Helical Twisting Number and Braiding Linkage Number of Solar Coronal Loops 

Markus J. Aschwanden Submitted: 20190227 08:49
Coronal loops in active regions are often characterized by
quasicircular and helically twisted (sigmoidal) geometries,
which are consistent with dipolar potential field models in
the former case, and with nonlinear forcefree field models
with vertical currents in the latter case. Alternatively,
Parkertype 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 verticalcurrent approximation of a nonpotential
magnetic field solution (that fulfills the divergencefree and
forcefree 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 μ_{NP} = 15° ±
3°. The resulting mean rotational twist angle is
ϕ = 49° ± 11°, which corresponds to
N_{twist}=ϕ/360° = 0.14±0.03 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} ≳ 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} ≥ 1) cannot explain
the observed stability of loops in a forcefree corona, nor
the observed low twist number. Parkertype nanoflaring can
thus occur in nonforcefree environments only, such as in the
chromosphere and transition region.
Authors: Markus J. Aschwanden
Projects: SDOHMI

Publication Status: ApJ (in press, February 27, 2019)
Last Modified: 20190227 12:12


