Standing Slow MHD Waves in Radiatively Cooling Coronal Loops

TL;DR

This paper analytically investigates how radiative cooling affects standing slow MHD waves in coronal loops, revealing that cooling decreases wave amplitude and influences damping rates, especially in hot loops.

Contribution

It provides an analytical framework for understanding the impact of radiative cooling on slow MHD wave damping in coronal loops, incorporating observed cooling profiles.

Findings

Cooling decreases wave amplitude over time.

Damping rates are affected by plasma temperature and cooling.

Hot coronal loops exhibit increased damping of slow waves.

Abstract

The standing slow magneto-acoustic oscillations in cooling coronal loops are investigated. There are two damping mechanisms which are considered to generate the standing acoustic modes in coronal magnetic loops namely thermal conduction and radiation. The background temperature is assumed to change temporally due to optically thin radiation. In particular, the background plasma is assumed to be radiatively cooling. The effects of cooling on longitudinal slow MHD modes is analytically evaluated by choosing a simple form of radiative function that ensures the temperature evolution of the background plasma due to radiation coincides with the observed cooling profile of coronal loops. The assumption of low-beta plasma leads to neglect the magnetic field perturbation and eventually reduces the MHD equations to a 1D system modelling longitudinal MHD oscillations in a cooling coronal loop. The cooling is assumed to occur on a characteristic time scale much larger than the oscillation period that subsequently enables using the WKB theory to study the properties of standing wave. The governing equation describing the time-dependent amplitude of waves is obtained and solved analytically. The analytically derived solutions are numerically evaluated to give further insight into the evolution of the standing acoustic waves. We find that the plasma cooling gives rise to a decrease in the amplitude of oscillations. In spite of the reduction in damping rate caused by rising the cooling, the damping scenario of slow standing MHD waves strongly increases in hot coronal loops.