Effect of subauroral polarization streams on the thermosphere: A statistical study

Using 2 years of coordinated CHAMP and DMSP observations we have investigated for the first time the relationship between subauroral polarization streams (SAPS), ionospheric Hall current (electrojet), upper thermospheric zonal wind, and mass density at subauroral regions in the dusk and premidnight sectors, separately for both hemispheres. For comparison, we have also analyzed the same parameters as a function of magnetic latitude (30°–80° magnetic latitude) during non‐SAPS periods. During periods of non‐SAPS, the neutral wind exhibits similar features as during SAPS events in the dusk to premidnight sector, streaming westward in the same direction as the plasma drift. Both neutral and plasma velocities peak at the same latitude regardless of SAPS occurrence. For higher geomagnetic activity both velocities are faster and the peaks shift equatorward. During non‐SAPS periods, the ratio between plasma and neutral wind velocity is on average 2.75 ± 0.4 in both hemispheres irrespective of geomagnetic activity. The neutral wind during SAPS events gets enhanced by a factor of 1.5/1.2 for Kp < 4 and 1.3/1.9 for Kp ≥ 4 in the Northern/Southern Hemisphere, respectively, as compared to non‐SAPS time. The velocity difference between SAPS and neutral wind is also larger during SAPS period than during non‐SAPS period, and the difference tends to increase with increasing geomagnetic activity. The peak latitude of the eastward auroral electrojet appears 1.5° poleward of the plasma drift during SAPS events, confirming the formation of SAPS equatorward of the high‐conductivity channel. These SAPS‐induced large winds can heat the upper thermosphere. As a result we observe a 10% enhanced mass density at 400 km altitude with respect to periods without SAPS. In addition a density anomaly peak occurs collocated with the SAPS, displaced from the electrojet peak. We regard this as an indication for efficient thermospheric heating by ion neutral friction.