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

TL;DR

This study reports the first simultaneous detection of multiple-periodic slow magnetoacoustic waves in coronal loops using Hinode/EIS, providing new insights into wave properties and coronal seismology.

Contribution

It introduces a novel application of coronal seismology based on multi-periodic wave observations in fan-like coronal loops.

Findings

Detection of two harmonics with periods of 12 and 25 minutes.

Measured wave propagation speed of 100-120 km/s.

Derived magnetic field inclination and plasma temperature near the loop footpoint.

Abstract

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 A line with the EUV Imaging Spectrometer (EIS) onboard Hinode. A new application of coronal seismology is provided based on this observation. We analyzed the EIS sit-and-stare mode observation of oscillations using the running difference and wavelet techniques. Two harmonics with periods of 12 and 25 min are detected. We measured the Doppler shift amplitude of 1-2 km/s, the relative intensity amplitude of 3%-5% and the apparent propagation speed of 100-120 km/s. 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$\pm$8 deg, a true propagation speed of 128$\pm$25 km/s and a temperature of 0.7$\pm$0.3 MK near the loop's footpoint from our measurements.