Torsional Alfv\'enic Oscillations Discovered in the Magnetic Free Energy During Solar Flares
Abstract
We report the discovery of torsional Alfv\'enic oscillations in solar flares, which modulate the time evolution of the magnetic free energy Ef(t), while the magnetic potential energy Ep(t) is uncorrelated, and the nonpotential energy varies as Enp(t) = Ep + Ef(t). The mean observed time period of the torsional oscillations is Pobs=15.1 3.9 min, the mean field line length is L=13535 Mm, and the mean phase speed is vphase =315 120 km s-1, which we interpret as torsional Alfv\'enic 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 NLFFF codes.