On the limitation of the current sheet approximation in estimation of the northward Bz associated field‐aligned currents

Abstract

This paper calls attention to the limitation of the current sheet approximation, which has commonly been used in deducing the field‐aligned currents (FACs) from the field‐perpendicular components of magnetic field disturbances detected from low‐altitude/midaltitude (below a few Earth radii) satellites. We focus our study on the current system involved in the northward interplanetary magnetic field associated FAC (so‐called northward Bz (NBZ)). Assuming a numerical model for the total system of the region 1, region 2, and NBZ FACs, plus the associated ionospheric currents, the dusk‐to‐dawn and sunward components of the resulting magnetic disturbance δBx and δBy are calculated. In contrast to the commonly adopted hypothesis that the field‐perpendicular magnetic disturbances as observed from the low‐altitude (a few hundred kilometers) satellites are attributed primarily to the FACs, it is found that the ionospheric currents can make a significant contribution to those disturbances. Particularly, in the central part of the region of NBZ FACs, δBx from the ionospheric currents dominates δBx from the FACs. Next, for a simple discussion on the limit of the current sheet approximation we consider the magnetic disturbances at altitudes higher than several hundred kilometers where the contribution (to δBy) from the ionospheric currents can be neglected. It is found that in general, the FAC intensities based on the current sheet model are significantly different from those actually given in the FAC distribution. Specific points to be noted are as follows: (1) On the dusk‐dawn lines passing the central part of the NBZ region the intensity of NBZ under the current sheet approximation is considerably smaller than the actual value, and the intensity ratio between region 1 and NBZ can be overestimated, by >50%, under that approximation. (2) On the dusk‐dawn lines near the edges (but just inside) of the NBZ region, the current sheet model breaks down even qualitatively; the current sheet model cannot infer the right locations of region 1 and NBZ FACs. (3) On the dusk‐dawn lines just outside of the NBZ region the so‐called “W”‐shaped profile in δBy, which can commonly be interpreted as a signature of the presence of NBZ FACs, emerges in the region where actually no NBZ FAC exists.