Hinode/EIS observations of propagating low-frequency slow magnetoacoustic waves in fan-like coronal loops

Abstract

Aims: We report the first observation of multiple-periodic propagating disturbances along a fan-like coronal structure simultaneously detected in both intensity and Doppler shift in the Fe xii 195 Å line with the EUV Imaging Spectrometer (EIS) onboard Hinode. A new application of coronal seismology is provided based on this observation.
Methods: We analyzed the EIS sit-and-stare mode observation of oscillations using the running difference and wavelet techniques.
Results: Two harmonics with periods of 12 and 25 min are detected. We measured the Doppler shift amplitude of 1-2 km s^-1, the relative intensity amplitude of 3%-5% and the apparent propagation speed of 100-120 km s^-1.
Conclusions: The amplitude relationship between intensity and Doppler shift oscillations provides convincing evidence that these propagating features are a manifestation of slow magnetoacoustic waves. Detection lengths (over which the waves are visible) of the 25 min wave are about 70-90 Mm, much longer than those of the 5 min wave previously detected by TRACE. This difference may be explained by the dependence of damping length on the wave period for thermal conduction. Based on a linear wave theory, we derive an inclination of the magnetic field to the line-of-sight about 59 ± 8°, a true propagation speed of 128 ± 25 km s^-1 and a temperature of 0.7 ± 0.3 MK near the loop's footpoint from our measurements.

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