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

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.