Application of the theory of damping of kink oscillations by radiative cooling of coronal loop plasma

Abstract

Aims. We present here a first comparative study between the observed damping of numerous fast kink oscillations and the theoretical model of their damping due to the cooling of coronal loops. The theory of damping of kink oscillations due to radiation of the solar plasma with a temporally varying background is applied here to all known cases of coronal kink oscillations. Methods. A recent dynamic model of cooling coronal loops predicts that transverse oscillations of such loops could be significantly damped due to the radiative cooling process (Morton & Erdélyi 2009, ApJ, 707, 750). The cooling of the loop plasma also has the consequence that the kink oscillation has a time-dependent frequency. The theory is applied to a relatively large number of known and reported examples of TRACE observations of damped kink oscillations. Results. We find that, for cooling timescales that are typical of EUV loops (500−2000 s), the observed damping of the transversal (i.e. kink) oscillations can be accounted for almost entirely by the cooling process in half of the examples. No other dissipative mechanism(s) seems to be needed to model the damping. In the remaining other examples, the cooling process does not appear to be able to account fully for the observed damping, though could still have a significant influence on the damping. In these cases another mechanism(s), e.g. resonant absorption, may be additionally required to account for the complete decay of oscillations. Also, we show that because of the dynamic nature of the background plasma, allowing for a time-dependent frequency provides a better fit profile for the data points of observations than a fit profile with a constant frequency, opening novel avenues for solar magneto-seismology.

Kink oscillations of cooling coronal loops with variable cross-section

“…Hence, it seems to be a viable assumption that while the plasma inside the loop is cooling, the temperature of the plasma outside the loop remains constant. Following Aschwanden & Terradas (2008) and Morton & Erdélyi (2010), we approximate the temperature evolution inside the loop by an exponentially decaying function,…”

Resonant damping of kink oscillations of cooling coronal magnetic loops

“…Similar to Paper I we also assume that the plasma temperature outside the loop, T 0 , does not change with time, while it decreases inside the loop due to the radiative cooling. Following Aschwanden & Terradas (2008) and Morton & Erdélyi (2010) we approximate the temperature evolution inside the loop by an exponentially decaying function,…”

“…They reported a 35% decrease in the oscillation period in the event reported by Nakariakov et al (1999). The problem of the period decrease has been also addressed by Morton & Erdélyi (2010) who found that the analytical profile with increasing frequency fits better the observational data than the profile with the constant frequency. On the other hand, the period decrease has not been found in any other papers analyzing observations of coronal loop kink oscillations, including the paper by Aschwanden & Schrijver (2011), which is not surprising at all.…”

The Effect of Cooling on Driven Kink Oscillations of Coronal Loops

“…As the majority of modelling of coronal loop oscillations is conducted using static background parameters (e.g., temperature, density), one opportunity to consider additional physics theoretically is to incorporate some form of time-dependence (see, for example, Dymova and Ruderman 2005;Al-Ghafri and Erdélyi 2013;Erdélyi et al 2014). Morton and Erdélyi [2009] considered the damping of coronal loops due to cooling through time and found, that for typical oscillatory periods, cooling could play a key role in explaining observed damping profiles (this was shown further in Morton and Erdélyi 2010). The idea that amplification of coronal loop oscillations could occur due to cooling within coronal loops that contained flow was first suggested by Ruderman [2011a].…”