A Million Miles to Earth: USC Instruments Find a Place in the Sun

A USC-designed and -built instrument installed in a solar observatory space vehicle is giving scientists long-sought, accurate values for a key component in the Earth's energy diet. The Solar and Heliospheric Observatory (SOHO), launched Dec. 2, 1995, 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.

A schematic of SOHO's "halo orbit." The orbit is designed to allow SOHO to observe the sun continuously, without ever passing into the Earth's shadow.

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

SOHO as it would look to an astronaut visiting it in its "halo orbit" above the Earth, with its flat solar panels deployed.

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. A complete paper on the results will appear soon in the Journal of Solar Physics.

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 Bundesanstalt, all of Germany; the University of Bern, Switzerland; University of New Hampshire; University of Maryland; University of Arizona; JPL; the Institute for Space Physics in Moscow; and the Physics Laboratory of the National Institute of Standards and Technology in Maryland.

Judge's primary USC collaborators are co-investigator Howard S. Ogawa and project manager Donald R. McMullin.