Nonlinear Effects in the Fast-Mode MHD Wave Propagation in the Solar Coronal Plasma

Astronomy Unit Seminars
Dr. Andrey Afanasyev (Irkutsk)
June 7th, 2013 at 14:30
GO Jones Room 410

Analysis of observations of the Sun in various spectral lines shows that magnetohydrodynamic (MHD) waves exist throughout the solar corona. Solar eruptions are sometimes accompanied by large-scale wave-like transients in the lower corona, the so-called EUV (or EIT) waves. Several different mechanisms are most likely responsible for such phenomena. We consider a class of EUV waves that are due to coronal fast-mode shock waves and model their propagation with the method of nonlinear geometrical acoustics. This method is based on the ray-tracing method (WKB approach) and allows one to take into account nonlinear properties of shock waves. Results of our modelling show the initial deceleration of an EUV wave as well as its damping and lengthening. These effects are due to nonlinearity of the coronal shock wave and agree with observations. Special attention is paid to the interaction of the waves with coronal magnetic null points. The wave is captured by the null point and the amplitude of the wave increases considerably due to the Alfven speed decrease as well as the wave front convergence in the neighbourhood of the null point. We model the propagation of a fast-mode MHD wave near a 2D magnetic null point using the nonlinear geometrical acoustics method. The results obtained show the following: i) nonlinear wave passes through the null point even in a cold plasma case like waves propagate in a warm plasma, and ii) unlike the linear cold plasma case, growth of the nonlinear wave amplitude is restricted, since nonlinear waves decay significantly when their intensities grow.

** N.B., the venue for this presentation is G. O. Jones 410 **