On the statistical detection of propagating waves in polar coronal holes

Abstract

Context. Waves are important to the study of dynamical processes in coronal holes and the acceleration of the fast solar wind. A spectral time series was taken with the SUMER spectrometer on-board SoHO on 20 October 1996. The observations were obtained in the N IV 765 Å transition region line and the Ne VIII 770 Å line of the low corona. Aims. We detect the presence of waves and study their characteristic properties in terms of their propagation speeds and direction. Previous statistical studies, undertaken with data from the CDS spectrometer, report the presence of waves in these regions. We extend this analysis using SUMER observations. Methods. Using Fourier techniques, we measured the phase delays between intensity oscillations, as well as between velocity oscillations, in our two lines over the full range of available frequencies. From this, we were able to measure the travel time of the propagating oscillations, hence the propagation speeds of the waves that produce the oscillations. Results. We detect the long period oscillations in polar coronal holes on the disc. For network bright locations within coronal holes, our results indicate the presence of compressional waves with a dominant period of $\approx $25 min. However, we also find power at many other different frequencies, so we are able to study oscillations over a full range of frequencies. We find evidence of propagating waves with a fixed time delay in the coronal hole. We find, moreover, that there is a difference in the nature of the wave propagation in the bright (``network''), as opposed to the dark (``internetwork'') regions, with the latter sometimes showing evidence of downwardly propagating waves that are not seen in the former. From a measurement of propagation speeds, we find that all measured waves are subsonic in nature. Conclusions. Waves with different characteristics are found to be present at different locations in the observed coronal hole. The measured propagation speeds are subsonic, indicating that the majority of them are slow magneto-acoustic in nature. These waves, measured in the lower atmosphere, could accelerate farther at higher altitudes and may be important for the acceleration of the fast solar wind.