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University of Southern California


Solar Hitchhiker

Developed by the University of Southern California Space Science Center, the Solar Extreme Ultraviolet Hitchhiker (SEH) is designed to obtain absolute solar EUV flux in the 200-1700 angstrom wavelength region utilizing four separate instruments: a neon ionization cell, a helium double ionization cell, a highly stable silicon photodiode, and a spectrometer assembly. SEH will be controlled from the GSFC payload operations control center during the mission.

The full disk extreme ultraviolet (EUV) solar radiation is a major energy source, whose magnitude is required in modeling the scattering, ionization, and heating of planetary atmospheres, moons, comets, and the inflowing interplanetary/instellar medium. Unfortunately, despite the importance of the solar EUV radiation, there is still considerable uncertainty in the absolute magnitude of the EUV flux and its long-term variation. This is especially true in the wavelength region shortward of the prominent He II 304 A line. The underlying causes for the uncertainty are associated with the difficulty of EUV detector calibration and the degradation of detector sensitivity common to the required open-faced detectors exposed to the prolonged UV radiation. It is for these reasons that SEH is investigating stable detectors of EUV emissions. One such inherent absolute detector is the rare gas ionization cell. Another type, the silicon photodiode, while not an intrinsic absolute spectral radiometer, does exhibit high quantum efficiency, i.e., the number of electron hole pairs produced per incident photon, that is well over 100% in the EUV and has been found to be stable when stored for several months at nominal ambient storage.

By measuring the absolute solar EUV flux, SEH provides a very important data point for the UVSTAR experiment, which is investigating the EUV emisisons of the Jupiter Io plasma torus. UVSTAR requires knowledge of the EUV flux input to the Juptier/Io system in order to fully understand the EUV dynamics of the plasma torus. By alternating the Shuttle orientation between the Io plasma torus and the sun, during the same orbit, SEH can provide near real-time measurement of the solar EUV flux during a UVSTAR Io plasma torus observation. Moreover, the SEH obtained solar EUV flux data will be used as a calibration source for other EUV observing instruments.

The SEH is developed and managed by the University of Southern California Space Science Centerand is sponsored by NASA HQ Code SLC. The principal investigator for SEH is Dr.Darrell Judge.

The SEH has previously flown as a sounding rocket experiment out of White Sands NM. Retrofited to fit into the Hitchhiker Carrier canister assembly, SEH truly represents a faster, better, cheaper approach to developing Shuttle instruments. As shown below during Integration at GSFC , the SEH instrument package is contained in a 5.0 and 2.0 cubic foot canister assembly; this assembly has a motorized door mounted on top of the stack and a small extension ring mounted at the bottom, enabling the instrument package to be fully contained and protected from the Shuttle environment. The SEH electronics package, including the original sounding rocket skin, is contained in the adjacent 5.0 cubic foot canister and provides the interface to the Hitchhiker Carrier system electronics.

1. Helium Double Ionization Cell

The Helium Double Ionization Cell, shown above, is one of three integral flux measurement instruments contained within the SEH experiment. The Helium Double Ionization cell is used to determine the helium photoionization rate. The photoionizatoin rate is them used to calculate the total number of photons in the emission lines between 50 - 504 Angstroms.
The Helium Double Ionization instrument contains a reservoir of helium that is released, via ground command, into the ionization cell. Photons enter the ionization cell and collide with helium atoms. Those photons in the emission line of interest are absorbed by the helium atom creating an electron and helium ion pair. A power supply collects the free electrons and collectors 1 and 2 collect the free helium ions. If too much helium has been introduced into the cell, then secondary and possibly tertiary collisions will occur between these electron ion pairs and the extra helium gas creating erroneous readings. Moreover, if too little helium is introduced into the cell, than photons will not collide with the helium gas again obfuscating the intended results. To circumvent these possibilities, a radiation pressure gauge, which utilizes a tritium source, provides a measure of helium in the cell and is sent via telemetry to the ground enabling realtime adjustments to the gas pressure.
Because the collectors are positively charged, electrons from the insturment ground plane flow to the collectors thru two electrometers - one each for each collector. The electrometer current provides a measure of the ionization rate. By comparing the difference in current between electrometers 1 and 2 the helium photoionization rate is determined, and hence the total photon count between 50 and 504 Angstroms.
2. Neon Rare Gas Ionization Cell

The Neon Rare Gas Ionization Cell, shown above, is the second of three integral flux measurement instruments contained in SEH. The Neon Rare Gas Ionization instrument measures the actual photon count between 50 and 575 Angstroms, unlike the Helium Double Ionization instrument which infers a total photon count from the photoionization rate. Photons entering the ionization cell collide with the neon atoms creating an electron neon ion pair. The negative collector attracts and collects the free neon ions. Electrons from the instrument ground plane flow to the collector thru an Ion Current Pico Ammeter producing a current that is proportional to the total number of photons between 50 and 575 Angstroms.

A pressure gauge is used to monitor the pressure in the cell to ensure that the correct amount of gas is introduced into the cell chamber.

3. Silicon Photo Diode
The SEH photodiode detector, shown above, has an aluminum film coating approximately 3000 Angstrom thick over the active area of the detector and is a phosphorus diffused (n on p) silicon photodiode. The photodiode wafer is fixed to a standard BNC connector and mounted to an aperture structure as shown. Near normal incidence solar photons enter a 5 mm aperture stop that provides a 16 deg field of view. The Al filter is positioned directly behind the aperture stop, and its mount is slotted for atmospheric pumpout. A field stop mask with a circular cross section is placed directly in front of the coated wafer. Both the Al filter and mask are electrically grounded to prevent signal drift. The EUV photons which are not filtered out (170-800 Angstroms) are absorbed by the silicon crystal creating electron hole pairs that are separated by the built-in junction field to produce the resulting photocurrent.
SEH photodiode detector
4. SEH Spectrometer
SEH Spectrometer
Unlike the three aforementioned instruments, the SEH Spectrometer, shown above, is not required to measure the absolute EUV photon count. Instead, the spectrometer measures the relative distribution of the EUV emissions. When the data from all four instruments is combined (see below graph), an emission line distribution vs. absolute magnitude image emerges, i.e. the absolute solar EUV/FUV Flux. Photons entering the spectrometer slit reflect off of a gold plated grating surface at varying angles as a function of their wavelength.
The redirected photons pass thru a filter and into the opening of a chevron microchannel plate. Photons entering the 6KV charged microchannel plate collide with the plates glass surface of the microchannelsinitiating an avalanch of electrons which impinge upon onto a resistive anode detector. The location of the deposited 'electron snowball' on the detector surface produces a resistance meaurement that is proportional to the photon wavelength.
SEH spectrometer graph
    website last updated 5/31/2007