In September 1999 and 2000 international joint observing campaigns were carried out to study the oscillatory behaviour of active regions. The main goal of these campaigns was to obtain comprehensive information on the interaction of solar oscillations and the magnetic field on small spatial and short temporal scales. A range of height in the solar atmosphere, from the deep photosphere to the corona, was covered by a variety of instruments and observing techniques.

A number of aspects of active region oscillations have already been addressed in several investigations which resulted from these campaigns: umbral oscillations in the transition region (TR) and corona (Muglach & O'Shea 2001; O'Shea et al. 2002a,b), intensity variations around sunspots (Georgakilas et al. 2002) and photospheric oscillations in umbrae and pores (Balthasar et al. 2000a,b). Muglach (2002) presents preliminary results of this work.

In this contribution I will concentrate on oscillations in the high photosphere and low chromosphere as observed with the Transition Region and Coronal Explorer (TRACE, Handy et al. 1999). I have studied the spatial distribution of oscillatory power in extended active regions (ARs) that contain sunspots, surrounding plage and patches of quiet sun with network and internetwork.

Table 1: Observational parameters: The first column gives the date of the data set and the second one the number of the active region. The images from 1999 contain two ARs, 8693 in the center of the FOV, which was the primary target of the campaign that day and 8699 at the top of the FOV. Column 3 gives the location of the center of the FOV away from disk center and Col. 4 the size of the FOV (as taken by the instrument). Column 5 gives the cadence of the images, calculated from the duration of the complete sequence divided by the number of images as indicated in Col. 6. Column 7 gives the Nyquist frequency, $\nu _{\rm Ny}$ in mHz, and Col. 8 the frequency resolution, $\Delta \nu$ in $\mu$ Hz. Column 9 finally indicates if white light images were taken interleaved with the UV observations.
date AR location FOV cadence # of $\nu _{\rm Ny}$ $\Delta \nu$ comment

[ $^{\prime \prime}$ from [ $^{\prime \prime}$ ] [s] images mHz $\mu$ Hz

center]

13/09/99 8693 -102, 93 128, 320 15.13 464 33.04 142.4 white light images

8699 every 600 s

29/09/00 9172 -207, 93 192, 256 30.05 486 16.64 68.5 no white light images

**Table 1:** Observational parameters: The first column gives the date of the data set and the second one the number of the active region. The images from 1999 contain two ARs, 8693 in the center of the FOV, which was the primary target of the campaign that day and 8699 at the top of the FOV. Column 3 gives the location of the center of the FOV away from disk center and Col. 4 the size of the FOV (as taken by the instrument). Column 5 gives the cadence of the images, calculated from the duration of the complete sequence divided by the number of images as indicated in Col. 6. Column 7 gives the Nyquist frequency, $\nu _{\rm Ny}$ in mHz, and Col. 8 the frequency resolution, $\Delta \nu$ in $\mu$ Hz. Column 9 finally indicates if white light images were taken interleaved with the UV observations.
date	AR	location	FOV	cadence	# of	$\nu _{\rm Ny}$	$\Delta \nu$	comment
		[ $^{\prime \prime}$ from	[ $^{\prime \prime}$ ]	[s]	images	mHz	$\mu$ Hz
		center]
13/09/99	8693	-102, 93	128, 320	15.13	464	33.04	142.4	white light images
	8699							every 600 s
29/09/00	9172	-207, 93	192, 256	30.05	486	16.64	68.5	no white light images

TRACE is a filtergraph instrument working in several EUV, UV and visible band-passes. The broad-band filters of TRACE provide a very clean intensity diagnostic that is not contaminated by crosstalk from velocity signals which might be the case for filtergrams like Ca II K or H $\alpha$ . In its near-Earth orbit TRACE produces image sequences that are free of atmospheric disturbances. They can cover considerable duration, but are still limited by the time-scale of evolution of the solar structures if one actually wants to make use of the full spatial resolution of 1 $^{\prime \prime}$ . Also, the spacecraft passes through the Earth's radiation belts which limits long-duration uninterrupted sequences (see below).

The field of view (FOV) of the TRACE CCD of 512 $^{\prime \prime} \times 512^{\prime \prime}$ covers a substantial fraction of the solar disk. However, the obtainable FOV is reduced due to a lack of solar rotation compensation and a limited telemetry which conflict with the need of high cadence and the demand for observations in several filters. On-board data compression can improve the situation, but it also increases the noise of the data (Aschwanden et al. 2000) as the compression algorithms are not lossless.

Earlier work on chromospheric dynamics deals with the sun outside active regions ("quiet sun''). An overview of this earlier ground-based work is given e.g. in Rutten & Uitenbroek (1991), while more recent work is described in Rutten (1999). The latest developments on sunspot oscillations are reviewed by Staude (1999) and Bogdan (2000).

Most earlier work was performed with spectrographs. They have the advantage of providing intensity as well as Doppler velocity to investigate the dynamics, but were limited to single slit positions on the disk and thus had very poor spatial coverage. In a few cases 2-d imaging was done (e.g. Kneer & v. Uexküll 1993) but they usually showed either spatially averaged power spectra or $k - \omega$ diagrams.

The first 2-d large-scale study of chromospheric AR dynamics comparable to this work was carried out by Braun et al. (1992). They used a 50 h sequence of full disk Ca II K images from the south pole. Interestingly, they found an enhancement of high frequency (3 min) acoustic power in the surroundings of ARs, while the actual locations of strong magnetic fields (sunspots) displayed a lack of power (at all frequencies) as has been known for a long time. Similar studies in Ca II K were later carried out by Toner & LaBonte (1993) and Thomas & Stanchfield (2000). They all found the same enhancements of high frequency power around the AR. Using lower temperature lines like Fe I (from ground) or Ni I (with MDI on SoHO) various groups searched for these features in the photosphere (Brown et al. 1992; Hindman & Brown 1998; Thomas & Stanchfield 2000; Donea et al. 2000; Jain 2001; Jain & Haber 2002). Power enhancements (also called power aureoles or halos) were present in 3 min velocity power maps. Their signature seems much weaker and patchier in the photosphere than in the Ca II K maps. No power enhancements were found in 5 min velocity maps and in photospheric continuum intensity maps. Hill et al. (2001) concluded that at least part of the patterns seen in acoustic power maps obtained from the ground can be due to the effect of seeing.

Recently, Krijger et al. (2001) carried out an analysis of TRACE data (similar to this one) restricted to the quiet sun, studying various network-internetwork issues. Note that the data sets of the two observing campaigns described in this article also included some true quiet sun (away from any ARs). As these results are in general compatible with the results of Krijger et al. (2001), they are not shown here.

This paper is structured in the following way: after this introduction a detailed description of the observations and the data reduction procedure are given in Sects. 2 and 3. Section 4 explains the results, which are discussed in Sect. 5. A summary and outlook are finally provided in Sect. 6.

1 Introduction