Abstract:
We analyzed 17 years (1993–2009) of horizontal winds measured by the medium frequency (MF) radar located
at Tirunelveli (8.7°N, 77.8°E) and 10 years (2005–2014) of horizontal winds measured by a meteor radar
located at Thumba (8.5°N, 77°E) to examine the seasonal, inter-annual, and solar cycle variability of the Quasi-
Two Day Wave (QTDW) in the mesosphere and lower thermosphere region. These two radars are nearly colocated,
but differ in their measurement technique. Comparison of the estimated QTDW amplitudes by the two
radars shows that the amplitudes are larger in the meteor radar than those in the MF radar. The difference
between the amplitudes is larger in May in the zonal component and in April and September in the meridional
one. Furthermore, the differences are larger in the meridional component. The QTDWs in both the radars show
a strong semi-annual oscillation (SAO). In addition, the meridional QTDW amplitudes of both the MF and
meteor radars show a distinct enhancement in the month of October. While the whole spectra of QTDWs
contribute to the SAO amplitudes, only 45–50 h waves contribute to the October enhancement. The amplitudes
of the QTDWs, in general, show large inter-annual variability. The QTDW amplitudes from both the radars
show modulation at period of quasi-biennial oscillation. The QTDWs of the MF radar show a small negative
correlation with solar activity while those of meteor radar do not show any correlation. The above aspects are
discussed in the light of current understanding of the QTDWs.
