The relation between subauroral polarization streams, westward ion fluxes, and zonal wind: Seasonal and hemispheric variations

Abstract

We investigate the seasonal variation of the relation between SAPS (subauroral polarization streams) peak ion velocity and zonal wind velocity with coordinated DMSP and CHAMP satellite observations in both hemispheres, separately for Kp < 4 and Kp ≥ 4 geomagnetic conditions. Both SAPS and westward zonal wind peak almost at the same latitude. However, SAPS and zonal wind peak velocities show significantly different seasonal variations. SAPS peak velocities are larger in winter as compared to summer, while zonal wind peak velocities have opposite behavior, larger in summer and smaller in winter, which is valid in both hemispheres and for both geomagnetic activity levels. We reveal, for the first time, that SAPS peak westward ion fluxes, the product of SAPS ion velocity and ion density, exhibit similar seasonal variation as the zonal wind. The results indicate that the effect of SAPS on the zonal wind depends strongly on local ion densities. A linear relationship between SAPS ion fluxes and zonal wind is derived for both hemispheres, which is practically independent of geomagnetic activity. The scaling factor between ion flux and wind velocity is about the same in both hemispheres. Conversely, the background westward wind velocity (bias value in linear equation) is by about 100 m/s higher in the Northern than in the Southern Hemisphere. We attribute this difference to the effect of the large offset between geographic and magnetic poles in the Southern Hemisphere.