Determination of a proxy for F region meridional neutral winds using in situ satellite ion density measurements

Abstract

A simple parameter has been developed to describe the measured latitudinal profiles of the ion density at 840 km near the magnetic equator. This parameter, which we call the “asymmetry index,” is found to be well correlated with empirical neutral wind model output. Such a correlation makes it possible to use the index as a proxy representing the relative strength and direction of the transequatorial neutral wind. Topside ion densities are measured routinely by in situ sensors aboard Defense Meteorological Satellite Program (DMSP) spacecraft near 0600, 0900, 1800 and 2100 hours local time. DMSP data over the full range of solar activity levels and available local times are considered and compared to model neutral winds. While the shape of the observed profile (ni versus latitude) is controlled both by dynamics and by chemistry and varies as a function of season, longitude, local time, solar cycle, and geomagnetic activity, the neutral wind is clearly a significant factor. The profile asymmetry index is not a direct measurement of the instantaneous wind but rather represents the integrated effects of vertical and interhemispheric plasma transport due to the neutral wind over several hours. The index is shown to duplicate well (data/model correlation coefficient R ∼ 0.8) the longitudinal and seasonal morphology of the neutral wind blowing along the geomagnetic meridian for all available local times during solar maximum conditions, but only near the 0900 and 1800 LT meridians during solar minimum. These results suggest that during these times the meridional neutral wind is the dominant influence on the ion density profile shape. Thus the asymmetry index is a useful parameter for the study of the dynamics of the low‐latitude ionosphere. In particular, such a parameter would be important for use in operational ionospheric models that require a real‐time neutral wind input and in the verification and testing of future thermospheric wind models.