Plasma Diffusive Flows in the Topside Ionosphere From Radio Occultation Measurements by COSMIC

O+ field-aligned diffusive velocities and fluxes in the topside ionosphere have been calculated from electron density profiles retrieved from COSMIC radio occultation measurements. The diffusive velocities and fluxes from October 2006 to October 2007 at low-latitude and mid-latitude have been statistically analyzed. The results show that daytime diffusive fluxes changed gradually from downward to upward as altitude increased. The transition height is below hmF2+80 km. The largest values of the upward diffusive fluxes and velocities during the daytime occur at geomagnetic latitudes from 10º to 20º above hmF2+80 km, whereas during the nighttime the maximum downward fluxes occur at geomagnetic latitudes from 30º to 40º. Diffusive fluxes are roughly symmetric around the magnetic equator during equinoxes. But they are asymmetric during solstices, the winter hemisphere has smaller fluxes than the summer hemisphere does. In addition, the diurnal variations have latitudinal dependences.