On the variability of the observed HF Doppler derived equatorial F-region plasma drifts during evening and morning hours and the chemical corrections therein

130-140The radar derived F-region vertical plasma drift measurements, especially during evening/morning hours are apparent mainly because of the additional contribution from photochemical loss and/or production. As a consequence, it is very important to delineate the role of chemistry from these drift measurements in order to have a meaningful/realistic interpretation of the actual movement of the plasma in the ionosphere. In this paper, two chemical schemes have been employed separately for evening and morning hours to estimate the apparent drifts produced solely due to the chemistry. This study has been carried out by taking into account the important chemical reactions over equator during these times and for the computation, the neutral density of N2, O2, and neutral temperature Tn have been obtained from the Mass Spectrometer and Incoherent Scatter (MSIS) model. The temporal, altitudinal and seasonal variations of the chemical contributions have been theoretically estimated. The observed F-region vertical plasma drift measurements obtained from a multi-frequency HF Doppler radar over Trivandrum (8.5°N, 77°E, 0.5°N dip latitude) are then corrected, by taking into account the reflection height variation. The study reveals that (i) the apparent drift due to chemistry is strongly altitude dependent and can indeed be as high as 10-15 m s-1; (ii) there is a clear cut seasonal pattern for these correction factors with a northern hemispheric (NH) summer (June) minima and a maxima during the NH winter solstice (December)