A FAR-ULTRAVIOLET IMAGER FOR THE INTERNATIONAL SOLAR-TERRESTRIAL PHYSICS MISSION

The aurorae are the result of collisions with the atmosphere of energe tic particles that have their origin in the solar wind, and reach the atmosphere after having undergone varying degrees of acceleration and redistribution within the Earth's magnetosphere. The global scale phen omenon represented by the aurorae therefore contains considerable info rmation concerning the solar-terrestrial connection. For example, by c orrectly measuring specific auroral emissions, and with the aid of com prehensive models of the region, we can infer the total energy flux en tering the atmosphere and the average energy of the particles causing these emissions. Furthermore, from these auroral emissions we can dete rmine the ionospheric conductances that are part of the closing of the magnetospheric currents through the ionosphere, and from these we can in turn obtain the electric potentials and convective patterns that a re an essential element to our understanding of the global magnetosphe re-ionosphere-thermosphere-mesosphere. Simultaneously acquired images of the auroral oval and polar cap not only yield the temporal and spat ial morphology from which we can infer activity indices, but in conjun ction with simultaneous measurements made on spacecraft at other locat ions within the magnetosphere, allow us to map the various parts of th e oval back to their source regions in the magnetosphere. This paper d escribes the Ultraviolet Imager for the Global Geospace Sciences porti on of the International Solar-Terrestrial Physics program. The instrum ent operates in the far ultraviolet (FUV) and is capable of imaging th e auroral oval regardless of whether it is sunlit or in darkness. The instrument has an 8 degrees circular field of view and is located on a despun platform which permits simultaneous imaging of the entire oval for at least 9 hours of every 18 hour orbit. The three mirror, unobsc ured aperture, optical system (f/2.9) provides excellent imaging over this full field of view, yielding a per pixel angular resolution of 0. 6 milliradians. Its FUV filters have been designed to allow accurate s pectral separation of the features of interest, thus allowing quantita tive interpretation of the images to provide the parameters mentioned above. The system has been designed to provide ten orders of magnitude blocking against longer wavelength (primarily visible) scattered sunl ight, thus allowing the first imaging of key, spectrally resolved, FUV diagnostic features in the fully sunlit midday aurorae. The intensifi ed-CCD detector has a nominal frame rate of 37 s, and the fast optical system has a noise equivalent signal within one frame of similar to 1 0 R. The instantaneous dynamic range is > 1000 and can be positioned w ithin an overall gain range of 10(4), allowing measurement of both the very weak polar cap emissions and the very bright aurora. The optical surfaces have been designed to be sufficiently smooth to permit this dynamic range to be utilized without the scattering of light from brig ht features into the weaker features. Finally, the data product can on ly be as good as the degree to which the instrument performance is cha racterized and calibrated. In the VUV, calibration of an an imager int ended for quantitative studies is a task requiring some pioneering met hods, but it is now possible to calibrate such an instrument over its focal plane to an accuracy of +/-10%. In summary, very recent advances in optical, filter and detector technology have been exploited to pro duce an auroral imager to meet the ISTP objectives.