Letter to the Editor

Propagating waves in polar coronal holes as seen by SUMER & EIS

¹ Indian Institute of Astrophysics, Bangalore 560034, India e-mail: dipu@iiap.res.in
² Max-Planck-Institut für Sonnensystemforschung (MPS), 37191 Katlenburg-Lindau, Germany
³ Joint Astronomy Programme, Indian Institute of Science, Bangalore 560012, India
⁴ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁵ Interferometrics, Inc., Herndon, VA 20171, USA
⁶ School of Space Research, Kyung Hee University, Yongin, Gyeonggi 446-701, Korea

Received: 13 March 2009
Accepted: 22 April 2009

Abstract

Context. To study the dynamics of coronal holes and the role of waves in the acceleration of the solar wind, spectral observations were performed over polar coronal hole regions with the SUMER spectrometer on SoHO and the EIS spectrometer on Hinode.

Aims. Using these observations, we aim to detect the presence of propagating waves in the corona and to study their properties.

Methods. The observations analysed here consist of SUMER spectra of the Ne viii 770 Å line ( MK) and EIS slot images in the Fe xii 195 Å line ( MK). Using the wavelet technique, we study line radiance oscillations at different heights from the limb in the polar coronal hole regions.

Results. We detect the presence of long period oscillations with periods of 10 to 30 min in polar coronal holes. The oscillations have an amplitude of a few percent in radiance and are not detectable in line-of-sight velocity. From the time distance maps we find evidence for propagating velocities from 75 km s^-1 (Ne viii) to 125 km s^-1 (Fe xii). These velocities are subsonic and roughly in the same ratio as the respective sound speeds.

Conclusions. We interpret the observed propagating oscillations in terms of slow magneto-acoustic waves. These waves can be important for the acceleration of the fast solar wind.