Auroral spirals, 2. Theory

A theoretical analysis of the magnetic fields surrounding field-aligned current sheets confirms that stable spirallike distortions (or, in some cases, folds) should occur at the free end. It is demonstrated that a Birkeland current sheet is completely analogous to a fluid shear layer, with the magnetic field and the current playing the roles of fluid streamlines and of vorticity, respectively. The rotational sense of the distortions indicates the direction of current. Spirals observed in the aurora imply upward current in all cases where this mechanism applies. The calculated threshold current density for distortion of an auroral arc is 2.5 × 10−6 A/m².