Waves in the solar transition region

The power spectra for line intensities of several lines formed in the upper transition region around 100000 to 250000 K are presented. A period of 5 min is clearly present in lines due to Oiii, Oiv, and Ov. In one dataset a period approaching 10 min is present for 40 min. The size of the emitting features is limited to 7 arc sec squared. In all datasets examined, there is excess power below 4 mHz everywhere along the slit, although the observed periods do not always come from the most intense regions. In 40% of instances clear periods are observable in the 2–5 mHz range with the largest power peak at 3.0 mHz. In all regions, the 5.0 mHz power peak is smaller. For the frequencies investigated there are no significant time delays in any of the datasets examined. This finding may not be entirely unexpected as the formation temperatures of Oiii (100000 K) and Ov (250000 K) may be too close in order to result in an observable phase shift.The observations are discussed in terms of trapped magnetic modes below the transition region and resonant absorption of MHD waves. For resonant absorption we derive from the observed period of 5 min and the observed extent of the structure a typical magnetic field strength of about 2 G. This value is in good agreement with results from MDI for quiet-Sun regions. Our results seem to imply that resonant waves can play a role in the heating of the quiet Sun. We discuss the effect of observing without tracking on the power spectrum and show that the effect is small.