Department of Geophysics, Faculty of Science, Kyoto University , Kyoto

The interplanetary magnetic field (IMF) B_y-dependent distribution of field-aligned currents in the cleft region is studied, using the magnetic field and plasma data from 47 passes of Dynamics Explorer (DE) 2. These orbits were chosen on the conditions that cusp/cleft particles are detected and that at the same time the IMF B_y and B_z components satisfy the criteria |B_y| ³ 5 nT and |B_z| £ 5 nT during the satellite's crossing of the relevant field-aligned current region. When B_y is positive (negative) in addition to satisfying these conditions, there is a strong eastward (westward) magnetic perturbation caused by a pair of field-aligned current sheets, consisting of an equatorward sheet with downward (upward) current and a poleward sheet having upward (downward) current. These B_y-dependent field-aligned currents in the equatorward and the poleward sheets are referred to as the low-latitude cleft current (LCC) and the high-latitude cleft current (HCC), respectively. The cusp/cleft electron precipitation region and the LCC region overlap with each other to a varying degree irrespective of the sign of B_y. For positive (negative) B_y, LCC has the same direction as the morning (afternoon) region 1 current or the afternoon (morning) region 2 current. Thus an interpretation has been given in the past that the LCC region is an extension of the region 1 or region 2 current system. However, in this paper we present an alternative view that the LCC region is not an extension of the region 1 or region 2 current system and that a pair of LCC and HCC constitutes the cleft field-aligned current regime. The proposed pair of cleft field-aligned currents is explained with a qualitative model in which this pair of currents is generated on the open field lines that have just been reconnected on the dayside magnetopause. The model assumes a quasi-steady reconnection operating within certain longitudinal width extending to both sides of the stagnation point on the dayside magnetopause. The reconnected flux tubes move under the influences of the field tension and the magnetosheath flow. When the magnetosheath B_y is positive, the northern hemisphere field lines reconnected on the eastward side of the stagnation point are pulled toward high latitudes, and the field lines reconnected on the westward side of the stagnation point are pulled along the dawnside magnetopause flank. The electric fields associated with these motions are present immediately inside the magnetopause (rotational discontinuity). This is the source region of LCC and HCC. The electric fields are transmitted along the field lines to the ionosphere, creating a poleward electric field and a pair of field-aligned currents when B_y is positive; the pair of field-aligned currents consists of a downward current at lower latitudes (LCC) and an upward current at higher latitudes (HCC). In the B_y negative case, the model explains the reversal of the field-aligned current direction in the LCC and HCC regions.

J. Geophys. Res., 98, No. A2, 1393-1407, Feb. 1993