Torsional slow-mode oscillations discovered in the magnetic free energy during solar flares
Markus J Aschwanden Submitted: 2020-01-27 15:08
We report the discovery of torsional Alfvénic oscillations in
solar flares, which modulate the time evolution of the magnetic free
energy E_f(t), while the magnetic potential energy E_p(t) is
uncorrelated, and the nonpotential energy varies as
Enp(t) = E_p + E_f(t). The mean observed time period of the
torsional oscillations is Pobs=15.1 ± 3.9 min, the mean field
line length is L=135±35 Mm, and the mean phase speed is vphase
=315 ± 120 km s-1, which we interpret as torsional
Alfvénic waves in flare loops with enhanced electron densities.
Most of the torsional oscillations are found to be decay-less,
but exhibit a positive or negative trend in the evolution of the
free energy, indicating new emerging flux (if positive),
magnetic cancellation, or flare energy dissipation (if negative).
The time evolution of the free energy has been calculated
in this study with the Vertical-Current Approximation
(Version 4) Nonlinear Force-Free Field (VCA4-NLFFF) code,
which incorporates automatically detected coronal loops in
the solution and bypasses the non-forcefreeness of the
photospheric boundary condition, in contrast to traditional
Authors: Aschwanden,M.J. and Wang,T.J.
Publication Status: ApJ (2020, Jan 27; in press)
Last Modified: 2020-01-29 13:13