Occurrence characteristics of subauroral westward plasma flows and lowest speed threshold of SAPS observed by the SuperDARN Hokkaido HF radar

Abstract:

Westward rapid plasma flows in the ionosphere at subauroral latitudes are called “Sub-Auroral Polarization Stream (SAPS) [Foster and Burke, 2002]”. SAPS is a manifestation of the Magnetosphere-Ionosphere (M-I) coupling. Therefore, it is important to know occurrence characteristics of SAPS in order to understand the details of M-I coupling system. As a result of the present analysis of SAPS using the SuperDARN Hokkaido HF radar, Kataoka et al. [2009] reported that positions of SAPS shifts toward lower latitude with decreasing Dst index. We investigate the characteristics of SAPS, with focus on the relationship between occurrence characteristics of SAPS and a variety of solar wind and geomagnetic parameters, using the SuperDARN Hokkaido HF radar with a field of view covering Far East Russia, which has been in operation since 2006. In particular, we identify the lowest limit of SAPS speed, which has not been discussed in the previous literatures. This is to examine the lowest threshold of electric field to generate SAPS as a result of M-I coupling. In order to investigate SAPS occurrence characteristics comprehensively, we analyzed events with wider ranges of velocity and MLAT than those in the previous studies. As a result of statistical analysis, we found two categories of westward flows that were reasonably separated with a speed threshold of ~150-200 m/s. For the faster flows above the threshold there is a clear correlation between MLAT and Dst index, whereas for the slower flows there is no such correlation. Similar correlation is found for MLT and AL index as well. The faster flows are considered to be SAPS, whereas the slower flows are probably associated with midlatitude F-region ionospheric irregularities not directly related to storms / substorms.