Characteristics of magnetoacoustic sausage modes

TL;DR

This study investigates how magnetoacoustic sausage mode oscillations in coronal loops depend on plasma-beta and density contrast, providing insights valuable for coronal seismology through numerical simulations and analytical models.

Contribution

It offers a detailed analysis of the dependence of sausage mode period and cut-off wavenumber on plasma-beta and density contrast, with validation from numerical simulations and analytical expressions.

Findings

Sausage mode period is weakly dependent on plasma-beta.

Cut-off wavenumber depends on plasma-beta and density contrast.

Harmonic ratio deviations are significant at low wavenumbers.

Abstract

Aims: We perform an advanced study of the fast magnetoacoustic sausage oscillations of coronal loops in the context of MHD coronal seismology to establish the dependence of the sausage mode period and cut-off wavenumber on the plasma-beta of the loop-filling plasma. A parametric study of the ratios for different harmonics of the mode is also carried out. Methods: Full magnetohydrodynamic numerical simulations were performed using Lare2d, simulating hot, dense loops in a magnetic slab environment. The symmetric Epstein profile and a simple step-function profile were both used to model the density structure of the simulated loops. Analytical expressions for the cut-off wavenumber and the harmonic ratio between the second longitudinal harmonic and the fundamental were also examined. Results: It was established that the period of the global sausage mode is only very weakly dependent on the value of the plasma-beta inside a coronal loop, which justifies the application of this model to hot flaring loops. The cut-off wavenumber kc for the global mode was found to be dependent on both internal and external values of the plasma-beta, again only weakly. By far the most important factor in this case was the value of the density contrast ratio between the loop and the surroundings. Finally, the deviation of the harmonic ratio P1/2P2 from the ideal non-dispersive case was shown to be considerable at low k, again strongly dependent on plasma density. Quantifying the behaviour of the cut-off wavenumber and the harmonic ratio has significant applications to the field of coronal seismology.