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The Ion Velocity Meter (IVM), a part of the Coupled Ion Neutral DynamicInvestigation (CINDI) instrument package on the Communication/NavigationOutage Forecast System (C/NOFS) spacecraft, has made over 5 yr ofin situ measurements of plasma temperatures, composition, densities, andvelocities in the 400–850 km altitude range of the equatorial ionosphere.These measured ion velocities are then transformed into a coordinate systemwith components parallel and perpendicular to the geomagnetic field allowingus to examine the zonal (horizontal and perpendicular to the geomagneticfield) component of plasma motion over the 2009–2012 interval. The generalpattern of local time variation of the equatorial zonal ion velocity is wellestablished as westward during the day and eastward during the night, withthe larger nighttime velocities leading to a net ionospheric superrotation.Since the C/NOFS launch in April 2008, F10.7 cm radio fluxes have graduallyincreased from around 70 sfu to levels in the 130–150 sfu range. Thecomprehensive coverage of C/NOFS over the low-latitude ionosphere allows usto examine variations of the topside zonal ion velocity over a wide level ofsolar activity as well as the dependence of the zonal velocity on apexaltitude (magnetic latitude), longitude, and solar local time. It was foundthat the zonal ion drifts show longitude dependence with the largest neteastward values in the American sector. The pre-midnight zonal drifts showdefinite solar activity (F10.7) dependence. The daytime drifts have a lowerdependence on F10.7. The apex altitude (magnetic latitude) variationsindicate a more westerly flow at higher altitudes. There is often a nettopside subrotation at low F10.7 levels, perhaps indicative of a suppressedF region dynamo due to low field line-integrated conductivity and a lowF region altitude at solar minimum

Topside equatorial zonal ion velocities measured by C/NOFS during rising solar activity