R. C. Catura

EIT: Extreme-UltraViolet Imaging Telescope for the SOHO Mission

The Extreme-ultraviolet Imaging Telescope (EIT) will provide wide-field images of the corona and transition region on the solar disc and up to 1.5 R⊙ above the solar limb. Its normal incidence multilayer-coated optics will select spectral emission lines from Fe IX (171 Å), Fe XII (195 Å), Fe XV (284 Å), and He II (304 Å) to provide sensitive temperature diagnostics in the range from 6 × 104 K to 3 × 106 K. The telescope has a 45 x 45 arcmin field of view and 2.6 arcsec pixels which will provide approximately 5-arcsec spatial resolution. The EIT will probe the coronal plasma on a global scale, as well as the underlying cooler and turbulent atmosphere, providing the basis for comparative analyses with observations from both the ground and other SOHO instruments. This paper presents details of the EIT instrumentation, its performance and operating modes.

EIT OBSERVATIONS OF THE EXTREME ULTRAVIOLET SUN

The Extreme Ultraviolet Imaging Telescope (EIT) on board the SOHO spacecraft has been operational since 2 January 1996. EIT observes the Sun over a 45 x 45 arc min field of view in four emission line groups: Feix, x, Fexii, Fexv, and Heii. A post-launch determination of the instrument flatfield, the instrument scattering function, and the instrument aging were necessary for the reduction and analysis of the data. The observed structures and their evolution in each of the four EUV bandpasses are characteristic of the peak emission temperature of the line(s) chosen for that bandpass. Reports on the initial results of a variety of analysis projects demonstrate the range of investigations now underway: EIT provides new observations of the corona in the temperature range of 1 to 2 MK. Temperature studies of the large-scale coronal features extend previous coronagraph work with low-noise temperature maps. Temperatures of radial, extended, plume-like structures in both the polar coronal hole and in a low latitude decaying active region were found to be cooler than the surrounding material. Active region loops were investigated in detail and found to be isothermal for the low loops but hottest at the loop tops for the large loops.Variability of solar EUV structures, as observed in the EIT time sequences, is pervasive and leads to a re-evaluation of the meaning of the term ‘quiet Sun’. Intensity fluctuations in a high cadence sequence of coronal and chromospheric images correspond to a Kolmogorov turbulence spectrum. This can be interpreted in terms of a mixed stochastic or periodic driving of the transition region and the base of the corona. No signature of the photospheric and chromospheric waves is found in spatially averaged power spectra, indicating that these waves do not propagate to the upper atmosphere or are channeled through narrow local magnetic structures covering a small fraction of the solar surface. Polar coronal hole observing campaigns have identified an outflow process with the discovery of transient Fexii jets. Coronal mass ejection observing campaigns have identified the beginning of a CME in an Fexii sequence with a near simultaneous filament eruption (seen in absorption), formation of a coronal void and the initiation of a bright outward-moving shell as well as the coronal manifestation of a ‘Moreton wave’.

Imaging the solar corona in the EUV

Abstract The SOHO (SOlar and Heliospheric Observatory) satellite was launched on December 2nd 1995. After arriving at the Earth-Sun (L1) Lagrangian point on February 14th 1996, it began to continuously observe the Sun. As one of the instruments onboard SOHO, the EIT (Extreme ultraviolet Imaging Telescope) images the Suns corona in 4 EUV wavelengths. The He II filter at 304 A images the chromosphere and the base of the transition region at a temperature of 5 − 8 × 104 K; the Fe IX–X filter at 171 A images the corona at a temperature of ∼ 1.3 × 106 K; the Fe XII filter at 195 A images the quiet corona outside coronal holes at a temperature of ∼ 1.6 × 106 K; and the Fe XV filter at 284 A images active regions with a temperature of ∼ 2.0 × 106 K. About 5000 images have been obtained up to the present. In this paper, we describe also some aspects of the telescope and the detector performance for application in the observations. Images and movies of all the wavelengths allow a look at different phenomena present in the Suns corona, and in particular, magnetic field reconnection.

Calibration of the EIT instrument for the SOHO mission

Optical characteristics in the wavelength range 15 - 75 nm of the EUV imaging telescope to be launched soon on the SOHO mission are discussed. Bandpasses and photometric sensitivity of the multilayered optics telescope have been measured by a dedicated synchrotron light source at Orsay, France.

The Extreme Ultraviolet Imaging Telescope On Board SOHO

The Extreme-Ultraviolet Imaging Telescope (EIT) will be launched on-board the Solar Heliospheric Observatory (SOHO) in 1996. Images in four narrow bandpasses at wavelengths ranging from 17 to 31 nm will be obtained using normal incidence multilayered optics deposited on quadrants of a Ritchey-Chretien Telescope. The design of the telescope is discussed in detail and actual performances measured on a 2/3 scale mock up are reported.

Performance of the engineering model x-ray mirror of the Solar X-ray Imager (SXI) for future GOES missions

We have measured the x-ray imaging performance of a grazing incidence telescope mirror, the HT #17, employing a hyperboloid-hyperboloid design. This design provides improved wide-field imaging compared to an optimally defocused Wolter Type I mirror. This improvement will be advantageous for future Geostationary Operational Environmental Satellite (GOES) missions that will provide full disk images of the sun with the Solar X-ray Imager (SXI). The x-ray measurements were made in the X-Ray Calibration Facility (XRCF) at Marshall Space Flight Center and the results are presented here.

Performance of back-illuminated Tektronix CCDs in the extreme ultraviolet

The quantum efficiency (QE) and flat field characteristics of back-illuminated 1024 X 1024 Tektronix CCDs have been measured in the extreme ultraviolet (EUV) between 44 and 1216 angstroms. These CCDs have been fabricated for the focal plane detector of the Extreme-ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observing spacecraft. The back-side surface of the EIT CCDs have been specially processed to enhance and stabilize the EUV QE. All requirements for QE are met by these devices, although a poorly understood variation of QE with temperature will complicate data analysis.

X-ray calibration of the GOES-N/O solar X-ray imagers

The X-ray calibration of the GOES Solar X-ray Imagers (SXI) was accomplished at the component level and at the instrument level. The CCD and thin film filters were characterized in the facilities at the Lockheed Martin Solar and Astrophysics Laboratory. The grazing incidence telescope mirrors and the completed instruments were calibrated at the X-ray Calibration Facility (XRCF) at NASA’s Marshall Space Flight Center. The XRCF consists of an X-ray source at one end of a 518 m long evacuated tube and a large vacuum chamber at the opposite end. The X-ray source has a variety of interchangeable anodes and filters to provide filtered characteristic K- and L-shell line emission in the range from 0.109 to 8.6 keV. The absolute Photometric calibration of the SXI telescopes is very important for analysis and interpretation of their data, and to monitor the long-term solar variations at X-ray wavelengths. This paper describes the results of these calibrations.