In this paper I have presented an analysis of time sequences of filtergrams taken by TRACE at 1700 Å, 1600 Å and 1550 Å. A detailed description of the data reduction and the statistical analysis is given. The main result is the presence of low power areas around the ARs in the 3 min intensity power maps. This lack of power which is also present around well-developed network is interpreted as due to the interaction of the underlying acoustic wave field with the inclined magnetic field of the AR (either due to wave conversion or wave reflection).
The results of this work leave ample space to further investigate this topic and clarify some of the following items.
One possibility to verify the above interpretation is the calculation of the actual magnetic field topology via a magnetic field extrapolation. For the Sep. 2000 data MDI magnetograms are available and such calculations are currently in progress.
As we have time-sequencies of these MDI data, we can also produce low photospheric power maps (similar to other MDI studies of that kind) and can thus make a direct, one-to-one comparison of the power distributions.
These first two points are currently under investigation and will be the topic of a follow-up paper (Muglach & Hofmann 2003).
From an observational point of view it would also be desirable to make such a comparison with Ca II K images, because a simultaneous and co-spatial data set should be able to solve the puzzle of the Ca II K power halos.
In a next step the current TRACE data can be used to calculate phase and coherence spectra of the waves to see how the phase relations change due to the magnetic field.
One weakness of the interpretation presented in this article is the assumed formation height, which is only valid for an averaged quiet sun. One would need detailed calculations of the formation height of the TRACE filters, especially for magnetized atmospheres.
Realistic numerical modelling of the wave propagation with a measured magnetic field distribution as input (and comparing the output with the power maps) would be very desirable, especially to get a better idea of the details of the interaction of the magnetic field with the oscillations (wave conversion and reflection).
Finally, a completely different test could be performed: using full Stokes spectro-polarimetry one can derive the complete magnetic field vector. Thus the field structure is determined at the formation level of the spectral line that is used and can be compared with simultaneously observed TRACE power maps.
Acknowledgements
This research is part of the TMR-ESMN (European Solar Magnetometry Network) supported by the European Commission. I would like to thank the TRACE instrument team and planners for their support during the observing campaigns. Special thanks go to T. Tarbell for his help in optimizing the TRACE observing program. I would also like to thank the members of the AIP J. Staude, H. Balthasar and A. Hofmann for their support during my stay there and members of the ESMN B. Fleck, E. O'Shea, R. Rutten, J. Krijger and P. Sütterlin for discussions and for providing some of their software. The magnetograms in Figs. 2 and 3 are courtesy of SoHO/MDI consortium. SoHO is a project of international cooperation between ESA and NASA.
Copyright ESO 2003