Periods and damping rates of fast sausage oscillations in multi-shelled coronal loops

TL;DR

This study investigates how fine structuring in coronal loops affects the periods and damping times of sausage oscillations, concluding that such effects are minimal and neglectable in seismological diagnostics.

Contribution

It introduces a model incorporating fine structuring into coronal loop oscillation analysis, demonstrating its limited impact on key oscillation parameters.

Findings

Fine structuring causes only minor changes in periods and damping times.

The steepness of the monolithic density profile significantly influences oscillation characteristics.

Neglecting fine structuring is justified in seismological applications.

Abstract

Standing sausage modes are important in interpreting quasi-periodic pulsations in the lightcurves of solar flares. Their periods and damping times play an important role in seismologically diagnosing key parameters like the magnetic field strength in regions where flare energy is released. Usually such applications are based on theoretical results neglecting unresolved fine structures in magnetized loops. However, the existence of fine structuring is suggested on both theoretical and observational grounds. Adopting the framework of cold magnetohydrodynamics (MHD), we model coronal loops as magnetized cylinders with a transverse equilibrium density profile comprising a monolithic part and a modulation due to fine structuring in the form of concentric shells. The equation governing the transverse velocity perturbation is solved with an initial-value-problem approach, and the effects of fine structuring on the periods $P$ and damping times $\tau$ of global, leaky, standing sausage modes are examined. A parameter study shows that fine structuring, be it periodically or randomly distributed, brings changes of only a few percent to $P$ and $\tau$ when there are more than about ten shells. The monolithic part, its steepness in particular, plays a far more important role in determining $P$ and $\tau$. We conclude that when measured values of $P$ and $\tau$ of sausage modes are used for seismological purposes, it is justified to use theoretical results where the effects due to fine structuring are neglected.