Damped Oscillations of Coronal Loops

Abstract

A mechanism of damped oscillations of a coronal loop is investigated. The loop is treated as a thin toroidal flux rope with two stationary photospheric footpoints, carrying both toroidal and poloidal currents. The forces and the flux-rope dynamics are described within the framework of ideal magnetohydrodynamics (MHD). The main features of the theory are the following: i) Oscillatory motions are determined by the Lorentz force that acts on curved current-carrying plasma structures and ii) damping is caused by drag that provides the momentum coupling between the flux rope and the ambient coronal plasma. The oscillation is restricted to the vertical plane of the flux rope. The initial equilibrium flux rope is set into oscillation by a pulse of upflow of the ambient plasma. The theory is applied to two events of oscillating loops observed by the Transition Region and Coronal Explorer (TRACE). It is shown that the Lorentz force and drag with a reasonable value of the coupling coefficient (c d ) and without anomalous dissipation are able to accurately account for the observed damped oscillations. The analysis shows that the variations in the observed intensity can be explained by the minor radial expansion and contraction. For the two events, the values of the drag coefficient consistent with the observed damping times are in the range c d ≈ 2 -5, with specific values being dependent on parameters such as the loop density, ambient magnetic field, and the loop geometry. This range is consistent with a previous MHD simulation study and with values used to reproduce the observed trajectories of coronal mass ejections (CMEs).

Physics of erupting solar flux ropes: Coronal mass ejections (CMEs)—Recent advances in theory and observation

“…Plasmas 24, 090501 (2017) quasi-stationary, occasionally buffeted by nearby eruptions causing characteristic damped oscillations. [176][177][178][179][180][181] Individual loops can show activities such as rapid variation in brightness associated with oscillations, which may be in both vertical and horizontal directions.…”

The possible role of vortex shedding in the excitation of kink-mode oscillations in the solar corona

“…The dashed thick curve shows the empirical best fitting exponentially decaying harmonic curve determined in Nakariakov et al (1999). of the corona. For example, recently, Chen & Schuck (2007) has suggested that the decay of vertically polarised kink oscillations of coronal loops could be attributed to aerodynamic drag with zero viscosity and resistivity, which provides the momentum coupling between the oscillating loop and the ambient coronal plasma. It is reasonable to expect both that a coronal loop embedded in the vertical flow should experience a periodic horizontal force due to vortex shedding, at least in the case where the external magnetic field is parallel to the plane of the loop, hence the axis of the generated vorticity, and that this force can be in resonance with natural kink oscillations of the loop.…”

Measurements with STEREO/COR1 data of drag forces acting on small-scale blobs falling in the intermediate corona

“…More recently, the same approach has been applied by Toriumi & Yokoyama (2011) to study the emergence of fluxtubes from the photosphere, by Haerendel & Berger (2011) to study the downflows in quiescent prominences, and by Chen & Schuck (2007) to study the damping of loop oscillations in the corona. Moreover, over the last few decades many authors applied the concept of a magnetic drag force to study the interplanetary propagation of solar eruptions (or coronal mass ejections -CMEs); CMEs can propagate in the intermediate corona (which extends from 1.5 to 5 solar radii) at velocities of up to 2500 km s −1 (Vourlidas et al 2002;Gopalswamy 2004), while at 1 AU velocities tend to be closer to that of the solar wind (SW; Gopalswamy 2007), around 500-800 km s −1 .…”