Non-Neutralized Electric Current of Active Regions Explained as a Projection Effect

TL;DR

This study demonstrates that observed non-neutralized electric currents in active regions can be explained as a geometric projection effect of twisted flux tubes, especially during early flux emergence stages.

Contribution

It reveals that projection effects can produce apparent non-neutralized currents in active regions, challenging previous interpretations of these currents as intrinsic.

Findings

Non-neutralized currents can result from projection effects of twisted flux tubes.

The non-neutralized current correlates with the flux tube's twist and peaks before magnetic flux.

Projection effects are significant during early flux emergence stages.

Abstract

Active regions (ARs) often possess an observed net electric current in a single magnetic polarity. We show that such "non-neutralized" currents can arise from a geometric projection effect when a twisted flux tube obliquely intersects the photosphere. To this end, we emulate surface maps of an emerging AR by sampling horizontal slices of a semi-torus flux tube at various heights. Although the tube has no net toroidal current, its poloidal current, when projected along the vertical direction, amounts to a significant non-neutralized component on the surface. If the tube emerges only partially as in realistic settings, the non-neutralized current will 1) develop as double ribbons near the sheared polarity inversion line, (2) positively correlate with the twist, and 3) reach its maximum before the magnetic flux. The projection effect may be important to the photospheric current distribution, in particular during the early stages of flux emergence.