  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 northern
coronal hole observed on the 20th October 1996, with the SUMER spectrometer
onboard 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 ~25 min. Using Fourier
techniques, we measured the phase delays between intensity as well as velocity
oscillations in the two chosen lines. 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. We found that there is a difference in
the nature of the propagation in bright ('network') and dark ('internetwork')
regions with the latter sometimes showing evidence for downwardly propagating
waves that is not seen in the former. As, in all cases, the measured
propagation speeds are subsonic, we concluded that the detected waves are slow
magnetoacoustic in nature.
