The damping profile of standing kink modes in coronal loops
The solar atmosphere, or corona, is an extremely hot plasma shaped by strong magnetic fields generated inside the Sun which emerge through its surface. Thanks to satellites which detect the extreme ultraviolet (EUV) light exclusively emitted by coronal plasma at temperatures of millions of degrees K we can now routinely observe the corona.
The magnetic loops have their footpoints anchored in the dense solar surface, so when displaced from their equilibrium they oscillate back and forth in a manner which can be described as a standing mode. In the late 1990s, the Transition Region And Coronal Explorer (TRACE) satellite detected these “kink” oscillations when loops were perturbed by large energy releases like flares or coronal mass ejections. These observations are important as they allow a form of seismology that reveals the properties of the coronal plasma from the behaviour of the oscillations.
More modern instruments allow us to study the corona in far greater detail than TRACE. The EUV imager onboard the Solar Dynamics Observatory (SDO) satellite has led to several discoveries of new types of wave behaviour. In our recently published paper (Pascoe et al. 2016, A&A, 585, L6) we looked at how the kink oscillations damp away. The popular explanation for the damping is mode coupling, which predicts that kink waves should be converted into another type of wave; Alfvén waves. It makes the further predictions that the damping envelope should have a Gaussian profile in faint (low density) loops, but an exponential shape in bright (high density) loops. The exponential damping behaviour in dense loops had been detected in previous observations, but only the latest high resolution data from SDO has allowed us to study oscillations in a number of low density loops for the first time, and so find examples of the Gaussian signature of mode coupling.
- Publication: Pascoe et al. 2016, A&A, 585, L6
- Caption to figure: EUV image of coronal loops taken with SDO (left). We measure the oscillation of the loop highlighted in red. The variation in intensity along the blue slit produces the time-distance map (top right), which shows the oscillation of the loop axis back and forth after a large eruption in the corona. The damping envelope (bottom right) is better described by a Gaussian profile (blue) rather than an exponential one (red), which provides us evidence that the damping mechanism is mode coupling.