A far ultraviolet imager for the International Solar-Terrestrial Physics Mission

Abstract

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

Far Ultraviolet Imaging from the Image Spacecraft: 1. System Design

“…Previously flown satellite imaging experiments have demonstrated the suitability of the vacuum ultraviolet spectral region for remote sensing observation of auroral precipitation (Anger et al, 1987;Frank et al, 1981;1988, Murphree et al, 1994Torr et al, 1995). In the wavelength region 120 -190 nm, a downward-viewing imager is minimally contaminated by scattered sunlight from clouds and the ground.…”

Analysis of auroral morphology: Substorm precursor and onset on January 10, 1997

“…The Ultraviolet Imager has a spectral bandpass between 120 and 200 nm [Torr et al, 1995]. In this study, image data taken with the LBH1 filter ( The size of the polar cap is strongly correlated with the substorm phases.…”

Isolating auroral FUV emission lines using compact, broadband instrumentation

“…The imager dimensions will also vary depending on the target of the mission it is used for. For example, in order to image the weakest emissions of interest at Earth (∼ 50 R, concentrated mainly in the 130.4 nm line) above a background of 8 R per 0.1 nm (consistent with with a UV spectrum from Torr et al 1995) to a significance of three sigma, a primary mirror of diameter ∼70 mm is required, slightly smaller than the 80 mm diameter JUDE mirror. The mirror size is currently limited by the low throughput of the 147 nm filter, so may potentially be reduced further by filter optimisation.…”

“…Better wavelength selection was achieved by the Polar UVI instrument using multilayer filters, which consist of alternating layers of high-and low-refractive index materials with carefully selected optical thicknesses such that light reflected from the boundaries between the layers experiences constructive interference at the wavelength of interest and destructive interference at other wavelengths. The Polar UVI oxygen filters each had 5 nm wide bandpasses and average out-of-waveband blocking of better than 3×10 −3 % (Zukic et al 1993), but the filters required a 45 • angle of incidence, increasing instrument complexity -the Polar UVI mass of 21 kg was substantially larger than the 7 kg (9.1 kg) total mass for the Viking (Freja) cameras (Torr et al 1995;Anger et al 1987;Murphree et al 1994). Good FUV wavelength discrimination may also be achieved with the use of imaging spectrometers, such as the Spectrographic Imager (SI) on IMAGE (Mende et al 2000; IMAGE also carried a Wideband Imaging Camera (WIC) based on the Viking/Freja Cassegrain design) but, as with the Polar UVI filters, this comes at the cost of increased instrument complexity and reduced throughput.…”