2 Observations

I have selected two data sets, one taken in 1999 and one in 2000, see Tables 1 and 2 for details of the observational parameters. Note that a number of additional data sets have been analysed containing both various active regions and also quiet sun. The results of these datasets are in agreement with the results presented in this paper and can be found on my home page (http://aipsoe.aip.de/~muglach/).

TRACE observed in three of its UV filters which are centered at 1550 Å, 1600 Å and 1700 Å. An image was taken in each of these filters consecutively and the time to cycle through the three filters was about 15 s in 1999 and 30 s in 2000. The cadence as given in Table 1 is the time difference between two images of the same wavelength and thus the temporal resolution of the sequences. Note that in 1999 the observing sequence also included white light images, but these were taken every 10 min. Thus, the 1999 sequence does not have a completely regular cadence. The cadence given in Table 1 is the mean cadence calculated from the duration of the complete sequence and the number of images. Also, the complete sequence from 1999 was about 4 h, but after about 2 h TRACE moved though the Earth's radiation belts and apart from heavy contamination by high energetic particles the cadence also became quite irregular. Additionally, since the size of the FOV in E-W direction was rather narrow the spot moved out of the FOV due to solar rotation. For this reason I finally chose a 2 h sequence for the current analysis. Thus, the sequence of 1999 has a higher Nyquist frequency ( $\nu_{\rm Ny}=33.04$ mHz) and less frequency resolution ( $\Delta \nu =142.4$ $\mu$Hz) compared to the sequence of 2000 ( $\nu_{\rm Ny}=16.64$ mHz and $\Delta \nu =68.5$ $\mu$Hz).

Also due to the different cadence the exposure times were changed between 1999 and 2000, being 5.6 s, 1 s, 2 s in 1999 and 9.7 s, 1.2 s, 3.4 s in 2000 for the three filters 1550 Å, 1600 Å, 1700 Å, respectively (see Table 2).

The 1700 Å filter covers mostly UV continuum, while the 1600 Å filter intensity is a combination of a continuum background, some small C I emission lines and some contribution of C IV. The 1550 Å filter is a rather narrowband filter centered at two strong emission lines of C IV (at 1548 Å and 1550 Å) (see Handy et al. 1999, Table 1 and Fig. 13).

According to the standard quiet sun VAL81 model (Vernazza et al. 1981) the disk center continuum intensity around 1700 Å originates from just below the temperature minimum region, 1600 Å from the temperature minimum and 1550 Å above the temperature minimum (which is located at h = 500 km). Of course, the 1550 Å filter includes the two strong C IV lines which originate at transition region temperatures (log T = 5.0). Thus, the 1700 Å images are the "cleanest'' diagnostic for the high photosphere/low chromosphere. Both 1600 Å and especially 1550 Å sample a combination of light from different heights and temperatures.

In the following sections I describe the details of the data reduction. Note that I do not use the standard SolarSoft data reduction procedures, as they are often not sufficient (the standard procedure would be to use the "trace_prep'' routine, which uses certain standard values for the various steps described below).