3. SUMER observations and data reduction
The data used here were obtained with SUMER on-board SOHO on 10 and 14 July 1996 (see Table 1). These datasets were taken in order to look for variations in electron density in the solar transition region, using the density sensitive line ratio of O IV 1399/1401. The pointing for our observations were centered on different regions in the Sun: one extended active region (AR), two `quiet' Sun regions (henceforth QS1 and QS2) and one region in the Northern coronal hole (CH). We used slit number six for the AR dataset ( arc sec2) and slit number four for the other datasets ( arc sec2). All the datasets were taken with a 20 s exposure time, and each region was observed over a period of approximately one hour and seven minutes. These observations were taken in a sit-and-stare mode with the rotational compensation turned off. This meant that for the CH an area of approximately arc sec2 was observed, since the rotational velocity in this region of the Sun is very low (1.5 arc sec in 67 minutes, see Fig. 2) 1. An area of arc sec2 was covered over the observation period for the QS datasets at disk centre, and arc sec2 for the AR dataset, (see Fig. 3 & Fig. 4).
Table 1. Description of observational data
Detector A was used for all the datasets and the observations were taken in first order. Due to very low signal-to-noise or problems with detector sensitivity at the ends of the slit image, some positions at the top and/or the bottom of the slit where clipped out. For the AR dataset thirty positions at the Northern end and four positions at the Southern end were clipped out, so that the final dimensions are arc sec2. For the CH dataset the final dimensions are arc sec2 after clipping low signal-to-noise pixels. For the QS the clipping depended on the dataset, and it was due to low signal-to-noise since the slit is centered in the detector. Four positions at the Southern end were clipped out for both datasets, so that for QS1 the dimensions where reduced to arc sec2, but for QS2 the dimensions where reduced further to arc sec2 after clipping out twenty-seven positions in the Northern end of the slit.
The O IV 1401.16 Å line is blended with the S I 1401.51 Å transition (see Judge et al. 1998, for reference wavelengths), although in most areas in the Sun the S I feature is considerably weaker than O IV . The S I line was appreciable only in the `quiet' Sun datasets. The O IV 1407/1401 ratio is also available from our data, but the O IV 1407.38 Å line is blended with the second order O III doublet at 703.85 Å, and some preliminary analysis with this ratio showed that unblending the two features was difficult.
Since the O IV lines we use here are not strong lines we used a binning in time of four minutes, plus a running mean along the slit of five pixels, to decrease the noise level of our data without losing a desirable spatial/time resolution. The low signal-to-noise of our data in the QS and CH regions made a reliable estimation of the electron density very difficult for some positions in our raster/temporal images. This, combined with the fact that the O IV 1399/1401 density-sensitive ratio is in the low density limit for a large fraction of the `quiet' Sun and coronal hole spectra, were the main reasons why for these regions a large part of our density estimates were set to the minimum theoretical value. Nevertheless, areas with measurable densities were found and they are discussed in Sect. 4.
3.2. Data reduction and calculation of errors
For the SUMER instrument, the process of data reduction involves three main steps: flat-fielding, de-stretching and radiometric calibration. Our dataset were automatically flat-field corrected on board. The de-stretching process is necessary in particular for the data located towards the edges of the detector due to various wavelength and spatial distortions (see Siegmund et al. 1994, Wilhelm et al. 1997). Other non-linearity effects that ought to be corrected in SUMER are dead-time effects and local gain depression. Dead-time effects of the detectors become significant for high total detector counts rates, for instance higher than 50 000 counts s-1. The local gain depression is critical for intense lines with more than 10 counts s-1 pixel-1. Detector noise is partly reduced by the flat field correction which corrects the readout noise and pixel-to-pixel variations.
The line fitting has been carried out using the CFIT_BLOCK subroutine (Haughan 1997). For all the datasets, only one Gaussian was used to fit either the O IV 1399 Å line or the O IV 1401 Å line. In the case of O IV 1401 Å, which has the weak line S I 1401.514 Å present in the QS and CH datasets, we checked using two Gaussians but found that the results were more reliable using only one. For the above corrections the basic IDL routines can be found from within the SUMER software tree. 2
The other source of noise in our data is the photon-related statistical noise, which obeys a Poisson distribution. Poisson noise in the data is calculated as the square root of the number of counts per pixel. For the estimation of the errors that affect our final results we have to include errors in the line fitting parameters and the propagation of these errors into the line ratio. Finally, the 1 uncertainty in the calculated values of the electron density are estimated from the theoretical curve (Fig. 1), by considering the corresponding 1 variation in the observed ratio.
The analysis of periodicities presented in Sect. 4 was carried out using the PERIODOGRAM.PRO routine given in the CDS software tree. This routine uses the method of Horne & Baliuna (1986) to calculate the periodogram.
© European Southern Observatory (ESO) 1999
Online publication: November 16, 1999