The aurorae are the result of collisions with the atmosphere of energetic 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 phenomenon represented by the aurorae therefore contains considerable information concerning the solar-terrestrial connection. For example, by correctly measuring specific auroral emissions, and with the aid of comprehensive models of the region, we can infer the total energy flux entering the atmosphere and the average energy of the particles causing these emissions. Furthermore, from these auroral emissions we can determine 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 are an essential element to our understanding of the global magnetosphereionosphere-thermosphere-mesosphere. Simultaneously acquired images of the auroral oval and polar cap not only yield the temporal and spatial morphology from which we can infer activity indices, but in conjunction with simultaneous measurements made on spacecraft at other locations within the magnetosphere, allow us to map the various parts of the oval back to their source regions in the magnetosphere. This paper describes the Ultraviolet Imager for the Global Geospace Sciences portion of the International Solar-Terrestrial Physics program. The instrument operates in the far ultraviolet (FUV) and is capable of imaging the auroral oval regardless of whether it is sunlit or in darkness. The instrument has an 8 ~ circular field of view and is located on a despun platform which

The optical constants of MgF(2) (bulk) and BaF(2), CaF(2), LaF(3), MgF(2), Al(2)O(3), HfO(2), and SiO(2) films deposited on MgF(2) substrates are determined from photometric measurements through an iteration process of matching calculated and measured values of the reflectance and transmittance in the 120-230-nm vacuum ultraviolet wavelength region. The potential use of the listed fluorides and oxides as vacuum ultraviolet coating materials is discussed in part 2 of this paper.

We report on the development of a new coating design for applications in the vacuum ultraviolet that yields significantly higher reflectivity over selectable bandwidths. We demonstrate that the concept can be used to fabricate high-performance narrow-band and broadband reflection filters, whose spectral properties can be greatly enhanced by utilizing several of these filters in tandem. For example, we have fabricated a narrow-band filter at the location of the 0 I 135.6-nm line with a 3.2-nm bandwidth, a peak transmittance of 39.3%, and out-of-band wavelength blocking of better than 10(-4)%. The principle of ourdesign approach is to use a combination of high (H) and low (L) refractive-index dielectric pairs so that H + L = lambda(r)/2, where H/L < 1. H and L designate the optical thicknesses of high- and low-index film materials. This kind of choice for the high-low ratio reduces the effects of absorption for the H films for which the extinction coefficient in the vacuum ultraviolet is much higher than for the low-index film material MgF(2). The reduced absorption of multilayers with H/L < 1 results in a significant increase in reflectivity compared with the classical quarter-wave stack for which H/L = 1.

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