2.3 PHOTON IMAGING
The IMAGE FUV spectrographic imager has the high spectral resolution, high sensitivity, and high throughput necessary to image the electron aurora, proton aurora, and the geocorona emissions. This imager is combined with a wideband imaging camera (Viking and Freja heritage) for measurement of the precipitating particle morphology with high spatial resolution and the greatest possible sensitivity, over the entire 134-160 nm spectral range. The intense, cold goecorona (Lyman emissions at 121.6 nm) and less intense, Doppler-shifted Lyman auroral emissions will be imaged separately, requiring a spectral resolution of 0.2 nm.
The FUV spectrographic imager has two slits, positioned parallel to the satellite spin axis. Slit 1 is a narrow slit, 1 pixel wide, with an equivalent wavelength resolution of 0.2 nm. The narrow slit width, combined with the 0.1 nm spectral resolution of the system, allows the imager to distinguish between, and separately measure, the central core of the Lyman [a] emissions (geocorona) and the Doppler-shifted components (proton aurora). Slit 2 is 15 pixels wide, which is equivalent to a bandwidth of 3 nm. This slit is used to separate the OI 135.6 nm emissions from the brighter and much scattered OI 130.4 nm emissions and has a Lyman [a] blocking transmission filter. On the detector, there are two regions of interest. Region 1 contains the Lyman [a] (121.6 nm) emissions from the geocorona and proton aurora through slit 1. Region 2 contains the 135.6 nm emissions from the electron aurora through slit 2. This region actually consists of about 80% OI 135.6 nm, about 20% 135.4 nm LBH, and a negligible amount (approx. 2%) of additional LBH at 140 nm that enters through slit 1.
The wideband imaging camera uses the basic design flown on the Viking and Freja satellites and observes auroral LBH emissions from 134 nm to 160 nm [Anger et al., 1987]. The emission lines in this region compete favorably with the dayglow even in sunlight. The larger field of view (see table 1) permits a long dwell (or integration) period and increases the apparent sensitivity.
The EUV instrument images resonantly scattered solar emissions from plasmaspheric He+ at 30.4 nm. Effective imaging of plasmaspheric He+ requires global "snapshots" in which the high apogee of the IMAGE mission and the wide FOV of the EUV imager provide, in a single exposure, a map of the entire plasmasphere from the outside with a sensitivity of 0.2 count/s-pixel-Rayleigh (R), a spatial resolution of 0.1 RE, and a time resolution of several minutes. The 30.4 nm feature is easy to measure because it is the brightest ion emission from the plasmasphere, it is spectrally isolated, and the background is negligible. Measurements are easy to interpret because the plasmaspheric He+ emission is optically thin, so its brightness is directly proportional to the He+ column abundance. Since the 1970's, EUV photometers in low-Earth orbits have produced "inside-out" images of the plasmasphere with a brightness f approx. 10 R.
The Image EUV instrument consists of three identical sensor heads. Each sensor head has a field of view of 30 x 30 degrees. The three sensors are tilted relative to one another to cover a fan-shaped instantaneous FOV of 90 x 30 degrees. As the satellite spins, the fan sweeps a 90 x 360 degree swath across the sky. In addition, each EUV sensor achieves throughput and the wide field of view by using a large entrance aperture and a single spherical mirror. A multi-layer reflective coating on the mirror selects a narrow 5 nm passband around the 30.4 nm line. To circumvent the red leak in the multi-layer mirror, the filter blocks H Lyman [a] contamination from the geocorona. The detector consists of two curved, tandem MCPs with an alkali halide front surface photocathode. The detector's spherical input surface minimizes the effects of spherical aberration. The sensitivity (accounting for the duty cycle inherent in a spinning spacecraft) is 0.2 count/(sec pixel) per Rayleigh, where the pixel size is taken to be 0.1 RE. By summing pixels to make a spatial resolution element (called a resel) of 0.5 RE, the count rate is 5 counts/(sec resel) per R.