Fast magnetoacoustic wave trains: from tadpoles to boomerangs

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 localised 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 multi-periodic (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.

CME-Driven and Flare-Ignited Fast Magnetosonic Waves Detected in a Solar Eruption

“…A number of studies theoretically supported that the waveguide-caused dispersion leads to the formation of quasi-periodic wave trains (e.g. Nakariakov et al, 2004;Li et al, 2018;Kolotkov et al, 2021). This view has been confirmed by observations (e.g.…”

Coronal Quasi-periodic Fast-mode Propagating Wave Trains

“…The typical feature of tadpole wavelet spectrum has been used as a characteristic signature for identifying the presence of possible QFP wave trains in both observational and numerical studies, when direct imaging of QFP wave trains in EUV were unavailable (e.g., Mészárosová et al, 2009bMészárosová et al, , 2013Karlický, Jelínek, and Mészárosová, 2011;Karlický, Mészárosová, and Jelínek, 2013;Jelínek, Karlický, and Murawski, 2012;Mészárosová et al, 2014). Recently, Kolotkov et al (2021) modeled the linear dispersively evolving of QFP wave trains in plasma slabs with varying steepness of the transverse density profile, in which they showed that the development of a QFP wave train evolved from an initial impulsive perturbation undergoes three distinct phases fully consistent with that qualitatively predicted by Benz (1983, 1984). In contrast to wave trains in smooth waveguides that produce the tadpole structures (Nakariakov et al, 2004), it is interesting that the wavelet power spectrum develops into a boomerang structure that has two pronounced arms in the longer-and shorter-period parts of the spectrum (see the right column of Figure 12).…”

“…The right column shows the time profile (top) and wavelet power spectrum (bottom) of a fully developed fast sausage wave train in a steep plasma waveguide. The three distinct developing phases of the wave train are indicated in the figure, and the wavelet spectrum shows a boomerang shape (Kolotkov et al, 2021).…”

Impulsively generated kink wave trains in solar coronal slabs

“…These fast wave trains are observed in various wavelength bands in the corona (Williams et al 2001;Li et al 2020, and references therein). They are examined analytically by Roberts et al (1984) and Oliver et al (2015), and numerically by e.g., Nakariakov et al (2004); Yu et al (2017); Kolotkov et al (2021). As demonstrated in e.g., Roberts et al (1984), the temporal signature of axisymmetric wave trains consists of distinct phases when observed sufficiently far from the exciter.…”