Excitation of kink oscillations of coronal loops: statistical study ^⋆

¹ Centre for Fusion, Space and Astrophysics, Department of Physics, University of Warwick, CV4 7AL, UK
e-mail: V.Nakariakov@warwick.ac.uk
² Space Research Institute (IKI), Russian Academy of Sciences, Profsoyuznaya St. 84/32, 117997 Moscow, Russia
e-mail: ivanzim@iki.rssi.ru
³ Astronomical Observatory at Pulkovo, Russian Academy of Sciences, 196140 St. Petersburg, Russia
⁴ School of Space Research, Kyung Hee University, 446-701 Yongin, Gyeonggi, Korea

Received: 11 September 2014
Accepted: 17 December 2014

Abstract

Context. Solar flares are often accompanied by kink (transverse) oscillations of coronal loops. Despite intensive study of these oscillations in recent years, the mechanisms that excite them are still not known.

Aims. We aim to clarify the excitation mechanisms for these kink oscillations of coronal loops.

Methods. We analysed 58 kink-oscillation events observed by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) during its first four years (2010–2014) with the use of the JHelioviewer. Association of these oscillation events with flares, lower coronal (r ≲ 1.4 R_⊙) eruptions and plasma ejections, coronal mass ejections (CMEs), and coronal Type-II radio bursts is studied.

Results. We find that 44 of these 58 oscillation events (76%) were associated with CMEs observed in the white light emission. Moreover, 57 events (98%) were accompanied by lower coronal eruptions/ejections (LCEs) observed in the extreme-ultraviolet band in the parental active regions. In the remaining event an LCE was not clearly seen, but it was definitely associated with a CME too. The main observational finding is that the kink oscillations were excited by the deviation of loops from their equilibria by a nearby LCE in 55 events (95%). In three remaining events, it was difficult to reliably determine the cause of the oscillations because of limitations in the observational data. We also found that 53 events (91%) were associated with flares. In five remaining events, the parental active regions were behind the limb and we could not directly see flare sites. It indicates that there is a close relationship between these two kinds of solar activity. However, the estimated speeds of a hypothetical driver of kink oscillations by flares were found to be lower than 500 km s^-1 in 80% of the cases. Such low speeds do not favour the association of the oscillation excitation with a shock wave, as usually assumed. That only 23 (40%) of the oscillation events were found to be associated with coronal Type-II radio bursts also goes against the shock wave mechanism for the excitation of kink oscillations.

Conclusions. The statistical analysis shows that the most probable mechanism for exciting the kink oscillations of coronal loops is the deviation of loops from their equilibrium by nearby eruptions or plasma ejections rather than a blast shock wave ignited by a flare.