

Abstract

The simultaneous measurements of zonal drift velocities, observed in the heights of 84–98 km in the Indian geomagnetic dip equatorial region by an medium frequency (MF, 1.98 MHz) spaced antenna and a high‐frequency (HF, 18 MHz) Doppler radars, are compared on selected few days in the solar maximum years of 1998, 1999, and 2000. The agreement between the two radar measurements is found to be good below about 88 km, where the neutral turbulence induced ionospheric irregularities are more predominant. Above 90 km, however, the agreement becomes poor and at the highest height of 98 km it becomes the least. At this height, more often the HF Doppler radar shows a westward drift of about 200 m/s whereas the MF spaced antenna radar values lie within ±10 m/s and sometimes attain maximum values of ±50 m/s. Detailed discussions are made on the possible sources of underestimation of the drift velocities measured by the MF radar and the nature of scattering irregularities that are produced because of large neutral turbulences and plasma instabilities. It is suggested that these neutral and plasma turbulences (particularly type II plasma irregularities) contribute in a different manner to different radar frequencies and techniques and hence very different drift velocities in the heights of 90–100 km particularly in the geomagnetic dip equatorial region. Discussions are also made on (1) the real atmospheric and ionospheric physical process prevailing in the 90–100 km region and (2) the technical aspects of the radars that limits them to measure only particular types of motion in this region.