Oscillations in active region fan loops: observations from EIS/Hinode and AIA/SDO

Abstract

Active region fan loops in AR 11076 were studied, in search of oscillations, using high cadence spectroscopic observations from Extreme-ultraviolet Imaging Spectrometer (EIS) on board Hinode combined with imaging sequences from the Atmospheric Imaging Assembly (AIA) on board Solar Dynamics Observatory (SDO). Spectra from EIS were analyzed in two spectral windows, Fe XII 195.12 Å and Fe XIII 202.04 Å along with the images from AIA in the 171 Å and 193 Å channels. We find short (<3 min) and long (≈9 min) periods at two different locations. Shorter periods show oscillations in all three line parameters and the longer ones only in intensity and Doppler shift but not in line width. Line profiles at both these locations do not show any visible blue-shifted component and can be fitted well with a single Gaussian function along with a polynomial background. Results using co-spatial and co-temporal data from AIA/SDO do not show any significant peaks corresponding to shorter periods, but longer periods are clearly observed in both 171 Å and 193 Å channels. Space-time analysis in these fan loops using images from AIA/SDO show alternate slanted ridges of positive slope, indicative of outward propagating disturbances. The apparent propagation speeds were estimated to be 83.5±1.8 kms/sup−1/ and 100.5±4.2 kms/sup−1/ , respectively, in the 171 Å and 193 Å channels. Observed short-period oscillations are suggested to be caused by the simultaneous presence of more than one MHD mode whereas the long periods are suggested to be signatures of slow magneto-acoustic waves. In case of shorter periods, the amplitude of the oscillation is found to be higher in EIS lines with relatively higher temperature of formation. Longer periods, when observed from AIA, show a decrease of amplitude in hotter AIA channels, which might indicate damping due to thermal conduction owing to their acoustic nature.