Side and rear views of the EUV imager on the payload deck during functional testing at SwRI.

IMAGE's Extreme Ultraviolet (EUV) imager will detect solar photons with a wavelength of 30.4 nm that have been resonantly scattered--i.e., absorbed and re-radiated at the same wavelength--by singly ionized helium (He+) in the Earth's plasmasphere. The imager consists of three identical sensor heads mounted one above the other in a common bracket; a common electronics module (the EUV Controller) to service the sensors and communicate with the IMAGE Central Instrument Data Processor (CIDP); and a high-voltage power supply.

Field of View

Each sensor head has a 30-degree cone-shaped field of view; the three sensors are tilted so that their fields-of-view overlap, giving the imager a fan-shaped instantaneous field of view of 30 degrees by 84 degrees. With each rotation of the spacecraft, the imager will complete a 360-degree sweep of the sky, resulting in a total field of view of 84 degrees by 360 degrees. When the spacecraft is at apogee (8 Earth radii from the center of the Earth), the main plasmasphere will fill approximately 13% of the EUV imager's field of view; at perigee, plasmaspheric emission will fill nearly the entire field.

Resolution

The EUV instrument's spatial resolution at apogee is ~0.6 degree or ~0.1 Earth radius per pixel. When the IMAGE spacecraft is farthest from the Earth, in other words, EUV can distinguish plasmaspheric features with scale sizes down to about 640 km. This resolving power will enable researchers to study fine-scale density structures that have been observed in the outer plasmasphere during active times and to determine whether these structures are a part of the main plasmasphere or detached from it. At this resolution, EUV images will also reveal variations in the radius of the plasmasphere. Such variations are typically on the order of a few tenths of an Earth radius.

Enhanced Reflectivity

A distinctive feature of the EUV instrument design is the use of a multilayered coating on the three sensor mirrors. This coating, which consists of six pairs of insulating and conducting layers, selects a narrow pass band around the target wavelength of 30.4 nm and significantly enhances the mirrors' reflectivity at that wavelength. This enhancement results from constructive interference experienced by the 30.4-nm photons as they are transmitted through the layers.

Integration Period

Despite the imager's enhanced reflectivity and efficiency, however, He+ emissions from some regions of the plasmasphere may be so weak that relatively long exposure times will be desirable to ensure an adequate signal-to-noise ratio. Thus each EUV sky map or image will be produced by integrating all the photons detected during five spacecraft rotations. Spin phase information from the CIDP (in the form of digital pulses at intervals of 0.1 degree in roll phase) will allow the EUV Controller to relate each detected photon to an absolute position in the sky and to build up a sky map, photon by photon, over the course of the five rotations. Since the spacecraft has spin period of two minutes, the total exposure time for each image will be 10 minutes. Under certain conditions--in the case of very bright emission features, for example--exposure times as short as two minutes (one spacecraft spin) may be used.

The lead investigator for the IMAGE EUV investigation is Bill R. Sandel, of the University of Arizona's Lunar and Planetary Laboratory. The EUV imager was built at the University of Arizona.

Instrument
Parameters
Wavelength: 30.4 nm
Bandpass: 5 nm
Sensitivity: 0.2 counts per second per Rayleigh per pixel
Field of View 30 degrees by 84 degrees (instantaneous)
84 degrees by 360 degrees (total)
Spatial Resolution: 640 km by 640 km at apogee (8 Earth radii)
Imaging Frequency: 1 image accumulated every 10 minutes (= 5 spacecraft rolls)
Instrument Mass: 15.6 kg
Power Consumption: 15.5 Watts
Thermal Range -25 to +40 degrees Celsius (operating)
-50 to +60 degrees Celsius (non-operating)

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