Interpreting Coronal-Loop Oscillations as the Modes of a 2D Waveguide
Abstract:The bright coronal loops that trace bundles of field lines within coronal arcades have been observed to oscillate with one or more preferred frequencies. Recent studies indicate that two distinct categories of oscillation occur: large-amplitude, transient oscillations that are initiated by flares and low-amplitude, “decayless” oscillations that can persist for as long as the loop remains visible. The preferred frequencies of these oscillations have previously been interpreted as the resonant frequencies of MHD fast waves that are trapped between the photospheric footpoints of a bundle of field lines in a 1-D cavity. The nascent field of coronal-loop seismology attempts to deduce loop properties, such as the magnetic-field strength, loop length, etc., by exploiting the information contained in the measured mode periods.
We present an alternative 2D model of the wave cavity whereby the waves can propagate across field lines and the entire magnetic arcade acts as a waveguide. Within this framework, the two types of oscillations, flare-induced waves and decayless oscillations, can both be attributed to MHD fast waves. The two components of the signal differ only because of the duration and spatial extent of the source that creates them. The flare-induced waves are generated by strong, localized sources of short duration, while the decayless background is excited by a diffuse, stochastic source. Further, the oscillatory signal induced by a flare can be interpreted as a pattern of interference fringes produced by waves that are launched from a compact source and have traveled diverse routes of various pathlength through the waveguide. The amplitude of the resulting fringe pattern decays in time without the need for local dissipation mechanisms. The details of the interference pattern depend on the shape of the arcade and on the spatial variation of the Alfvén speed within the arcade. We explore these details with a view to understanding the excitation mechanism of the oscillations and the seismological implications of the interference process.