C/NOFS Observations of Reversed Zonal E x B Drifts Below the Equatorial Ionospheric F-Peak at Sunset and Their Implications for the Generation of Large Scale Instabilities

Abstract:

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 350-400 km near sunset. During these orbits, data from the electric field probes consistently show evidence of reversed zonal drifts -- i.e., westward drifts at the lower altitudes -- whenever the satellite journeys below the F-peak near sunset. Similar to observations of sheared zonal drifts in the evening sector by sounding rockets and the Jicamarca radar, these observations demonstrate that zonal plasma drifts, sheared with respect to altitude at and below the F-peak, are routinely observed and suggest that this shear may be a source of large scale, Kelvin-Helmholtz-like instabilities that could seed equatorial “spread-F”. Indeed, in addition to the shear in the DC electric field data, the C/NOFS data set also provides direct observations of large scale (100’s of km) and medium scale (10’s of km) undulations of the plasma density on the bottomside ledge of the sunset equatorial ionosphere and their associated shorter scale irregularities. These undulations are generally devoid of smaller scale structures in the early evening, particularly when the satellite is still in the sunlit ionosphere, yet appear at later local times with associated, fully-developed smaller scale structures along the same orbit. We present examples of both the sheared zonal plasma flow and the large scale undulations and discuss their implications for our understanding of the unstable equatorial ionosphere.