Extreme Ultraviolet Photometer System (XPS)
The XPS instrument will take readings in the “far” ultraviolet, with wavelengths much shorter than seen by SOLSTICE, and down into the “soft” X-ray region. The instrument is essentially an array of 12 thumbtack-sized, identical photodiodes that give off an electrical signal each time x-ray or ultraviolet radiation strikes them. Filters are placed over the diodes to allow the instrument to take readings in 5-10 nm increments. One of the diodes, for instance, will be equipped with a filter that allows only light with 12-20 nm wavelengths to pass through. Another diode is covered by a filter that allows only 17–25 nm wavelengths to pass. Five such filtered diodes together will be used to measure wavelengths between 1-31 nm. A sixth diode will have a filter that screens out all but one very specific wavelength at 121.6 nm, known as Lyman-alpha radiation. Lyman-alpha radiation, which is emitted by ionized hydrogen in the Sun’s corona, is an especially important wavelength to monitor because it is a very strong emission line coming from a transition to the lowest energy “ground” state of hydrogen, and therefore it makes a good “probe” of the Sun’s corona, the solar wind, and even hydrogen flowing into our solar system from elsewhere in the Milky Way Galaxy.

Scientists will use XPS to study the Sun’s corona and transition zone by monitoring solar radiation in extreme ultraviolet and low-energy x-ray wavelengths. (Image courtesy Solar Radiation and Climate Experiment Project)

Rarely do these shorter wavelengths penetrate the lower layers of the atmosphere, so they have little effect anywhere below 70 kilometers in altitude where weather develops and life is found. These wavelengths do, however, impact the very outermost regions of the atmosphere such as the mesosphere, the thermosphere, and the ionosphere. Changes in these wavelengths due to solar cycles can affect the chemical composition and the temperature of these regions, which in turn can disrupt satellite operations and radio and satellite communications.

The readings can also give scientists detailed information about what goes on in the Sun’s atmosphere. The Sun’s atmosphere consists of two zones known as the corona and the transition zone. The corona is a cloud of ions that burns at 1 million degrees Celsius, or roughly 200 times hotter than the temperature at the surface of the Sun. Between the Sun and the corona is a transition zone. Though the corona and the transition zone have been studied in increasing detail since the 1970s, their complex variations during the solar cycle are not adequately characterized. SORCE will provide further insight into these fluctuations in the outer solar atmosphere. The 1-31 nm range of XPS is sensitive to changes in the corona, and the Lyman-alpha monitors the transition zone.

Calibration of the XPS will be completed in part by the six diodes that do not take daily measurements. Three of these diodes will not be covered by filters, and they will be used to check on the condition of the silicon window that covers the photodiodes. The three remaining diodes will be covered by filters, and they will take redundant measurements of the Sun once a week. These readings will be compared to those taken daily by the six diodes measuring ultraviolet and x-ray radiation. Finally, sub-orbital flights will be launched once a year with identical XPS photodiodes on board. By contrasting these measurements taken by the sub-orbital flights with those of the instrument aboard SORCE, the scientists should be able to tell if the SORCE instrument’s performance has changed. If so, they can then make the compensating adjustments to the data they receive.

References:
Intergovernmental Panel on Climate Change, 2001: Summary for Policymakers, A Report of Working Group 1 of the Intergovernmental Panel on Climate Change, Geneva, Switzerland, pp. 2-13.

Related Sites:
SORCE Project
Solar and Earth Radiation

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Solar Radiation and Climate Experiment (SORCE)
Introduction
Earth’s Energy Balance
Solar Variability
The Sun and Global Warming
Uncertainties in Solar Measurements

The SORCE Satellite
Total Irradiance Monitor (TIM)
Spectral Irradiance Monitor (SIM)
Solar Stellar Comparison Experiment (SOLSTICE)
Extreme Ultraviolet Photometer System (XPS)

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Reflected Solar Radiation
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