Authors: Brian T. Welsch (UW-Green Bay), Peter W. Schuck (NASA-GSFC), Mark G. Linton (NRL-DC)

Flares and coronal mass ejections (CMEs) are powered by magnetic energy stored in coronal electric currents. Since photospheric vector magnetograms are the only routine measurements of the Sun’s vector magnetic field currently available, the nature of coronal electric currents is often inferred indirectly from magnetograms.  The horizontal components of the magnetogram’s vector field (the components tangent to the photosphere) can be decomposed into toroidal and poloidal components. The toroidal component is related via Ampère’s law to vertical electric currents, i.e., currents flowing across the photosphere.   The poloidal component is due entirely to currents flowing above or below the photosphere, which we approximate as a zero-thickness plane. The vertical component of the magnetogram’s vector field (the component perpendicular to the photosphere) is due entirely to horizontal currents flowing above or below the photosphere.  By applying Gauss’s separation method, introduced into the solar physics literature by Schuck et al. (2022), to the photospheric field’s poloidal and vertical components, the portion of these fields due to exterior currents (z > 0), denoted B^>(x,y), can be unambiguously distinguished from the portion due to interior currents (z < 0), denoted B^<(x,y). We use the short hand “coronal currents” to refer to the sources of B^>, though currents in the chromosphere and transition region also contribute to B^>. In some flare-productive active regions, we find that B^> exhibits large-scale (tens of Mm), spatially coherent structures that are consistent with significant horizontal coronal currents above sheared-field polarity inversion lines (PILs).  We refer to these structures in B^> as the photospheric imprints of coronal currents.   We also analyze changes in toroidal fields and photospheric imprints associated with the X2.2 flare of 2011/02/15 in AR 11158.  The most clear flare-driven change is an increase in the field’s toroidal component, implying intensification of photospheric currents as a result of magnetic relaxation in this flare – a possibly counterintuitive finding.