Impulsively Generated Wave Trains in Coronal Structures. I. Effects of Transverse Structuring on Sausage Waves in Pressureless Tubes

2017

ApJL

Quasi-periodic rapidly propagating wave trains are frequently observed in extreme ultraviolet observations of the solar corona, or are inferred by the quasi-periodic modulation of radio emission. The dispersive nature of fast magnetohydrodynamic waves in coronal structures provides a robust mechanism to explain the detected quasiperiodic patterns. We perform 2D numerical simulations of impulsively generated wave trains in coronal plasma slabs and investigate how the behavior of the trapped and leaky components depend on the properties of the initial perturbation. For large amplitude compressive perturbations, the geometrical dispersion associated with the waveguide suppresses the nonlinear steepening for the trapped wave train. The wave train formed by the leaky components does not experience dispersion once it leaves the waveguide and so can steepen and form shocks. The mechanism we consider can lead to the formation of multiple shock fronts by a single, large amplitude, impulsive event and so can account for quasi-periodic features observed in radio spectra.

Section: Introductionsupporting

confidence: 69%

Dispersive Evolution of Nonlinear Fast Magnetoacoustic Wave Trains

Pascoe

Goddard

2017

ApJL

“…For example, the detected characteristic timescale of about one second occurs in the dispersive formation of quasi-periodic fast magnetoacoustic wave trains (e.g., Roberts et al 1984;Nakariakov et al 2004). Theoretical modeling demonstrated that the mean period is approximately equal to the width of the fast magnetoacoustic waveguide, e.g., the minor radius of the loop or width of the current sheet (e.g., Shestov et al 2015;Yu et al 2017, for recent results). Thus, for a characteristic length scale of 1500km, which is a typical width of an EUV loop, and a typical coronal active region fast speed of 1000km s −1 , the oscillation period is about the detected values of the quasi-periods.…”

Section: Discussionmentioning

confidence: 98%

Quasi-periodic Pulsations in a Solar Microflare

Anfinogentov

Storozhenko

et al. 2018

ApJ

Irregular time evolution of the radio emission generated in a B2-class microflare (SOL2017-01-25T10:15), occurring on 2017 January 25 in active region 12,628, is studied. The microflare was apparently initiated by an appearance of an s-shaped loop, observed in the EUV band. The radio emission is associated with the nonthermal electrons detected with Ramaty High Energy Solar Spectroscopic Imager, and originates simultaneously from two opposite footpoints of a magnetic fan structure beginning at a sunspot. According to the active region geometry, the footpoints are situated in the meridional direction, and hence are observed by RATAN-600 simultaneously. The radio emission intensity signal, as well as the left-hand and right-hand circular polarization signals in the lowfrequency band (3-4GHz) show good correlation with each other, with the average characteristic time of the variation 1.4±0.3s. The polarization signal shows a time variation with the characteristic time of about 0.7±0.2s. The irregular quasi-periodic pulsations of the radio emission are likely to be caused by the superposition of the signals generated at the local electron plasma frequencies by the interaction of nonthermal electrons with the plasma at the footpoints. In this scenario, the precipitation rate of the nonthermal electrons at the opposite footpoints could be modulated by the superposition of fundamental and second harmonic modes of sausage oscillations, resulting in the observed different characteristic times of the intensity and polarization signals. However, other mechanisms, e.g., the oscillatory regime of loop coalescence or magnetic null point oscillation could not be rigorously excluded.

“…This dependence on the driver duration is extremely important in attempts to unlock the seismological potential of these waves. Varying other parameters such as the spatial scale of the driver, and the transverse density profile of the waveguide can have similar effects on the final wave train signature (e.g., Nakariakov et al 2005;Pascoe et al 2013Pascoe et al , 2017Yu et al 2017). Therefore an effort to clearly distinguish these factors in both theoretical and observational studies should be made.…”

Section: Discussionmentioning

confidence: 99%

“…Since then wavelet analysis has been frequently used to visualise the obtained wave train signatures in both observational and numerical studies. Further theoretical studies have focused on different perpendicular density profiles of the waveguide (e.g., Yu et al 2017;Li et al 2018b), wave train formation in current sheets (e.g., Jelínek & Karlický 2012;Mészárosová et al 2014), accounting for 2D and cylindrical effects (Pascoe et al 2013;Shestov et al 2015), amplitudes entering the non-linear regime (Pascoe et al 2017), and analytical estimations (e.g., Oliver et al 2015).…”

Section: Introductionmentioning

confidence: 99%

Fast magnetoacoustic wave trains with time-dependent drivers

Goddard

Pascoe

2019

A&A

Context. Frequent observations of quasi-periodic rapidly-propagating wave trains in coronal structures have been made in the last decade. The dispersive evolution of fast magnetohydrodynamic waves propagating in coronal waveguides can provide a physical interpretation for many of these observations. Aims. Previous studies have considered the generation of fast wave trains by impulsive drivers which deposit energy instantaneously. The signatures of dispersively formed wave trains must depend on the temporal nature of the driver. We investigate the effect of varying the temporal width of the driving perturbation. Methods. 2D magnetohydrodynamic numerical simulations of impulsively generated wave trains in a guiding field-aligned density enhancement were performed with the novel addition of a time-dependant driver. Results. The final spatial and spectral signatures of the guided wave trains are found to depend strongly on the temporal duration of the initial perturbation. In particular, the wavelength (or frequency) of highest spectral amplitude is found to increase (decrease) with increasing temporal duration, whereas the spectral width decreases. Additionally, the efficiency of generation of fast wave trains is found to decrease strongly with increasing temporal width of the driver, with a cut-off at twice the internal Alfvén crossing time.