Wave amplitude modulation in fan loops as observed by AIA/SDO

Abstract

Aims. We perform a detailed analysis to understand the evolution and dynamics of propagating intensity disturbances observed in a fan loop system. Methods. We performed multiwavelength time-distance analysis of a fan loop system anchored in an isolated sunspot region (AR 12553). The active region was observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. We measured the phase speeds of the propagating intensity disturbances by employing cross-correlation analysis, and by obtaining the slopes in xt-plots. We obtained original and detrended light curves at different heights of the time-distance maps and characterised them by performing Fourier and wavelet analysis, respectively. Results. The time-distance maps reveal clear propagation of intensity oscillations in all the coronal extreme ultraviolet (EUV) channels except AIA 94 and 335 Å. We determine the nature of the intensity disturbances as slow magneto-acoustic waves by measuring their phase speeds. The time-distance maps, as well as the detrended light curves, show an increase and decrease in the amplitude of propagating 3 min oscillations over time. The amplitude variations appear most prominently in AIA 171 Å, though other EUV channels also show such signatures. The Fourier power spectrum yields the presence of significant powers with several nearby frequencies in the range of 2–3 min (5–8 mHz), along with many other smaller peaks between 2–4 min. Wavelet analysis shows an increase and decrease of oscillating power around 3 min simultaneous to the amplitude variations. We obtain the modulation period to be in the range of 20–30 min. Conclusions. Our results provide the viability of occurrence of phenomenon like “Beat” among the nearby frequencies giving rise to the observed amplitude modulation. However, we cannot at this stage rule out the possibility that the modulation may be driven by variability in an underlying unknown source.

MHD Waves in open coronal structures

“…These results compelled the authors to conclude that the coronal slow waves are externally driven by the global solar oscillations (p-modes) in the photosphere. A recent study by Sharma et al (2020) also show similar amplitude modulation in slow waves observed in a sunspot across different layers of the solar atmosphere possibly confirming the wider applicability of this interpretation. Utilising high-resolution multi-wavelength observations of a sunspot from 16 different channels, Zhao et al (2016) studied the propagation of slow waves from photosphere to corona.…”

Magnetohydrodynamic Waves in Open Coronal Structures

“…These results compelled the authors to conclude that the coronal slow waves are externally driven by the global solar oscillations (p-modes) in the photosphere. A recent study by Sharma et al (2020) also shows similar amplitude modulation in slow waves observed in a sunspot across different layers of the solar atmosphere possibly confirming the wider applicability of this interpretation. Utilising high-resolution multi-wavelength observations of a sunspot from 16 different channels, Zhao et al (2016) studied the propagation of slow waves from photosphere to corona.…”

Upflows in the Upper Solar Atmosphere

“…There are also a couple of studies reporting a temperature-dependent PD velocity (Krishna Prasad, Banerjee, and Singh, 2012; Uritsky et al, 2013), which is a characteristic of slow-mode magneto-acoustic waves. However, several other studies did not reveal any obvious temperature dependence (Tian et al, 2011b;Sharma et al, 2020). A statistical study by Kiddie et al (2012) showed that the temperature dependency appears to be clear only for PDs rooted in sunspots.…”