Time signatures of impulsively generated coronal fast wave trains

Abstract

Impulsively generated short‐period fast magneto‐acoustic wave trains, guided by solar and stellar coronal loops, are numerically modelled. In the developed stage of the evolution, the wave trains have a characteristic quasi‐periodic signature. The quasi‐periodicity results from the geometrical dispersion of the guided fast modes, determined by the transverse profile of the loop. A typical feature of the signature is a tadpole wavelet spectrum: a narrow‐spectrum tail precedes a broad‐band head. The instantaneous period of the oscillations in the wave train decreases gradually with time. The period and the spectral amplitude evolution are shown to be determined by the steepness of the transverse density profile and the density contrast ratio in the loop. The propagating wave trains recently discovered with the Solar Eclipse Coronal Imaging System (SECIS) instrument are noted to have similar wavelet spectral features, which strengthens the interpretation of SECIS results as guided fast wave trains.

Radio fiber bursts and fast magnetoacoustic wave trains

“…This can be also seen in Fig Because the wavelet tadpoles are interpreted as signatures of the fast sausage magnetoacoustic wave trains (e.g. Nakariakov et al 2004), the similarity of both the drifts indicates that the fiber bursts are physically connected with these magnetoacoustic wave trains. Based on these results we propose that the analyzed dm-fiber bursts are generated by the fast sausage magnetoacoustic wave which modulates the radio emission of superthermal electrons trapped in a flare loop or in current sheet.…”

“…2. Although a real wave evolution is complex (see Roberts et al 1984;Nakariakov et al 2004, and also the following MHD model), for a simplification we take the velocity of this wave perturbation as the Alfvén speed v A computed along the loop. The ratios of wave perturbations is taken as b z /B 0 = ρ 1 /ρ 0 1, where B 0 and ρ 0 mean the local magnetic field and density in the Aschwanden's models.…”

“…The trains exhibit three phases: (1) periodic phase (long-period spectral components arrive as the first ones at the observation point); (2) quasiperiodic phase (both long-and short-period spectral components arrive and interact); and finally (3) decay phase (Roberts et al 1983, and1984). As shown by Nakariakov et al (2004), wavelet spectra of these trains correspond to the wavelet tadpoles (tail plus head). Detailed numerical simulations of these magnetoacoustic (fast sausage) wave trains and corresponding tadpoles in the density slab, as well as in the current sheet, were performed by .…”

Broadband microwave sub-second pulsations in an expanding coronal loop of the 2011 August 10 flare

“…This characteristic wavelet tadpole pattern consists of the long-period spectral components (tadpole tail with a characteristic period P ) propagating faster than the medium and short-period ones (tadpole head). Numerical simulations of these tadpole patterns can be found in Nakariakov et al (2004) and Mészárosová et al (2014), for instance. Observed fast sausage magnetoacoustic waves with these tadpoles were presented by Katsiyannis et al (2003) and Mészárosová et al (2009a, b;2011a, b;2013).…”

“…Each of these fast sausage magnetoacoustic waves form wave trains that propagate along the waveguide. The time evolution of these trains forms a tadpole wavelet pattern with a narrow tail that precedes a broadband head (Nakariakov et al 2004). The start of the wave decay phase corresponds to the tadpole head maximum.…”