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Register a new userWave amplitude modulation in fan loops as observed by AIA/SDO
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Durgesh Tripathi Professor
Publication date
2020
fields
Physics
and research's language is
English
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We perform 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 measure the phase speeds of the propagating intensity disturbances by employing cross-correlation analysis, as well as by obtaining the slopes in xt-plots. We obtain original as well as de-trended light curves at different heights of the time-distance maps and characterize them by performing Fourier and Wavelet analysis, respectively. The time-distance maps reveal clear propagation of intensity oscillations in all the coronal EUV channels except AIA 94 and 335~{AA}. We determine the nature of the intensity disturbances as slow magnetoacoustic waves by measuring their phase speeds. The time-distance maps, as well as the de-trended light curves, show an increase and decrease in the amplitude of propagating 3-min oscillations over time. The amplitude variations appear most prominent in AIA 171~{AA}, though other EUV channels also show such signatures. Fourier power spectrum yield the presence of significant powers with several nearby frequencies between 2--3 minutes (5--8 mHz), along with many other smaller peaks between 2--4 minutes. 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 minutes. 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 of an underlying unknown source.
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A comprehensive study of the physical parameters of active region fan loops is presented using the observations recorded with the Interface Region Imaging Spectrometer (IRIS), the EUV Imaging Spectrometer (EIS) on-board Hinode and the Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager (HMI) on-board the Solar Dynamics Observatory (SDO). The fan loops emerging from non-flaring AR~11899 (near the disk-center) on 19th November, 2013 are clearly discernible in AIA 171~{AA} images and those obtained in ion{Fe}{8} and ion{Si}{7} images using EIS. Our measurements of electron densities reveal that the footpoints of these loops are approximately at constant pressure with electron densities of $log,N_{e}=$ 10.1 cm$^{-3}$ at $log,[T/K]=5.15$ (ion{O}{4}), and $log,N_{e}=$ 8.9 cm$^{-3}$ at $log,[T/K]=6.15$ (ion{Si}{10}). The electron temperature diagnosed across the fan loops by means of EM-Loci suggest that at the footpoints, there are two temperature components at $log,[T/K]=4.95$ and 5.95, which are picked-up by IRIS lines and EIS lines respectively. At higher heights, the loops are nearly isothermal at $log,[T/K]=5.95$, that remained constant along the loop. The measurement of Doppler shift using IRIS lines suggests that the plasma at the footpoints of these loops is predominantly redshifted by 2-3~km~s$^{-1}$ in ion{C}{2}, 10-15~km~s$^{-1}$ in ion{Si}{4} and $~$15{--}20~km~s$^{-1}$ in ion{O}{4}, reflecting the increase in the speed of downflows with increasing temperature from $log,[T/K]=4.40$ to 5.15. These observations can be explained by low frequency nanoflares or impulsive heating, and provide further important constraints on the modeling of the dynamics of fan loops.
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