Helical Twisting Number and Braiding Linkage Number of Solar Coronal Loops

Abstract

Coronal loops in active regions are often characterized by quasi-circular and helically twisted (sigmoidal) geometries, which are consistent with dipolar potential field models in the former case, and with nonlinear force-free field models with vertical currents in the latter case. Alternatively, Parker-type nanoflare models of the solar corona hypothesize that a braiding mechanism operates between unresolved loop strands, which is a more complex topological model. In this study we use the vertical-current approximation of a nonpotential magnetic field solution (that fulfills the divergence-free and force-free conditions) to characterize the number of helical turns $N_{twist}$ in twisted coronal loops. We measure the helical twist in 15 active regions observed with AIA and HMI/SDO and find a mean nonpotentiality angle (between the potential and nonpotential field directions) of $\mu_{NP} = 15^\circ \pm 3^\circ$. The resulting mean rotational twist angle is $\varphi = 49^\circ \pm 11^\circ$, which corresponds to $N_{twist}=\varphi/360^\circ = 0.14\pm0.03$ turns with respect to the untwisted potential field, with an absolute upper limit of $N_{twist} \lapprox 0.5$, which is far below the kink instability limit of $|N_{twist}| \gapprox 1$. The number of twist turns $N_{twist}$ corresponds to the Gauss linkage number $N_{link}$ in braiding topologies. We conclude that any braided topology (with $|N_{link}| \ge 1$) cannot explain the observed stability of loops in a force-free corona, nor the observed low twist number. Parker-type nanoflaring can thus occur in non-forcefree environments only, such as in the chromosphere and transition region.