On the statistical detection of propagating waves in polar coronal hole
Waves are important for the heating of the solar corona and the acceleration of the solar wind. We have examined a long spectral time series of a southern coronal hole observed on the 25th February 1997, with the SUMER spectrometer on-board SoHO. The observations were obtained in a transition region N IV 765 A line and in a low coronal Ne VIII 770 A line. Our observations indicate the presence of compressional waves with periods of 18 min. We also find significant power in shorter periods. Using Fourier techniques, we measured the phase delays between intensity as well as velocity oscillations in the two chosen lines over a frequency domain. From this we are able to measure the travel time of the propagating oscillations and, hence, the propagation speeds of the waves producing the oscillations. As the measured propagation speeds are subsonic, we conclude that the detected waves are slow magneto-acoustic in nature.