On the subphotospheric origin of coronal electric currents

Using photospheric vector magnetograms from the Haleakala Stokes Polarimeter and coronal X-ray images from the Yohkoh Soft X-Ray Telescope (SXT), we infer values of the force-free held parameter a at both photospheric and coronal levels within 140 active regions. We determine the value of a for a linear force-free field that best fits each magnetogram in a least-squares sense. We average values from all available magnetograms to obtain a single mean photospheric alpha-value [alpha(p)] for each active region. From the SXT images we estimate a in the corona by determining (pi/L) sin gamma for individual loops, where gamma is the observed shear angle of X-ray loops of length L. We then average these values of alpha to obtain a single coronal a value, [alpha(c)], for each active region. In active regions for which the photospheric alpha-map is predominantly of one sign, we find that the values of [alpha(p)] and [alpha(c)] are well correlated. Only for active regions in which both signs of a are well represented, and in which our method of analysis therefore breaks down, are the values of [alpha(p)] and [alpha(c)] poorly correlated. The former correlation implies that coronal electric currents typically extend down to at least the photosphere. However, other studies imply subphotospheric origin of the currents, and even current systems, that are observed in the photosphere. We therefore conclude that the currents responsible for sinuous coronal structures are of subphotospheric origin.