4 Results

   
4.1 Topological context

The dynamics on the sun are closely related to the structure of the atmosphere. The solar magnetic field is the structuring agent.

Figure 2 (left) shows a temporal average of the 4 hours of co-aligned 1700 Å images (Sep. 2000) and next to it a single magnetogram taken by the Michelson Doppler Imager (MDI, Scherrer et al. 1995) on-board SoHO, giving the distribution of line-of-sight magnetic flux density of the same area (Fig. 2 right). AR 9172 contains a bipolar region with a moderately sized regular sunspot and surrounding plage area.

Figure 3 shows the same for the data from Sept. 1999. AR 8693 in the center of the FOV was the main target of the campaign on that day. A pore and plage of opposite polarity were north of the main spot. The AR on top of Fig. 3 was very active and showed a lot of brightenings during the 2 h sequence, which were visible in all filters (as can be seen in the movies on the web).

We can differentiate between three different magnetic domains within the FOV. (i) The dark structures in the 1700 Å images are sunspots and pores, with sunspot umbra and penumbra well recognizable. (ii) The bright structures around the spots are plage and further away from the AR is the bright chromospheric network. Immediately around the spot we can see radial spiky features. They are the signature of outward moving bright dots and can be identified in the magnetograms as moving magnetic features. For the spots, pores, plage and network we can find a one-to-one relation with the black and white features in the magnetogram. Note that MDI uses Ni I 6768 Å, which is formed around 100-200 km (Bruls 1993). Thus, the magnetic topology given here with an MDI filter magnetogram refers to the low photosphere.

All of these structures harbour strong (kG) magnetic fields in the photospheric layers of the solar atmosphere and differ primarily in size. Field strengths between 2000 and 4000 G are common in sunspot umbrae with a radially outward decrease. Observationally and semi-empirically determined intrinsic field strengths of the network are 1500 G at the $\tau=$ 1 level (Stenflo 1973; Muglach & Solanki 1992). Various authors get the same values for the plage (Keller et al. 1990; Hasan et al. 1998; Bellot Rubio et al. 2000). Both plage and network can be considered as made of an assembly of small-scale magnetic elements, which all have very similar properties. The different flux density values in MDI are primarily due to different filling factors (plage is more densely populated with these fluxtubes than the network).

(iii) The dark cell like structures in the TRACE images are the internetwork cells. The internetwork (IN) is evenly grey in the magnetograms, which is partly due to the chosen contrast of the figures. Nevertheless, the flux values are very low (around zero), as the IN contains very little longitudinal magnetic flux. Thus, earlier works on chromospheric dynamics consider the IN as magnetic field free regions. This also makes wave propagation calculations much easier, as magnetic fields are usually not part of these calculations (e.g. Carlsson & Stein 1997). The true magnetic structure of the internetwork as well as its relation to internetwork dynamics are still under debate, see e.g. the recent papers of Lites et al. (1999) and Sivaraman et al. (2000) for diverging results on the magnetic nature of Ca II grains. Concerning internetwork field strengths, Lin (1995) used full Stokes vector polarimetry in the infrared and finds a broad distribution of field strengths peaking around 600 G. This confirms earlier findings from visible data (e.g. Keller et al. 1994).

Still weaker field values are obtained using Hanle effect diagnostics which give only a few Gauss for a turbulent (mixed polarity) magnetic field (e.g. Stenflo et al. 1998). A small scale mixed polarity topology is also supported by the observation and inversion of asymmetric and unusually shaped Stokes V profiles (e.g. Sigwarth 2001; Socas-Navarro & Sánchez Almeida 2002).

As can be seen in the magnetograms the ARs are bipolar, e.g. in Fig. 2 the inversion line of the two polarities runs diagonal to the left of the sunspot. Filaments were located along the inversion lines as could be seen in H$\alpha$ images on the same day.

   
4.2 Power maps

Figure 7: Spatial distribution of intensity oscillatory power for the Sept. 2000 TRACE data (1600 Å channel). The left image shows the integrated power between 2.3-4.3 mHz (5 min range) and the right one between 5.5-7.5 mHz (2-3 min range). Only those power peaks have been included that are statistically significant according to the randomisation test, which means that all power with a probability less than 95% has been set to 0 and is thus not included when calculating the frequency bins (cf. Sect. 3.8.1). Dark areas have less power than bright ones.

Figure 8: Spatial distribution of intensity oscillatory power for the Sept. 2000 TRACE data (1550 Å channel). The left image shows the integrated power between 2.3-4.3 mHz (5 min range) and the right one between 5.5-7.5 mHz (2-3 min range). Only those power peaks have been included that are statistically significant according to the randomisation test, which means that all power with a probability less than 95% has been set to 0 and is thus not included when calculating the frequency bins (cf. Sect. 3.8.1). Dark areas have less power than bright ones.

