SECONDARY FAST MAGNETOACOUSTIC WAVES TRAPPED IN RANDOMLY STRUCTURED PLASMAS

ABSTRACT Fast magnetoacoustic waves are an important tool for inferring parameters of the solar atmosphere. We numerically simulate the propagation of fast wave pulses in randomly structured plasmas that mimic the highly inhomogeneous solar corona. A network of secondary waves is formed by a series of partial reflections and transmissions. These secondary waves exhibit quasi-periodicities in both time and space. Since the temporal and spatial periods are related simply through the speed of the fast wave, we quantify the properties of secondary waves by examining the dependence of the average temporal period ( ) on the initial pulse width (w0) and studying the density contrast ( ) and correlation length (Lc) that characterize the randomness of the equilibrium density profiles. For small-amplitude pulses, does not alter significantly. Large-amplitude pulses, on the other hand, enhance the density contrast when is small but have a smoothing effect when is sufficiently large. We found that scales linearly with Lc and that the scaling factor is larger for a narrower pulse. However, in terms of the absolute values of , broader pulses generate secondary waves with longer periods, and this effect is stronger in random plasmas with shorter correlation lengths. Secondary waves carry the signatures of both the leading wave pulse and the background plasma. Our study may find applications in magnetohydrodynamic seismology by exploiting the secondary waves detected in the dimming regions after coronal mass ejections or extreme ultraviolet waves.