Contribution of Field Strength Gradients to the Net Vertical Current of Active Regions

Abstract

We examined the contribution of fi eld strength gradients for the degree of net vertical current ( NVC ) neutralization in active regions ( ARs ) . We used photospheric vector magnetic fi eld observations of AR 11158 obtained by Helioseismic and Magnetic Imager on board SDO  and Hinode . The vertical component of the electric current is decomposed into twist and shear terms. The NVC exhibits systematic evolution owing to the presence of the sheared polarity inversion line between rotating and shearing magnetic regions. We found that the sign of shear current distribution is opposite in dominant pixels ( 60% – 65% ) to that of twist current distribution, and its time pro fi le bears no systematic trend. This result indicates that the gradient of magnetic fi eld strength contributes to an opposite signed, though smaller in magnitude, current to that contributed by the magnetic fi eld direction in the vertical component of the current. Consequently, the net value of the shear current is negative in both polarity regions, which when added to the net twist current reduces the direct current value in the north ( B z > 0 ) polarity, resulting in a higher degree of NVC neutralization. We conjecture that the observed opposite signs of shear and twist currents are an indication, according to Parker, that the direct volume currents of fl ux tubes are canceled by their return currents, which are contributed by fi eld strength gradients. Furthermore, with the increase of spatial resolution, we found higher values of twist, shear current distributions. However, the resolution effect is more useful in resolving the fi eld strength gradients, and therefore suggests more contribution from shear current for the degree of NVC neutralization.