Authors: Brian T. Welsch (Univ. of Wisc. - Green Bay), Yang Liu (Stanford Univ.)
Electric currents flowing in coronal magnetic fields store the energy that is released in solar flares and coronal mass ejections (CMEs). Little is known, however, about the structure and development of electric currents in the near-surface solar interior and solar atmosphere, because vector magnetic fields have only been routinely and reliably observed at the photosphere. Does the emergence of current carrying fields play a role in energy build up prior to flares & CMEs? How do currents change as result of flares & CMEs? Applying Gauss’s separation method to observed photospheric magnetic fields shows promise for addressing these and related questions. This procedure decomposes a 2D map of a three-component magnetic field, B(x,y), into three components, based upon the location of each component’s source currents: (i) B<(x,y), due to currents flowing in the interior, (ii) Btoroidal(x,y), due to currents flowing across the photosphere, and (iii) B>(x,y), due to currents flowing in the solar atmosphere. We refer to B<(x,y) and B>(x,y) as the photospheric imprints of interior and exterior currents, respectively. Because strong flares and CMEs originate along polarity inversion lines (PILs, where the vertical photospheric field changes sign), applying Gauss’s method there can provide especially useful insights about evolution of currents near flares and CMEs. These considerations motivate us to analyze evolution in the near-PIL structure of B<(x,y) and B>(x,y) around the times of strong (> X-class) eruptive solar flares. For AR 11158, over a time range from a few hours preceding the eruption to just after it, we find significant increases in both (i) area-averaged, cross-PIL gradients of both B<z and B>z , and (ii) area-averaged amplitudes of extrema in the components of B<h and B>h perpendicular to the PIL. For both interior and exterior fields, these changes are consistent with increased horizontal electric currents near the photosphere. Such increases might occur via the emergence of additional current-carrying fields from the interior, or via converging motions that cause intensification of horizontal currents already present.