Abstract
ABSTRACT Rapidly propagating fast magnetoacoustic wave trains guided by field-aligned plasma non-uniformities are confidently observed in the Sun’s corona. Observations at large heights suggest that fast wave trains can travel long distances from the excitation locations. We study characteristic time signatures of fully developed, dispersive fast magnetoacoustic wave trains in field-aligned zero-β plasma slabs in the linear regime. Fast wave trains are excited by a spatially localized impulsive driver and propagate along the waveguide as prescribed by the waveguide-caused dispersion. In slabs with steeper transverse density profiles, developed wave trains are shown to consist of three distinct phases: a long-period quasi-periodic phase with the oscillation period shortening with time, a multiperiodic (peloton) phase in which 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 due to the waveguide dispersion, and is shown to be different for axisymmetric (sausage) and non-axisymmetric (kink) modes. In wavelet analysis, this corresponds to the transition from the previously known tadpole shape to a new boomerang shape of the wave train spectrum, with two well-pronounced arms at shorter and longer periods. We describe a specific previously published radio observation of a coronal fast wave train, highly suggestive of a change of the wavelet spectrum from a tadpole to a boomerang, broadly consistent with our modelling. The applicability of these boomerang-shaped fast wave trains for probing the transverse structuring of the waveguiding coronal plasma is discussed.
Highlights
The highly filamented nature of the plasma of the solar corona plays a crucial role in magnetohydrodynamic (MHD) wave processes ubiquitously observed in the corona with spaceborne and ground-based instruments in multiple bands (e.g., Roberts 2000)
We present the first quantitative demonstration of the formation of a fast wave train consisting of these three distinct phases
In a smooth waveguide with p = 1, the wave group speed is a monotonic function of the parallel wavenumber k z as shown by Fig. 1, so that each parallel harmonic k z excited by the initial perturbation propagates at its own speed
Summary
The highly filamented nature of the plasma of the solar corona plays a crucial role in magnetohydrodynamic (MHD) wave processes ubiquitously observed in the corona with spaceborne and ground-based instruments in multiple bands (e.g., Roberts 2000). Pascoe et al (2017b) demonstrated that the accumulation of nonlinear effects in the dynamics of the fast wave trains trapped in coronal plasma slabs is highly inhibited by strong waveguide dispersion, so that even for the relative perturbation amplitudes of several tens of percent the nonlinear steepening of the wave does not occur. The latter result clearly justifies the applicability of the linear MHD theory to modelling fast wave trains in coronal plasma structures.
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