Correlation of the subauroral polarization streams (SAPS) with the Dst index during severe magnetic storms

The subauroral polarization stream (SAPS) is the enhanced poleward electric field/ westward plasma flow in the region equatorward of the auroral electron precipitation boundary in the dusk-midnight sector. An important characteristic of SAPS is its great intensification during magnetic storms. However, the relationship of SAPS characteristics to the strength of magnetic storms is not well known. Using Defense Meteorological Satellite Program (DMSP) satellite measurements, we have examined the location (magnetic latitude) of the SAPS channel during four severe magnetic storms with minimum Dst values between - 271 and - 422 nT. We find that the latitude of the SAPS channel varies directly with the magnitude of Dst. For example, in one storm case, SAPS moved from similar to 60 degrees magnetic latitude at the storm sudden commencement with Dst of near zero to similar to 38 degrees at the end of the storm main phase with Dst of - 422 nT. We have identified more than 300 locations of SAPS and corresponding Dst values during the four storms. The most important finding is that the SAPS latitude is linearly correlated with Dst, and an empirical formula is derived. If the SAPS channel is mapped along the magnetic field lines to the magnetospheric equatorial plane, the mapped SAPS location corresponds to the inner edge of the ion plasma sheet and is close to the peak of the ring current. The result of this paper suggests a promising method of using ionospheric satellite and radar measurements of SAPS to monitor the evolution of the plasma sheet inner boundary and ring current during magnetic storms.