Phase speed and frequency-dependent damping of longitudinal intensity oscillations in coronal loop structures observed with AIA/SDO

Abstract

Longitudinal intensity oscillations along coronal loops that are interpreted as signatures of magneto-acoustic waves are observed frequently in different coronal structures. The aim of this paper is to estimate the physical parameters of the slow waves and the quantitative dependence of these parameters on their frequencies in the solar corona loops that are situated above active regions with the Atmospheric Imaging Assembly (AIA) onboard Solar Dynamic Observatory (SDO). The observed data on 2012-Feb-12, consisting of 300 images with an interval of 24 seconds in the 171 $\rm{\AA}$ and 193 $\rm{\AA}$ passbands is analyzed for evidence of propagating features as slow waves along the loop structures. Signatures of longitudinal intensity oscillations that are damped rapidly as they travel along the loop structures were found, with periods in the range of a few minutes to few tens of minutes. Also, the projected (apparent) phase speeds, projected damping lengths, damping times and damping qualities of filtered intensities centred on the dominant frequencies are measured in the range of $\rm{C_s}\simeq 38-79~ \rm {km~s^{-1}}$, $\rm{L_d}\simeq 23-68 ~\rm{Mm }$, $\rm{\tau_d}\simeq 7- 21 ~\rm {min}$ and $\rm{\tau_d/P}\simeq 0.34- 0.77$, respectively. The theoretical and observational results of this study indicate that the damping times and damping lengths increase with increasing the oscillation periods, and are highly sensitive function of oscillation period, but the projected speeds and the damping qualities are not very sensitive to the oscillation periods. Furthermore, the magnitude values of physical parameters are in good agreement with the prediction of the theoretical dispersion relations of high-frequency MHD waves ($>1.1~ \rm{mHz}$) in a coronal plasma with electron number density in the range of $\rm{n_e}\simeq 10^{7} - 10^{12} ~\rm{cm^{-3}}$.