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University of Southern California


Missions-SOHO: A Million Miles From Earth

A USC SSC designed and fabricated instrument, the Solar EUV Monitor (SEM) onboard the Solar and Heliospheric Observatory (SOHO) is giving scientists long-sought, accurate values for a key component in the Earth's energy diet, extreme ultraviolet (EUV) radiation.

The Solar and Heliospheric Observatory (SOHO), launched Dec. 2, 1995 (right panel), contains a sophisticated package designed to measure the flux of extreme ultraviolet light (EUV) and X-rays produced by the sun, as well as energetic particles.

SOHO's unique, stable observation position, 1 million miles from the Earth along the Earth-sun line at a "Lagrange point" where the graviational pulls of the Earth and sun balance, allows uninterrupted solar observations (see panel below).

"We have observed a 27-day modulation of the solar flux associated with the rotation period of the sun," said solar astronomer Darrell L. Judge, under whose direction one of the key solar EUV-monitoring instruments on the spacecraft, the CELIAS/SEM, was built. "Such variability is only observable through continuous monitoring of the sun, such as SOHO provides."

The instrument's name is an acronym for Charge, Element and Isotope Analysis System/ Solar Extreme-ultraviolet Monitor.

The EUV spectrum is blocked by the Earth's atmosphere, making ground-based measurements impossible. Scientists have previously measured the flux of this radiation either by suborbital rockets rising briefly above the atmosphere, or by satellites that pass behind the Earth and thus out of view of the sun for part of their orbits.

Solar rotation and polar flows of the Sun as deduced from measurements by MDI. The cutaway reveals rotation speed inside the Sun. The left side of the image represents the difference in rotation speed between various areas on the Sun. Red-yellow is faster than average and blue is slower than average. The light orange bands are zones that are moving slightly faster than their surroundings. The new SOHO observations indicate that these extend down approximately 20,000 km into the Sun. Sunspots, caused by disturbances in the solar magnetic field, tend to form at the edge of these bands.-SOHO

"SOHO permits the accumulation of an around-the-clock database over an extended period of time," impossible with the earlier platforms, Judge noted.

The CELIAS/SEM, first turned on on Dec. 16, was briefly turned off for operational reasons Dec. 20, and has operated continuously since then, providing extremely high-quality data.

The observations taken from Dec. 16, 1995, to now have all been obtained during solar minimum, the time when the sun is at its lowest level of activity, and have found larger-than-expected variations in the amount of ultraviolet light produced by the sun.

During this period the CELIAS/SEM observed a flare episode in which the soft X-ray flux increased by a factor of 100. "We look forward to observations of the sun during a more active period," Judge said.

The solar EUV data is important in planetary science studies because earlier studies have shown that EUV absorption in the upper atmosphere "produces heating, ionization, and excitation of atomic and molecular species, thus leading to complex chemical and transport processes in the affected atmospheric regions," according to a report on the experiment presented in June, 1996, at a solar workshop.

Scientists have attempted to model both planetary and solar atmosphere processes, but definitive tests of the models depend on more accurate measurements of the EUV flux than have so far been available.

The SOHO project is being carried out by the European Space Agency (ESA) and the U.S. National Aeronautics and Space Administration (NASA) as a cooperative effort.

Other institutions involved in the CELIAS/SEM observations on board the SOHO spacecraft include the Max Planck Institute for Extraterrestrial Physics, Max Planck Institute for Aeronomy, the Braunschweig Technical University and the Physikalish-Technishe Bundes- anstalt, all of Germany; the University of Bern, Switzerland; University of New Hampshire; University of Maryland; Univer- sity of Arizona; JPL; the Institute for Space Physics in Moscow; and the Physics Laboratory of the National Institute of Standards and Technology in Maryland.

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    website last updated 5/31/2007