Equatorial vertical plasma drift velocities and electron densities inferred from ground-based ionosonde measurements during low solar activity

Average values of ionosonde hmF2 data acquired from an African equatorial station have been used to determine vertical plasma drift (Vz) measurements during period of low solar activity. Pre-noon peak was around 1000 h LT for all seasons. The peak daytime F2 drift is higher during the equinoctial months with an average of 18.1 m/s than the solsticial months (14.7 m/s). At nighttime, Vz is characterized first by upward enhancement around 1900 h LT with a range of 0.3-8.0 m/s, then by a downward reversal. The highest enhancement was recorded in December solstice and start earliest during the March equinox. The peak reversal values are 13.3, 10.7, 9.0 and 4.2 m/s for December Solstice, September Equinox, March Equinox and June Solstice respectively. The observed simultaneous post-sunset rise in hmF2 and in vertical E x B drift together with a sharp drop in NmF2 at all season infer that electrons moving away from the equator are at a region of low recombination loss rate. The abrupt faster drift of the plasma away from the equator as indicated by the pre-reversal enhancement (PRE) in upward plasma drift is responsible for the sharp drop in NmF2 immediately after sunset. Some past results were also confirmed in this work. (C) 2013 Elsevier Ltd. All rights reserved.