Abstract:
In the present communication, the role of neutral dynamical processes in governing the
morphology and seasonal variations of daytime VHF radar echoes from the mesosphere and the
collision-dominated lower E region is discussed. While the seasonal variations of mesospheric echo
occurrence show semiannual variation with their maxima in the equinoxes, the occurrence of E region echoes
shows strong annual variation peaking in the summer with two secondary maxima identical to semiannual
variation of the mesospheric echoes. The occurrence rates of the E region echoes, however, display
year-to-year variability. Semiannual variations in the occurrence of low-latitude mesospheric echoes are
closely linked with similar variations in the occurrences of temperature inversion, large wind shear, tides, and
gravity wave activities in the same height region. In the case of E region echoes, the summer maximum and
equinoctial secondary maxima are found to be linked with similar variations in wind and wind shear, Es
activity, tidal, and gravity wave activities. Further, the summer maximum is found to be strongly correlated
with that of meteor flux and nonmigrating diurnal tide. This finding is significant given the fact that such wind
field in the presence of metallic ions could form both Es and field-aligned irregularities. The present results
show that tides and gravity waves are the most important dynamical factors governing the morphology and
seasonal variations of the mesospheric as well as lower E region echoes observed over Gadanki. These results
are discussed in the light of current understanding on the dynamical coupling of the mesosphere and lower E
region through tides and gravity waves.
