Interferometer Observations of Ion and Neutral Dynamics in the Polar Cusp

Abstract

Thermospheric winds, ion drifts, and temperatures, determined by ground-based optical Doppler velocimetry in the dayside of the winter polar cap, are presented and discussed in context with the systematic errors of measurement and the possible influence of cusp-related geophysical phenomena. The thermospheric wind observed from Spitsbergen is predominantly westward on the dayside of the polar cap when plotted in geographic coordinates. In geomagnetic coordinates, the flow is more poleward but retains a westward zonal component on the average. Separation of the data by cusp latitude shows that the westward component is stronger when it is poleward of the station. The cusp shows clearly as a temperature maximum from noon-midnight passes of the DE satellite, but is not noticeable in the same way from the ground, making a zonal pass underneath. Observations of vertical winds are discussed along with the implications on the uncertainty of reduction of observational data to horizontal components. Winds are found to be non-uniform particularly near the minimum velocity seen in passage through the evening vortex.

Optically measured ion drifts have the expected characteristic with the change of sign of the IMF By component. For By positive, the zonal component is westward near magnetic noon. For By negative, it is eastward. The neutral wind is also a function of the By component, but response is asymmetric. This result demonstrates the strong influence of ion-neutral momentum coupling on the thermospheric wind and the variation in its effectiveness to produce circulation cells depending on the dawn and dusk ion convection cells. Results are quoted which indicate that in winter conditions near the terminator, there is a 200—400 m/s downward field aligned ion flow below the F peak. This is in qualitative agreement with the predictions of the Sheffield ionospheric model (Allen et al., 1984).