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Ultraviolet Auroral Imager

NSSDCA ID: 1992-064A-06

Mission Name: Freja
Principal Investigator:Dr. John Sandy Murphree


Freja's auroral imager experiment carried two ultraviolet cameras; one sensitive to the LBH band at wavelength 135-145 nm, and the other to the atomic oxygen line at 130.4 nm. The design was predicated by the emphasis on the micro-structure/physics of auroral displays. Each camera had a Cassegrain, spherical mirror of diameter 2.66 cm, and focal length 2.24 cm; the field of view was 25 x 20 degrees. The optical axes were perpendicular to the spin axis, which pointed to Sun. The primary image on the spherical photocathode was mapped as electron streams by the backing layer of Microenhanced Plate (MCP). The electron stream was further accelerated by a 4800 V potential to provide a mapped visual image on a curved phosphor plate. This visual image was finally mapped, via fiber optics, to a 288 x 385 pixel CCD. The response of each pixel corresponded to an angular resolution of 0.0783 x 0.0783 degrees which, at apogee, corresponded to a 5 km resolution. The key factor for providing blur-free images from a spinning spacecraft was the transfer of the charges in the CCD array to a parallel CCD array rapidly (at 780 Hz) in a pre-programmed sequence, so that each pixel of the parallel array corresponded exactly to the same point on an auroral arc. The charges were allowed to accumulate during the overall exposure time of 0.37 s (within each spin period of 6 s), before digitizing. The signal-to-noise ratio, for an aurora of 1 KR in the filtered bands, was about two and could be improved to about seven by adding charges from four (2 x 2) adjacent pixels. The cameras were operated in one of four modes: (1) 2 x 2 pixel averaging through a field of view of 25 x 20 degrees. (2) a broader spin-direction angle of about 105 degrees (i.e., horizon to horizon of the oval), with 2 x 2 averaging; (3) as in (1), but with single pixel digitizing that provided the highest resolution; or (4) 2 x 2 pixel averaging of composite images from both cameras. Only in mode (2) was it possible to image all auroral forms below; in the other modes the chosen imaging segment of the atmospheric surface could fall away from auroral forms, unless the aiming (i.e., starting instant of imaging during each spin) was helped by ground-based all-sky cameras. Each image could be down linked in real-time at 44 or 88 Kbps. Alternatively, several images could be stored on-board in a 952 KB memory before downlinking. Such stored images could number as many as 36, depending upon the operating mode. The instrument is inoperative since about 25 December 1992.

Alternate Names

  • F-5
  • Freja/UltravioletAuroralImager

Facts in Brief

Mass: 10.8 kg
Power (avg): 5.7 W
Bit rate (avg): 66 kbps

Funding Agency

  • National Research Council of Canada (Canada)


  • Space Physics: Ionospheric Studies
  • Space Physics: Magnetospheric Studies
  • Space Physics: Thermospheric Studies

Additional Information

Questions and comments about this experiment can be directed to: Coordinated Request and User Support Office



NameRoleOriginal AffiliationE-mail
Prof. Gordon RostokerCo-InvestigatorUniversity of
Dr. Leroy L. CoggerCo-InvestigatorUniversity of
Dr. Asgeir BrekkeCo-InvestigatorAuroral
Dr. John Sandy MurphreePrincipal InvestigatorUniversity of

Selected References

  • Andre, M., The Freja scientific satellite, Swedish Inst. of Space Phys., Oct. 1992.
  • Lundin, R., and G. Haerendel, The Freja satellite mission, EOS, 74, No. 29, 321, doi:10.1029/93EO00394, July 1993.
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