SpringerLink
Forum Springer Astron. Astrophys.
Forum Whats New Search Orders


Astron. Astrophys. 360, 1148-1156 (2000)

Previous Section Next Section Title Page Table of Contents

4. Observations

The Coronal Diagnostic Spectrometer (hereafter CDS) is an imaging spectrograph, whose primary objective is the study of the solar corona through line and continuum emission between 150 Å and 780 Å. The main achievements of CDS are therefore the large number of lines that can be observed simultaneously, the monochromatic imaging capability and the possibility of long duration studies.

CDS is composed of two distinct spectrometers sharing the same telescope: the Grazing Incidence Spectrometer (hereafter GIS) covering four spectral ranges: 151-221, 256-341, 393-492 and 659-785 Å; and the Normal Incidence Spectrometer (hereafter NIS) covering the ranges 307-379 and 513-633 Å. Full details of the CDS instrument may be found in Harrison et al. (1995). In the present work use is made both of the Normal Incidence Spectrometer and of the Grazing Incidence Spectrometer in order to maximise the number of ions whose lines are observed in the spectrum.

The observations analyzed in the present work have been obtained by CDS on 19 November 1996 and consist of a full spectrum scan of NIS and a full spectrum scan of GIS. As the two instruments cannot be used simultaneously, the GIS spectrum has been observed before the NIS spectrum. The NIS field of view is [FORMULA] arcsec2 and is centered at [FORMULA]; the GIS field of view is [FORMULA] arcsec2 and is included in the NIS one.

The NIS field of view is displayed in Fig. 4 overlaid to an MDI magnetogram which shows the bipolar structure of the active region: the NIS (and GIS) field of view include only the positive polarity of the region. Fig. 3 and Fig. 5 are monochromatic images of the observed active region at He i 537 Å (around [FORMULA] K) and Mg x 625 Å (2.5[FORMULA] K).

[FIGURE] Fig. 3. CDS intensity map of the observed region observed by the He i 537 Å line. The intensity closely follows the magnetic field pattern. The contours of the portions selected for the analysis are overplotted on the map.

[FIGURE] Fig. 4. MDI magnetogram of the observed active region. The CDS field of view is overlaid on the magnetogram: the CDS observation covers only the positive portion of the bipolar magnetic region.

[FIGURE] Fig. 5. CDS intensity map of the observed region observed by the Fe ix 171 Å (left) and Mg x 625 Å (right) lines. The intensity closely follows the magnetic field pattern. The contours of the portions selected for the analysis are overplotted on the map.

Two smaller portions of the emitting region have been selected to be analyzed, and are overlaid in Fig. 3 and Fig. 5. They correspond to a region of enhanced magnetic field corresponding to He i enhanced emission, and to a small bright point with enhanced emission in ions formed at around [FORMULA] K (visible, for example, with the Fe ix 171 Å line).

The GIS and NIS images have been coaligned using the lines between 308 and 340 Å which are observed both in the GIS 2 and NIS 1 channels. The NIS observation has been cleaned from cosmic rays and from line tilting; in the GIS observation ghosting regions have been identified and, where possible, corrected. Only GIS lines which were unambiguously identified as free of ghosts have been included in the present study. For more details on "ghosts" in the GIS spectrum, see Breeveld (1996) and Landi et al. (1999b).

Data have been calibrated according to the standard CDS calibration available in the CDS software (as of autumn 1999). A large amount of lines ([FORMULA] 70) has been used, belonging to ions sensitive to the temperature range from 105 K to more than 106 K. Table 1 lists the ions whose lines have been used in the present study. A complete list of the observed lines can be found in Landi et al. 2000. Table 1 shows that lines from high-FIP (O, Ne) and low-FIP (Mg, Al, Si, Ca, Fe) elements were detected in the present dataset; S xiv is at the boundary between high and low-FIP elements, since its FIP is 10.4 eV.


[TABLE]

Table 1. Ions whose lines are used in the present analysis.


Previous Section Next Section Title Page Table of Contents

© European Southern Observatory (ESO) 2000

Online publication: August 23, 2000
helpdesk.link@springer.de