Decayless low-amplitude kink oscillations: a common phenomenon in the solar corona?

Abstract

Context. We investigate the decayless regime of coronal kink oscillations recently discovered in the Solar Dynamics Observatory (SDO)/AIA data. In contrast to decaying kink oscillations that are excited by impulsive dynamical processes, this type of transverse oscillations is not connected to any external impulsive impact, such as a flare or coronal mass ejection, and does not show any significant decay. Moreover the amplitude of these decayless oscillations is typically lower than that of decaying oscillations. Aims. The aim of this research is to estimate the prevalence of this phenomenon and its characteristic signatures. Methods. We analysed 21 active regions (NOAA 11637-11657) observed in January 2013 in the 171 Å channel of SDO/AIA. For each active region we inspected six hours of observations, constructing time-distance plots for the slits positioned across pronounced bright loops. The oscillatory patterns in time-distance plots were visually identified and the oscillation periods and amplitudes were measured. We also estimated the length of each oscillating loop. Results. Low-amplitude decayless kink oscillations are found to be present in the majority of the analysed active regions. The oscillation periods lie in the range from 1.5 to 10 min. In two active regions with insufficient observation conditions we did not identify any oscillation patterns. The oscillation periods are found to increase with the length of the oscillating loop. Conclusions. The considered type of coronal oscillations is a common phenomenon in the corona. The established dependence of the oscillation period on the loop length is consistent with their interpretation in terms of standing kink waves.

Undamped transverse oscillations of coronal loops as a self-oscillatory process

“…Small-amplitude undamped kink oscillations are observed continuously without any visible driver. Similar to the oscillations in the decaying regime, the period of undamped kink oscillations was found to scale with the loop length (Anfinogentov et al 2015). Assuming that the small-amplitude undamped kink oscillations are subject to the same damping mechanism as the large-amplitude, rapidly decaying kink oscillations, there should be some mechanism that continuously supplies the oscillating loops with energy to compensate the damping.…”

“…Wang et al 2012;Nisticò et al 2013). Using the observations of undamped oscillations in 21 non-flaring active regions (NOAA 11637-11657) observed in January 2013 in the 171 Å channel of SDO/AIA (Anfinogentov et al 2015), we constructed scalings of the oscillation amplitudes with the period and loop length (Fig. 2).…”

“…Nakariakov et al (2009), Nisticò et al (2013), and Anfinogentov et al (2015) suggested describing the displacement a(t) of the loop top by a driven damped harmonic oscillator equation, where δ is the damping coefficient, Ω K is the natural frequency of the kink oscillation, and f (t) is an external driving force that compensates damping caused by dissipation and/or mode conversion. The damping coefficient can be obtained empirically from the large-amplitude, rapidly damped oscillatory regime.…”

“…In the former regime the loop displacement reaches several minor radii of the loop and, typically, the oscillation decays in a few oscillation cycles. In the latter regime the amplitude is usually smaller than the minor radius of the loop, and remains constant or low-varying for a number of oscillation cycles (Anfinogentov et al 2013(Anfinogentov et al , 2015. The same loop may oscillate in both these regimes, that is before and after some impulsive excitation .…”

Spatially resolved observation of the fundamental and second harmonic standing kink modes using SDO/AIA

“…Goddard et al (2016) studied the distributions of the oscillation parameters, such as the derived kink speed, as well as the damping of the oscillations. In contrast to this strongly damped regime, low amplitude decay-less kink oscillations have also been detected Anfinogentov et al 2013) and confirmed to be a common phenomenon in the corona (Anfinogentov et al 2015).…”