Standing sausage modes in coronal loops with plasma flow^⋆

Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, School of Space Science and Physics, Shandong University at Weihai, 264209 Weihai, PR China
e-mail: bbl@sdu.edu.cn

Received: 29 December 2013
Accepted: 18 June 2014

Abstract

Context. Magnetohydrodynamic waves are important for diagnosing the physical parameters of coronal plasmas. Field-aligned flows appear frequently in coronal loops.

Aims. We examine the effects of transverse density and plasma flow structuring on standing sausage modes trapped in coronal loops, and examine their observational implications in the context of coronal seismology.

Methods. We model coronal loops as straight cold cylinders with plasma flow embedded in a static corona. An eigen-value problem governing propagating sausage waves is formulated and its solutions are employed to construct standing modes. Two transverse profiles are distinguished, and are called profiles E and N. A parameter study is performed on the dependence of the maximum period P_max and cutoff length-to-radius ratio (L/a)_cutoff in the trapped regime on the density parameters (ρ₀/ρ_∞ and profile steepness p) and the flow parameters (its magnitude U₀ and profile steepness u).

Results. For either profile, introducing a flow reduces P_max obtainable in the trapped regime relative to the static case. The value of P_max is sensitive to p for profile N, but is insensitive to p for profile E. By far the most important effect a flow introduces is to reduce the capability for loops to trap standing sausage modes: (L/a)_cutoff may be substantially reduced in the case with flow relative to the static one. In addition, (L/a)_cutoff is smaller for a stronger flow, and for a steeper flow profile when the flow magnitude is fixed.

Conclusions. If the density distribution can be described by profile N, then measuring the sausage mode period can help deduce the density profile steepness. However, this practice is not feasible if profile E more accurately describes the density distribution. Furthermore, even field-aligned flows with magnitudes substantially smaller than the ambient Alfvén speed can make coronal loops considerably less likely to support trapped standing sausage modes.