Abstract

Observations of the Sun at Extreme Ultra-Violet or X-Ray wavelengths from space-borne instruments show that the solar corona is a highly dynamic region. The coupling between the coronal magnetic field and the low density fully-ionized plasma, makes the corona an “elastic” medium, able to sustain the formations and propagation of waves, interpreted in term of magnetohydrodynamics theory. Moreover, the advent of high spatial and temporal resolution instruments (e.g., COMP , Hinode, STEREO , SDO), have shown that waves are truly ubiquitous in the atmosphere.

A large variety of modes are triggered in standing and propagating form, including transverse oscillations of coronal loops, more properly defined as “kink” waves. Excited by a flare or an eruption nearby, kink oscillations are usually observed to damp in about 3—4 periods. The physical mechanism responsible for the rapid damping has been subject to a number of theoretical studies, and the general accepted interpretation is the phenomenon of resonant absorption: linear coupling of the kink mode to unresolved torsional motions at a very narrow resonant layer.

Here, we provide the state-of-art in the observations of wave phenomena in the solar corona, focusing on kink oscillations of coronal loops observed by the Solar Dynamics Observatory (SDO). The presence of almost persistent low-amplitude (close to the spatial resolution of the telescopes) transverse motions, in addition to the well-known large-amplitude and decaying mode is shown. The behaviour is consistent with the empirical model of a damped linear oscillator excited by a continuous low-amplitude harmonic driver and/or by an impulsive high-amplitude driver. The properties of this type of transverse oscillations make them interesting object of study in the framework of coronal heating.