Letter to the Editor

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

¹ Department of Physics, Catholic University of America, 620 Michigan Avenue, Washington, DC 20064, USA e-mail: wangtj@helio.gsfc.nasa.gov
² NASA Goddard Space Flight Center, Code 671, Greenbelt, MD 20771, USA
³ Space Science Division, Naval Research Laboratory, Washington, DC 20375, USA

Received: 19 May 2009
Accepted: 28 July 2009

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 , a true propagation speed of  km s^-1 and a temperature of  MK near the loop's footpoint from our measurements.