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Astron. Astrophys. 358, 1097-1108 (2000)

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2. The CDS observation scheme

CDS consists of a Wolter-Schwartschild II grazing incidence telescope feeding two spectrometers. A single exposure represents one spectrum for the region of the Sun imaged through a selected slit and images may be constructed simultaneously in all emission lines by rastering the image, i.e. through interlacing exposures with the movement of a flat scan mirror. The field of view of the telescope is 4x4 arcminutes and images of this size can be readily built up. To produce larger images we must repoint the entire instrument using a dedicated pointing system and construct images from a mosaic of the 4x4 arcminute frames. This process is time consuming, but is a necessary feature of such a spectrometer system.

Thus, CDS is not able to image the full Sun with a cadence appropriate for studies of the kind required for the investigation of CME onsets, e.g. well under 1 hour. Therefore, the instrument is used to monitor a likely CME onset region which is much smaller than the solar disc, and the LASCO observations are used to concentrate on the overlying corona, to identify and study CMEs which propagate from that region. Other instruments have been used to supplement the CDS and LASCO observations, during the running of the observing campaigns, to allow detailed studies of associated solar activity and the interplanetary influences. These are beyond the scope of the current report.

The basic tool we use for CDS is a 4x12 arcminute scan along a chosen limb (that is, a stack of 3 4x4 arcminute frames). An example is given in Fig. 1 which displays a set of such images in the He I 584 Å line on 16 July, 1997 (CDS observation run 8449). He I has a characteristic temperature up to 20,000 K.

[FIGURE] Fig. 1. A sequence of 4x12 arcminute images of the western limb taken in the 20,000 K He I 584 Å line on 16 July, 1997. The top left image was taken from 10:02 UT, with time running from left to right and down. The image cadence is 50 minutes with the last image starting at 22:32 UT. There is a data drop-out in the central portion of the fourth image.

As with all space instrumentation, CDS has limited telemetry (11.3 kbit s-1) which means that we have to trade between the fraction of the spectral range returned and the image cadence time. For this particular work we require a cadence of under 1 hour and for this large 4x12 arcminute field with an appropriate exposure time at each location of the rastered image (10 seconds) we find a cadence of 50 minutes if we return data from just 6 emission lines. The spectral lines selected, and the wavelengths across these lines which are returned in the telemetry are listed in Table 1. The line profiles are sampled over 15 bins across the range given in the table. To produce the fastest possible rasters, bright lines were selected (to minimise the exposure time). Table 1 also lists the characteristic temperatures of the selected lines.


Table 1. The emission lines used for this study

The CDS instrument allows a choice of slit and for this sequence we make use of a 4x240 arcsecond slit which is rastered though 60 locations to build up the basic 4x4 arcminute image.

The He I 584 Å line views the coolest plasmas and is, in effect, our chromospheric indicator. The O V line is a bright line at transition region temperatures and the Mg IX line is a bright coronal line. The Fe XVI line is used to identify high temperature plasmas and is particularly bright in active regions but not usually bright in the quiet Sun. The Si X line pair intensity ratio is a useful density diagnostic for plasmas at about 1.3 million K. Thus, in summary, the 6 lines cover a large temperature range simultaneously (2 x 104 - 2 x 106 K), allowing us to determine the contribution and response to the ejection processes of plasmas at very different temperatures in the solar atmosphere, and we have a useful density diagnostic capability. In addition, the line shapes can be used to detect plasma flows in the various temperature regimes. Thus, with these basic spectroscopic tools we can embark on a rather unique examination of the mass ejection onset process, with supporting observations, in particular, from the LASCO coronagraph.

The CDS/LASCO campaign described has been run on numerous occasions since late 1996. Early campaigns did not acquire CME onset data due to the rarity of CME events at that time. More recent runs have `caught' CME onsets and some preliminary reports have been given by Harrison (1997a,b)

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© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000