Slow Magnetoacoustic Waves in Smoothly Nonuniform Coronal Plasma Structures

Numerical simulations of a propagating slow magnetoacoustic wave guided by a field-aligned low- β plasma nonuniformity are performed in terms of ideal magnetohydrodynamics, aiming at modeling propagating extreme ultraviolet (EUV) emission disturbances observed in the solar corona. The perpendicular profiles of the equilibrium density and temperature are smoothly nonuniform, resulting in smoothly nonuniform profiles of the sound and tube speeds. It is found that an initially plane wavefront perpendicular to the magnetic field experiences a growing deformation with the distance from the driver. The segments of the wavefront located at higher sound speed regions propagate along the field faster. This results in progressively increasing phase mixing. At some distance from the wave driver, at a certain perpendicular cross-section of the nonuniformity, there are opposite phases of the wave. As local perpendicular phase and group speeds are opposite to each other, the slow wave energy tends towards regions of the higher local sound speed. This effect increases with the increase in the plasma- β . Thus, plasma nonuniformities with temperature decreases are slow magnetoacoustic anti-waveguides, while those with temperature increases are waveguides. In the optically thin radiation regime, typical for the EUV emission from the solar corona, phase mixing of slow waves leads to apparent damping of the waves. This damping is not connected with any dissipative process, and is caused by the destructive interference of slow perturbations with different phases, integrated along the line of sight. The apparent damping depends on the combination of magnetic-field strengths, plasma- β , and viewing angles. This effect could be responsible for nonsystematic dependencies of the damping length upon the oscillation periods and the plasma temperature, appearing in observations.