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Astron. Astrophys. 364, 835-844 (2000)

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2. Instrument and observations

The observations were made with the SUMER spectrograph on 1996 June 20 above the active region NOAA 7974 at the solar east limb, starting at 20:11 UT. Fig. 1 shows the position and extension of the SUMER raster superimposed on the [FORMULA] 304 Å image of the eastern limb of the Sun taken at 19:41 UT with the EIT ultraviolet imager (Delaboudiniere et al. 1995).

[FIGURE] Fig. 1. SUMER raster superimposed on a section of the [FORMULA] 304 Å EIT image taken at 19:41 UT, showing the active region NOAA 7974 on the limb and its neighbour NOAA 7973 in photonegative representation. (Courtesy, EIT/SOHO consortium)

A full description of SUMER and its performance are given in Wilhelm et al. (1995, 1997) and Lemaire et al. (1997). The instrument is an ultraviolet telescope and spectrometer with a wavelength resolution element of 42-44 mÅ over the range 800-1610 Å (in first order). The spatial resolution is about 1" in the N-S direction, while in the E-W direction it depends on the slit width. In the observations analyzed in the present work a slit size of 4"x300" was used. The raster started 40" off-limb, above the position of the bright [FORMULA] protrusion seen in the EIT image, and the slit was stepped eastward for 133". At each position, two 40 Å wide spectra were obtained, one centered on the [FORMULA] 999 Å line, and the other on the [FORMULA] 1191 Å line. The integration time per spectrum was increased exponentially with position from 250 s at 40" to 867 s at 173". The total raster time was about 10 hr. The lines used in the present study are reported in Table 1.


Table 1. Spectral lines considered in the present work. The [FORMULA] line is observed in second order.

The observations analyzed in the present work include both plasma from active region structures and outside these structures. Fig. 2 shows the intensity maps obtained from [FORMULA] and [FORMULA] lines. [FORMULA] shows a highly structured behaviour, indicating that this ion is present only inside in a relatively cool structured area belonging to the active region at the limb. Outside this area the residual [FORMULA] emission is mainly due to scattered light. [FORMULA] presents a fairly uniform map with no signature of the active region.

[FIGURE] Fig. 2. [FORMULA] 1196 Å (top) and [FORMULA] 1191 Å (bottom) intensity maps of the emitting region.

We will refer to the low temperature plasma inside the structures as "prominence" plasma, and to the hotter coronal plasma as "active region corona". In the present work these two plasmas have been analysed separately.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001