Abstract
The question of whether there is sufficient magnetic twist in solar active regions for the onset of the kink instability is examined using a blind test of analysis methods commonly used to interpret observational data. Photospheric magnetograms are constructed from a recently developed numerical simulation of a kink-unstable emerging flux rope with nearly constant (negative) wind. The calculation of the best-fit linear force-free parameter αbest is applied, with the goal of recovering the model input helicity. It is shown that for this simple magnetic structure, three effects combine to produce an underestimation of the known helicity: (1) the influence of horizontal fields with lower local α values within the flux rope; (2) an assumed simple relation between αbest and the winding rate q does not apply to nonaxis fields in a flux rope that is not thin; and (3) the difficulty in interpreting the force-free twist parameter measured for a field that is forced. A different method to evaluate the magnetic twist in active region flux ropes is presented, which is based on evaluating the peak α value at the flux rope axis. When applied to data from the numerical simulation, the twist component of the magnetic helicity is essentially recovered. Both the αbest and the new αpeak methods are then applied to observational photospheric vector magnetic field data of NOAA AR 7201. The αbest approach is then confounded further in NOAA AR 7201 by a distribution of α that contains both signs, as is generally observed in active regions. The result from the proposed αpeak approach suggests that a larger magnetic twist is present in this active region's δ-spot than would have been inferred from αbest, by at least a factor of 3. It is argued that the magnetic fields in localized active region flux ropes may indeed carry greater than 2π winds, and thus the kink instability is a possible trigger mechanism for solar flares and coronal mass ejections.
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