Upward propagating high-frequency Alfvén waves as identified from dynamic wave-like spicules observed by SOT on Hinode

¹ Department of Geophysics, Peking University, Beijing, PR China e-mail: [chuanyitu;jshept]@pku.edu.cn
² Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany e-mail: marsch@mps.mpg.de

Received: 10 August 2008
Accepted: 6 February 2009

Abstract

Aims. We identify high-frequency Alfvén waves propagating upward in the solar chromosphere and transition region from observation by Solar Optical Telescope (SOT) onboard Hinode.

Methods. The spicule shape is enhanced through application of a normal radial gradient filter and an un-sharp mask on the images taken by SOT. The displaced position of the spicule is at each height obtained by tracing the maximum intensity after image processing. The dominant wave period is obtained by the FFT method applied to the time variations of the displaced position at a certain height. The phase speed is estimated with the help of a cross-correlation analysis of two temporal sequences of the displaced positions at two heights along the spicule.

Results. We find in four cases that the spicules are modulated by high-frequency (≥0.02 Hz) transverse fluctuations. Such fluctuations are suggested to be Alfvén waves that propagate upward along the spicules with phase speed ranges from 50 to 150 km s^-1. Three of the modulated spicules show clear wave-like shapes with short wavelengths less than 8 Mm.

Conclusions. Our work identified directly upward propagation of Alfvén waves in the solar chromosphere and transition region. In addition to the recently reported Alfvén waves with very long wavelength and wave period, we find here four examples of Alfvén waves with shorter wavelengths and periods. These findings shed new light on the wave origin and on coronal and solar-wind heating.