  Observations carried out with the Magneto-Optical Filter at Two Heights
(MOTH) experiment show upward-traveling wave packets in magnetic regions with
frequencies below the acoustic cut-off. We demonstrate that the frequency
dependence of the observed travel times, i.e. the dispersion relation shows
significant differences in magnetic and non-magnetic regions. More importantly,
at and above the layer where the Alfven speed equals the sound speed we do not
see the dispersion relation of the slow acoustic mode with a lowered cut-off
frequency. Our comparisons with theoretical dispersion relations do not suggest
this is the slow acoustic wave type for the upward low-frequency wave. From
this we speculate that partial mode conversion from the fast acoustic to the
fast magnetic wave might take place.