Figure 9: Spatial distribution of intensity oscillatory power for the Sep. 1999 TRACE data in 1700 Å, 1600 Å and 1550 Å (from left to right). The top images show the integrated power between 2.3-4.3 mHz (5 min range) and the bottom ones between 5.5-7.5 mHz (2-3 min range). Only those power peaks have been included that are statistically significant according to the randomisation test, which means that all power with a probability less than 95% has been set to 0 and is thus not included when calculating the frequency bins (cf. Sect. 3.8.1). Dark areas have less power than bright ones.

Figures 4-6 show the result of the data reduction and the application of the various statistical tests. I use the data from the 1700 Å filter (of the 2000 campaign) as an example. Figure 4 left shows the spatial distribution of power in the 5 min frequency band, on the right is the map of 3 min power. Each image is linearly scaled, the brighter the region, the higher the power, black means that there is no power. The iterative randomisation test has been applied to the data, so only power peaks that are found to be statistically significant are included. Figure 5 shows the same as Fig. 4, but the 99% Groth level (cf. 3.8.2) has been subtracted from the raw power spectra. Figure 6 shows the analogous maps of the 3 $\times$ 3 pixels summed data (including the randomisation test after summing). Comparing the power distributions of Figs. 4-6 one can see that they all look very similar. The contrast in Fig. 5 is a bit higher than in Fig. 4. The decrease in spatial resolution is easily visible in Fig. 6, but the general large-scale pattern in the power distribution is still clearly present. Thus, all three tests give consistent results.

The sunspot displays several well-known features. The umbra is completely black in all maps which means that there is no significant power. This is an effect of the low count-rate that we have in the dark umbra at full spatial resolution. When summing up a larger number of pixels we find the signature of umbral oscillations in the 3 min range as demonstrated in Muglach & O'Shea (2001). These oscillations are well known from ground-based observations (see e.g. Lites 1992 for a review). They have also been observed in EUV spectra by other space instruments such as SUMER and CDS on-board SoHO (e.g. Brynildsen et al. 2002, see review by Staude 1999). In the penumbra one can see significant power in the 5 min range. There is also the signature of radially outward moving waves, which can be confirmed by displaying the sequence of images as a movie. Georgakilas et al. (2002) have analysed these waves in great detail and I refer the interested reader to this paper.

Outside the sunspot there is plage, network and internetwork. It is well known from ground-based observations of e.g. the Ca II K and H line that there is a distinct difference in wave frequencies between quiet sun network and internetwork: the network is dominated by oscillations with periods of 5 min and longer, while the internetwork displays additional power in the 3 min range (e.g. Rutten & Uitenbroek 1991). This is confirmed in Figs. 4-6. The brightest patches in the 5 min map can be found at the locations of the network and plage, when comparing Figs. 4-6 with Fig. 2. The nature of the network oscillations is still not known, although the fact that the network (and plage) consists of small-scale strong-field magnetic elements hints at a connection to these fluxtubes. The 3 min internetwork oscillations (as observed in the Ca II H line) have been successfully reproduced by acoustic waves that travel into the chromosphere and form shocks in the higher layers (Carlsson & Stein 1997).

Figures 4-6 show another interesting and new feature, especially in the 3 min map: in the surroundings of the spot, plage and network the 3 min power is considerably lower than in the internetwork away from the AR. This is quite a large-scale effect and produces a dark area around the AR. The low field region in the plage around $x=30^{\prime \prime}$ and $y=150^{\prime \prime}$ almost completely disappears in the 3 min maps, which means that there is hardly any significant power. When comparing this region with the magnetogram one can see that it lies exactly at the magnetic neutral line (between the black and white of the two polarities). The same is true for a large internetwork cell centered at $x=60^{\prime \prime}$ and $y=100^{\prime \prime}$, which shows a gradient of power radially away from the sunspot. The network regions further away from the spot show a darkening immediately around them as well. This darkening is also slightly recognizable in the 5 min maps but is quite pronounced in the 3 min maps. None of the AR studies listed in the introduction mention such an effect, so this is a new finding in large-scale AR dynamics. Nevertheless, first hints of this can also be found around quiet sun network structures and are mentioned by Judge et al. (2001) and Krijger et al. (2001), which is in agreement with my own quiet sun data. McIntosh et al. (2001) argue that some of the observations of Judge et al. (2001) and others can be explained by differences in the magnetic field topology which can influence the oscillation pattern found in the higher atmosphere. This will be discussed in more detail in a follow-up paper (Muglach & Hofmann, in preparation). Figures 7 and 8 show the power maps of the 1600 Å and 1550 Å filters respectively. The same patterns can be found in them as well. Additionally, an increased influence of transition region dynamics is visible with increased formation temperature (or smaller wavelength of the filter) as bright horizontal structures in the plage region and in the penumbra of the spot (primarily seen in the 5 min maps). These are due to short-term brightenings typical for TR dynamics. Finally, Fig. 9 summarizes the results of the data from 1999, 5 min maps are on top, 3 min maps at the bottom, 1700 Å, 1600 Å and 1550 Å from left to right. The resulting power distribution is completely consistent with the results described above. As mentioned earlier AR 8699 at the top of the FOV was very active during the observing time, which can be seen as white areas in the power maps, getting more extended the higher the formation temperature.