Longitudinal variations in the equatorial vertical drift in the topside ionosphere

[1] The vertical ion drift is important for understanding and modeling the electrodynamics at low latitudes. Measurements from the ion drift meter on the Defense Meteorological Satellite Program ( DMSP) F15 are used to examine longitudinal variations in the vertical ion drift at the dip equator in the topside ionosphere. Local time was restricted to 0930 and the data were organized by month for 2001 and 2002. Two features were found contributing to the longitudinal variations in the electrodynamics. Meridional winds contribute to the equatorial vertical ion drift at the 830 km magnetic apex due to their dynamo action at the foot points of the magnetic field lines in the F region at magnetic latitudes near 15 degrees. Electric fields producing a downward perturbation drift at 0930 are produced by hemispheric differences in the dynamo current due primarily to the different orientation of the magnetic meridian relative to the terminator. This produces the seasonally dependent variation as a function of longitude that is observed. A wavenumber-4 longitudinal variation also appears to be present throughout the year but is more influential during equinox conditions. This variation also shows a seasonal cycle, shifting east during northern summer and west during northern winter. Further analysis would be required to isolate the characteristics of a wavenumber-4 driver from the wavenumber-4 component of the Fourier series used here to fit the data.