Evidence of Medium Scale (~50 km) Undulations Observed at Sunset in the Equatorial Ionosphere by Electric Field and Plasma Density Probes on the C/NOFS Satellite Below the F-Peak in Conjunction with Larger Scale (~500 km) Depletions

Electric field and plasma density observations gathered on the C/NOFS satellite are presented in cases where the ionosphere F-peak has been elevated above the satellite perigee of 400 km near sunset. During these passes, data from the electric field and plasma density probes on the satellite frequently show evidence of "medium scale" (40-80 km) undulations in conjunction with, yet distinct from, series of periodic, larger scale (200-800 km) density depletions. The observations suggest that a second category of wavelike variations of the bottomside plasma density may be important for the subsequent development of the larger scale density depletions and their associated "spread-F" irregularities. The electric fields associated with the medium scale undulations are typically a few mV/m with density variations of a few percent in which upwards E x B drifts are associated with their depletions. The undulations are observed both before and after local sunset and are typically observed, in the satellite frame as it journeys from west to east, prior to the onset of the larger scale depletions. We present examples of these medium scale undulations and discuss their implications for driving the larger scale depletions, possibly in a manner similar to that discussed by Hysell and Kudeki [2004], Kudeki et al. [2007], and others.