Fast magnetoacoustic wave trains in magnetic funnels of the solar corona

Abstract

Context. Fast magneto-acoustic waves are highly dispersive in waveguides, so they can generate quasi-periodic wave trains if a localised, impulsive driver is applied. Such wave trains have been observed in the solar corona and may be of use as a seismological tool since they depend upon the plasma structuring perpendicular to the direction of propagation. Aims. We extend existing models of magnetoacoustic waveguides to consider the effects of an expanding magnetic field. The funnel geometry employed includes a field-aligned density structure. Methods. We performed 2D numerical simulations of impulsively generated fast magneto-acoustic perturbations. The effects of the density contrast ratio, density stratification, and spectral profile of the driver upon the excited wave trains were investigated. Results. The density structure acts as a dispersive waveguide for fast magneto-acoustic waves and generates a quasi-periodic wave train similar to previous models. The funnel geometry leads to generating additional wave trains that propagate outside the density structure. These newly discovered wave trains are formed by the leakage of transverse perturbations, but they propagate upwards owing to the refraction caused by the magnetic funnel. Conclusions. The results of our funnel model may be applicable to wave trains observed propagating in the solar corona. They demonstrate similar properties to those found in our simulations.

Observation of a high-quality quasi-periodic rapidly propagating wave train using SDO/AIA

“…These waves also form wave trains (e.g. Sych et al 2012), but propagate along the apparent magnetic field lines at essentially sub-sonic speeds, and their periods are typically longer than 120 s. Pascoe et al (2013) developed an advanced numerical model for the dispersive evolution of fast MHD waves in an expanding magnetic field generated by an impulsive, spatially localised energy release with a field-aligned density structure which produced results consistent with the observations of Yuan et al (2013).…”

“…Pascoe et al (2013) studied the dispersive evolution of fast MHD waves in an expanding magnetic field with a field-aligned density structure. To adapt our modelling to the current observation, we extended the model of Pascoe et al (2013) to a curved coronal loop filled in with enhanced plasma density. The gravitational force is not included in the model.…”

“…The magnetic field used in our model is based on the potential field used by Pascoe et al (2013), but with a horizontal shift of π/2, which places the closed field lines at the centre of the numerical domain rather than the open ones,…”

Spatially resolved observation of the fundamental and second harmonic standing kink modes using SDO/AIA

“…Rather, it is based on the relationship to other periodicities and comparison with those expected for known wave modes when realistic loop and plasma properties are assumed. Zhang et al (2015) reported an observation of a coronal funnel with SDO/AIA which demonstrates fast and slow waves propagating along the same field-aligned path, consistent with numerical simulations of Pascoe et al (2013b; Nisticò et al (2014). Measurement of the propagation speeds allowed the plasma-β to be estimated as approximately 0.01.…”

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

“…Mészárosová et al (2013) found these fast magnetoacoustic waves to propagate in the fan structure of the coronal magnetic null point. These studies were supported by MHD numerical simulations conducted by Jelínek & Karlický (2012), Pascoe et al (2013), and Mészárosová et al (2014), for instance.…”