Ion temperature troughs in the equatorial topside ionosphere
It is noted that the retarding potential analyzer aboard OGO 6 sometimes records pronounced minima of ion temperature when the satellite crosses the magnetic equator and that the variation of ion temperature along the satellite path takes the form of a trough about 20 to 30 deg wide in latitude and up to 1200 K in depth. Observations of night-time, daytime, and dawn-dusk ion-temperature troughs are discussed along with ion concentration and composition in the troughs, ion drift velocities, and comparisons with Jacchia's (1971) thermospheric model. An explanation of trough morphology is given in terms of thermospheric winds which produce a transequatorial plasma flow along geomagnetic field lines. The effect of such a plasma flow on 630-nm nightglow is considered, and it is shown how ion composition affects the extent of ion-temperature troughs. Some questions for further study are suggested.