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Helical Twisting Number and Braiding Linkage Number of Solar Coronal Loops View all abstracts by submitter

Markus J. Aschwanden   Submitted: 2019-02-27 08:49

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 Ntwist 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 μNP = 15° ± 3°. The resulting mean rotational twist angle is ϕ = 49° ± 11°, which corresponds to Ntwist=ϕ/360° = 0.14±0.03 turns with respect to the untwisted potential field, with an absolute upper limit of Ntwist ≈ 0.5, which is far below the kink instability limit of |Ntwist| ≳ 1. The number of twist turns Ntwist corresponds to the Gauss linkage number Nlink in braiding topologies. We conclude that any braided topology (with |Nlink| ≥ 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.

Authors: Markus J. Aschwanden
Projects: SDO-HMI

Publication Status: ApJ (in press, February 27, 2019)
Last Modified: 2019-02-27 12:12
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