Three‐dimensional equilibria for the extended magnetotail and the generation of field‐aligned current sheets

Using the magnetotail equilibrium theory and a solution method outlined by Birn (1987), we have constructed self‐consistent three‐dimensional models for the quiet average magnetotail beyond about 20 R E distance but earthward of a potential distant neutral line, which take into account the decrease of the tail flaring with distance. We find that this effect is coupled with the presence of magnetic shear and thus with field‐aligned electric currents. These currents have the signature of region 1 currents, toward the Earth on the dawnside and away on the duskside, and contribute about 5 × 10 5 A to the total Birkeland current. They are strongly concentrated near the plasma sheet‐lobe boundary and increase toward the flanks of the tail. Associated with the field‐aligned currents and the corresponding magnetic field shear there is a bulging effect that tends to deform a circular cross section of the tail near the Earth into one that has bulges in the low‐latitude boundary region. We argue that this effect may be the cause for increased interaction with the solar wind in these regions, producing interconnected fields and tailward flowing plasma on magnetospheric‐like fields in the low‐latitude boundary layer, and deforming this boundary region into the observed dog bone shape of the plasma sheet cross section.