Is Non-Neutralized Electric Current a Projection Effect?

Abstract:

Active regions often possess an observed net electric current in a single photospheric magnetic polarity. We show how such "non-neutralized" current can arise from a simple projection effect. To this end, we emulate flux emergence by kinematically lifting a twisted, semi-torus flux tube across the solar surface. Although the isolated tube has no net toroidal (axial) current, its poloidal (azimuthal) current, when projected to the vertical direction, amounts to a significant non-neutralized component. This requires the tube axis to be trapped at low altitudes: a realistic and critical condition overlooked by early theoretical arguments. The projection effect leads to a non-neutralized current that 1) develops near the sheared polarity inversion line, 2) increases and plateaus in sync with the magnetic flux, and 3) positively correlates with the twist. All are consistent with recent observations. We discuss the implications on models for solar flux emergence and eruption.