Quasi-parallel electron beams and their possible application in inferring the auroral arc's root in the magnetosphere

Abstract. In this study we investigate the upgoing electron beams at the topside ionosphere and their counterpart feature, the bidirectional quasi-parallel electron beams (QPEB) in the equatorial magnetosphere, with highlight on their potential application in estimating the location of the arc's root (AR) in the magnetotail central plasma sheet (CPS). We infer from FAST data that the upgoing electron beam is often found in the equatorward vicinity of the inverted-V arc. On the premise of such a scenario, we propose a method to estimate the location of the AR from available magnetospheric measurements by assuming that the tailward boundary of the QPEB demarcates the earthward boundary of the AR. We report two events with THEMIS observations of QPEBs in the magnetotail CPS, and demonstrate how to use the QPEB features, together with the magnetic signatures of the current circuit constituted by the QPEB and arc, to estimate the earthward boundary of the AR. We find that the estimated earthward boundary of AR is situated at the periphery of a quasi-dipolar magnetosphere characterized by a strong B_z gradient. This finding is consistent with previously existing proposals on the possible AR location in the tail (e.g., Lui and Burrows, 1978; Sergeev et al., 2012).