Where Is Region 1 Field-Aligned Current Generated?

摘要

We investigate generation regions of the Region 1 field-aligned currents (FACs) by using global magnetohydrodynamics (MHD) simulation for steady and purely southward interplanetary magnetic field (IMF). Unlike conventional means, we consider Alfvén traveling time and background motion of plasma. We trace “packets,” which are supposed to carry the perturbations associated with FACs, backward in time from the ionosphere. The following criteria are used to identify generation regions. (a) Conversion between field-aligned and field-perpendicular currents takes place (as required from current continuity). (b) Plasma performs negative work against the magnetic tension force (as required from energy conservation). (c) Rate of change in FACs is nonzero (as required from Ampère's and Faraday's laws). Flank (low-latitude) magnetopause is identified to be a major generator region, in which solar wind-originated plasma pulls the newly reconnected magnetic field lines. Magnetosphere-originated plasma, which is indirectly pulled by the solar wind-originated plasma, also participates in the generation. The involvement of the reconnection can explain the general dependence on the southward IMF. The generation of Region 1 FACs is associated with field-aligned gradient of space charge ?_(||)(??E), where E is the electric field, but the contribution from the term (?~2E)|| is also significant because of curved magnetic field lines. The flank generator can reasonably explain the latitudinal and the longitudinal extensions of the Region 1 FACs because of the finite travel time of the Alfvén waves under convective motion. The generation region is located apart from the instantaneous magnetic field lines extending from the ionosphere.