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GISSMO:

The Gas Ionization Solar Spectral MOnitor (GISSMO) is an optics-free solar EUV spectrometer. It is designed to monitor solar EUV emissions in the wavelength range from 7 to 40 nm.

The GISSMO AssemblyThe instrument design features a gas cell in which solar EUV photons ionize Ne, a rare earth gas, producing an ion and a photoelectron of energy equal to the ionizing photon's energy minus the ionization potential of Ne. Subsequent measurement of the photoelectron's energy, coupled with a knowledge of neon's cross section for photon absorption, yields information about the original photon's energy and flux. Indeed, a related technique is employed by the National Institute of Standards and Technology for absolute EUV flux calibration.

The GISSMO AssemblyA photoelectron, created by a solar EUV photon within the ionization region, passes through entrance slits into a toroidal electrostatic analyzer where it is focused onto an imaging detector. By varying the voltage on the electrostatic analyzer plates, photoelectrons of differing energies - corresponding to solar photons of differing wavelengths - are allowed through the instrument. Additionally, higher energy photoelectrons are focused to large radii rings on the detector, while those of lesser energy are focused toward the center of the detector. In this manner, spectral information within a band of energies is recorded radially along the detector, while different bands of energies may be selected electronically by varying the analyzer plate potentials. The instrument itself is cylindrically symmetric, allowing for full 2pi azimuthal detection of photoelectrons.

The Installed GISSMOThe Installed GISSMO and Plumbing The resolution of this spectrometer, varies in the range from 25 to 50 depending on the photon energy. At longer wavelengths, photoelectron energies are small and the earth's magnetic field reduces the electrostatic analyzer resolution to about 25. For wavelengths below about 200angstroms (using Ne gas) however, this effect is lessened and the resolution approaches 50. GISSMO's strength is that it's sensitivity should remain constant over large (solar cycle) time scales; its weakness, in it's current design, is that its resolution is not as high as that of grating based spectrometers. However, GISSMO was designed as a proof-of-concept instrument for a short duration rocket flight and its resolution was purposely traded in favor of increased sensitivity. In principle, the resolution of a gas ionization spectrometer can be arbitrarily high. Furthermore, while GISSMO requires gas flow through its ionization region to function, gas is consumed at only 10 cubic cm. per second. At this rate, a one liter supply of high pressure Ne would be sufficient to enable GISSMO, if placed in orbit, to produce one absolutely calibrated solar spectrum per day over an entire solar cycle. GISSMO will be operated in this manner on the mission.

 
26 May 1999
Center for Space Physics
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