Hinode/EIS Observation of the Alfvénic Fluctuations in the Quiet Sun

Abstract Alfvénic fluctuations in the corona are regularly observed in EUV imaging and infrared spectroscopic data. We report here on the Doppler velocity fluctuations within the data from the Extreme-Ultraviolet Imaging Spectrometer (EIS) on board Hinode. The capabilities of EIS provide an opportunity to detect fluctuations in the solar atmosphere and confirm the previous results from ground-based spectroscopic data using EIS’s EUV spectra and in seeing-free conditions. We analyzed Doppler velocity time series for a particular date of observation, 2007 September 27, above the limb. The Fourier transform of the observed signals show the Doppler velocity has a broad spectrum distributed as a power law with a power enhancement around 3–4 mHz. This result is consistent with previous observational results obtained with Coronal Multi-Channel Polarimeter data, and hence we interpret the Doppler velocity fluctuations as Alfvénic in nature. We performed nonlinear regression on the power spectra to determine the index of the power law and to parameterize the properties of the enhancement. We find good agreement between the estimated values and those from previous studies. Furthermore, correlation analysis shows that the observed oscillations are spatially coherent perpendicular to the magnetic field on the length scale L ⊥ ∼ 8 Mm, providing an estimate on the energy injection scale of a bundle of coherently oscillating loops.