Motion of near-Earth neutral line in substorm recovery phase

A primary characteristic of the plasma sheet boundary layer(PSBL) is the presence of high speed field-aligned ion beams with abrupt low-speed cutoffs. Onsager et al.[\textit{JGR,96},20,999,1991] proposed a model of the formation of the PSBL which assumed a steady spatially extended source of plasma, together with steady equatorward ExB convection due to reconnection at a downtail neutral line. Elphic et al. [\textit{JGR,100},1857,1995] applied this model to ISEE 2 observations to estimate the downtail distance of the neutral line. The estimated locations were more than 65 R_E downtail from the Earth, which seem to correspond to the distant neutral line(DNL). In this study, we estimated the distance of a near-Earth neutral line(NENL), which is inferred to be formed during substorms, by applying the velocity filter effect model of plasma sheet boundary layer(PSBL). The method was applied to counter-streaming ion beams observed by Geotail during the substorm event at 11:02 UT on Octorber 5, 2000. In the best case,we can estimate the downtail distance of the NENL with an accuracy of 10%. The estimated values range from 20 to 55 R_E downtail, and increased with the total pressure decrease. This is consistent with previous statistical studies [\textit{Nagai et al.,JGR,101},4419,1998;\textit{Miyashita et al.,JGR,108},1353,2003]. In contrast to the past studies, however, our approach allows us to monitor the distance of the NENL continuously and to address its dynamics in the course of substorms.