The SUMER Investigation

The Solar Atmosphere

In the outer solar atmosphere, the plasma temperature, density, flow characteristics and wave activity vary strongly as a function of height and of horizontal position. Complex features, such as active regions, prominences, magnetic loops, bright points, and explosive and turbulent events, will influence the actual structure of the atmosphere. Generally, the temperature rises abruptly from values of about 10 000 K in the chromospheric layer, which occupies the first 2 000 km above the photosphere, to values in excess of 1 000 000 K in the corona, across a transition region only a few hundred kilometers thick. The existence of a hot corona requires a persistent non-radiative energy input to compensate for radiative and conductive losses and for the solar wind expansion.

The Experiment

The experiment "Solar Ultraviolet Measurements of Emitted Radiation" is designed for the investigation of

plasma flow characteristics, turbulence and wave motions;
plasma densities and temperatures;
structures and events associated with solar magnetic activity in the chromosphere, the transition zone and the corona.

Specifically, SUMER will

measure profiles and intensities of extreme ultraviolet (EUV) lines emitted in the solar atmosphere ranging from the upper chromosphere to the lower corona;
determine line broadenings, spectral positions and Doppler shifts with high precision and accuracy;
provide stigmatic images of selected areas of the Sun in the EUV with high spatial, temporal and spectral resolution and
obtain full images of the Sun and the inner corona in selectable EUV lines, corresponding to a temperature range from 10 000 to 2 000 000 K.

Parts of the information gathered with the SUMER instrument are available in our Pictures section; a list of Publications is also available.

SUMER Performance Characteristics

Single parabolic mirror telescope. Pointing and scanning will be performed by a mirror mechanism, which has a 2-dimensional scan capability of ± 32 arcmin. One step is equivalent to 0.76 arcsec; half-steps are possible. The east-west scan mechanism has to be operated in high-current mode (provided by the redundancy concept of SUMER) since autumn 1996. In order to maintain this capability, east-west raster scans are no longer permitted with SUMER. Pointing and rotation compensation procedures, however, can be employed without restrictions.
Normal incidence spectrometer with the grating in a Wadsworth mount.
Wavelength range: 500 to 1610 Å.
Scans can be performed within 21 s over the full range.
Two 2-dimensional microchannel plate detectors with cross delay-line anodes in photon-counting mode; KBr and bare microchannel plate photocathode.
Spatial resolution: 1.2-1.5 arcsec; the pixel size corresponds to approximately 1 arcsec.
Spectral resolution: lambda/dlambda = 19 000 to 40 000, where dlambda corresponds to the pixel size. Sub-pixel accuracy will be achievable for the determination of line shifts and widths.
Typical minimum sampling times: 3 to 10 s.
Doppler velocities down to 1 km/s can be measured seperately for each spatial resolution element.
Rear slit camera for monitoring the slit position on the solar disk.

The optical design is based upon two parabolic mirrors, a plane mirror and a spherical concave grating, all made of silicon carbide. The first off-axis telescope parabola mirrors the Sun on the spectrometer entrance slit. The second off-axis parabola collimates the beam leaving the slit. This beam is then deflected by the plane mirror onto the grating. Two detectors, located in the focal plane of the grating, collect the monochromatic images of the spectrometer entrance slit. Coverage of the full spectral range of the instrument requires a wavelength scan performed by rotating the plane mirror. A baffle system, consisting of an entrance aperture, light traps, an aperture stop, a pre-slit and a Lyot stop, completes the design.

The Optical Design in Numbers

(1 arcsec at L1 corresponds to 715 km on the Sun.)


The Telescope:
   Focal length                 1302.77 mm at 75°C
   Off-axis angle               4.5°
   Equivalent f-number          10.67
   Plate scale in slit plane    6.316 µm/arcsec
   Total dynamic field-of-view  64 × 64 arcmin²
   Smallest step sizes          0.38 arcsec
      (N-S and E-W)

The Slits:                      1   × 300, 
                                1   × 120, 
                                0.3 × 120, 
                                4   × 300 arcsec²

The Rear-Slit Camera:
   Wavelength range             5770 - 6520 Å
   Array size                   512 (spatial) pixels
   Pixel size                   25 µm

The Spectrometer:
   Wavelength ranges            
      Detector A                390 -  805 Å (2nd order) *
                                780 - 1610 Å (1st order)
      Detector B                330 -  750 Å (2nd order) *
                                660 - 1500 Å (1st order)
   Collimator focal length      399.60 mm
      Off-axis angle            7°
   Grating radius               3200.78 mm
   Grating ruling               3600.42 lines/mm
   Magnification factor **      4.092 at  800 Å
      in detector plane         4.407 at 1600 Å

The Detectors:
   Array size                   1024 (spectral) × 360 (spatial) pixels
   Pixel size (mean value)      26.5 × 26.5 µm²
   Angular scale                1.03 arcsec/px at  800 Å
                                0.95 arcsec/px at 1600 Å
   Spectral scale               22.3 mÅ/px at  500 Å (2nd order) *
                                21.0 mÅ/px at  800 Å
                                45.2 mÅ/px at  800 Å (1st order)
                                41.9 mÅ/px at 1600 Å

* In the range below 500 Å the sensitivity is very low, because of three normal-incidence reflections. Strong lines have, however, been observed in this regime during the calibration phase and under operational conditions. Some lines have been observed in 3rd order.

** The magnification factor is the ratio of the effective grating focal length and the collimator focal length.

IED, 18.Dec.98