7 Conclusions

We have used ultraviolet image sequences from TRACE to provide a detailed view of quiet-Sun brightness oscillations in the upper photosphere and low chromosphere. The view is made comprehensive through combining space-time and Fourier representations of various sorts and through comparison with the well-modelled spectral behaviour of Ca IIH. The view is also well-defined because the ultraviolet passbands yield a simpler picture than the Dopplershift-sensitive Ca IIH core and are not too sensitive to magnetic canopy variations, and because TRACE's image sequences co-align precisely, do not suffer from seeing, and provide excellent statistics by combining large field with long duration.

A major motivation for this study was to diagnose high-frequency waves (well above f=10 mHz) that might heat the internetwork and network chromospheres. We have not found those, but instead have detected:

• Well-defined acoustic phase-difference properties between different passbands including signatures of wave steepening (Figs. 18, 19).

• Phase-difference contrast between the p-mode and pesudo-mode ridges and the interridges in two-dimensional (k_h,f) phase-difference spectra (Fig. 24).

• Strong evidence for ubiquitous internal gravity waves (Figs. 18 and 24).

• A mesh background pattern to the three-minute oscillation, which we attribute primarily to gravity-wave interference (Figs. 5, 11). The spatio-temporal appearance of internetwork grains is dominated by constructive interference between the acoustic and gravity wave patterns (Fig. 11).

• Three-minute brightness modulation aureoles around network (Figs. 14-16).

• Narrower modulation shadows around network - but only when normalised by the mean brightness (Fig. 17).

• Frequent presence of "persistent flashers'', network-like features "on the loose'' that presumably have magnetic anchoring and seem to represent isolated fluxtubes (Fig. 9).

• Wide zones of "intermediate'' pixels between network and internetwork that have higher time-averaged brightness than the internetwork "cell'' centers (Fig. 4). They contain more flashers and correspond closely to the mottle-like extensions seen in low-frequency C IV modulation which presumably delineate low-canopy topology (Fig. 14).

These findings define obvious desires for further study. Some issues, such as the properties and nature of power aureoles and the effects of canopy geometry, will gain from sampling more solar scenes in similar fashion but with variety in the degree of activity. The role of granular dynamics in setting the acoustic oscillation and the background mesh patterns may be studied through combination with photospheric imaging and Doppler mapping, by TRACE and MDI or with ground-based telescopes at higher angular resolution. The persistent flashers may be identified and studied using high-resolution magnetograms. The phase relationships portrayed here provide a valuable testbed for numerical simulation of the chromospheric three-minute oscillation. Internal gravity waves will be explored in a future paper using the October 14 white light data. Finally, simultaneous passband sampling as envisaged for the Solar Diagnostics Observatory may push the phase detectability limit to higher frequencies than reached here.

Acknowledgements

We are indebted to T. J. Bogdan, B. Fleck, P. G. Judge, O. V. Khomenko, R. I. Kostik and N. G. Shchukina for comments. J. M. Krijger's research is funded by The Netherlands Organization for Scientific Research (NWO). The TRACE sequences were taken as part of SOHO Joint Observing Program JOP72 proposed and led by P. G. Judge. J. M. Krijger and R. J. Rutten thank the Leids Kerkhoven-Bosscha Fonds for travel support and the Lockheed-Martin Solar and Astrophysics Laboratory at Palo Alto, the High Altitude Observatory at Boulder, the Jurusan Astronomi and Bosscha Observatory of the Institute of Technology Bandung, and their colleagues at these institutions for hospitality. The Utrecht-Naples collaboration is part of the European Solar Magnetometry Network supported by the European Commission under contract ERBFMRXCT980190. B. W. Lites acknowledges partial support from NASA Grant W-19.328. Careful reading by referee Dr. W. Curdt improved the paper.