Relationship between Large-, Meso-, and Small-Scale Field-Aligned Currents and their Current Carriers

Abstract

Carriers of the dayside large-scale field-aligned currents (FACs) are discussed. Since the gyro-radius of the current carriers are smaller than the size of small-scale FACs (a pair of upward and downward FACs associated with inverted-V potential structure as shown in Figure 1), the current carriers of large-scale FAC could be controlled by small-scale (and hence meso-scale) FACs. We restrict the discussion to only a few regions. (1) Although the current carries are electrons in most cases, the framework of the large-scale FAC system is sometimes determined by positive ions, especially in the cusp. (2) There is a dawn-dusk asymmetry in the relationship between the large-scale FACs and the current carriers. A substantial fraction of the Region-1 FAC is probably composed of many small-scale paired FACs which are associated with the inverted-V structure (see Figure 1), whereas the Region-2 FAC is carried by CPS electrons in the morning sector and by thermal electrons in the afternoon sector. (3) Meso-scale FAC is formed by individual ion injections, but it has no relation to the large-scale FACs even inside the cusp region. Thus the large-scale cusp FACs (and the cusp itself) are formed by a steady mechanism but not by the meso-scale injections such is FIEs. This rules out the FTE cusp model.