Wednesday

Rapidly propagating fast magnetoacoustic wave trains, also known as quasi-periodic fast-propagating (QFP) waves, are confidently observed at different heights of the Sun’s corona. Observations at large heights suggest that fast wave trains can travel long distances from regions of the primary energy releases exciting them. We study characteristic time signatures of fully developed fast magnetoacoustic wave trains guided along the magnetic field by dense plasma slabs in the linear regime. Fast wave trains are modelled as an impulsively excited ensemble of harmonic waves, whose propagation along the waveguide is governed by the waveguide-caused dispersion. With the increase in the slab transverse density steepness, the wave trains are shown to develop three distinct phases of their time profiles: a long-period quasi-periodic phase with the oscillation period gradually shortening with time, a multi-periodic (peloton) phase in which the harmonics with distinctly different periods co-exist, and a short-lived periodic (Airy) phase. The appearance of these phases is attributed to a non-monotonic dependence of the fast wave group speed on the parallel wavenumber, and is shown to be different for axisymmetric (sausage) and non-axisymmetric (kink) modes. In the Morlet wavelet spectra, this evolution corresponds to the transition from the previously known tadpole-shaped to a boomerang-shaped structure, with two well-pronounced arms at shorter and longer periods. We describe a specific previously published observation of a coronal fast wave train in the radio band, manifesting the change of the wavelet spectrum shape from a tadpole to a boomerang, which is fully consistent with our modelling. The availability of such a direct observational confirmation indicates the applicability of boomerang-shaped fast wave trains for seismological probing of the transverse structuring of the waveguiding coronal plasma structures.