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Winter 2001

Student-built satellite detects extra X-rays from the Sun

by Bob Henson

Data from the Student Nitric Oxide Explorer (SNOE) satellite have verified what solar physicists have long suspected: the Sun emits far more energy at very low wavelengths than once thought. The finding is not only a step toward improved models of solar output—it also confirms that putting science into student hands can yield benefits for research as well as for education.

Although the main goal of SNOE is to monitor nitric oxide levels in the lower thermosphere, the X ray findings are news in themselves. NCAR scientist Stanley Solomon coauthored a paper on the X-ray results with former SNOE student Scott Bailey (then at the University of Colorado, now a research assistant professor at Hampton University) and SNOE investigator Thomas Woods (CU). The paper appears in the 1 June 2001 issue of Geophysical Research Letters.

Stan Solomon. (Photo by Carlye Calvin.)

About 100 students helped bring SNOE to life. The satellite was built at CU's Laboratory for Atmospheric and Space Physics in the mid-1990s under the direction of principal investigator Charles Barth and the deputy PI and project manager, Solomon, who was then at CU.

Launched from a Pegasus rocket on 26 February 1998, SNOE has collected observations every six hours at several wavelength bands in the poorly monitored soft X-ray range between 2 and 20 nanometers (nm). Within this range, the data show up to four times more solar irradiance than indicated in a standard model. "This confirms contentions developed from several lines of evidence, but to a larger degree than previously suspected," says Solomon.

The soft X rays are an important part of what Solomon calls space climate. As he points out, "Many people use the term 'space weather' to mean particle-driven changes in the space environment." As these charged particles emerge from solar storms and bombard Earth's outer atmosphere, there's a parallel spike in the level of soft X rays. The spike is an indication of the highly variable nature of the Sun's output at such low wavelengths. The total solar irradiance varies by only about 0.1% across the 11-year solar cycle, but the soft X ray output rises and falls by a factor of five. "These changes have a huge effect on the density and composition of the ionosphere," says Solomon.

Before SNOE, a handful of rocket shots had provided data across the 2–20 nm range. The Atmospheric Explorer–E satellite gathered limited data down to 15 nm from 1976 to 1980. The late solar physicist Hans Hinteregger (Air Force Geophysical Laboratory) leaned on these scant clues when he put together a 1981 model of solar output at the low end of the spectrum. "People have been using some variant of this model ever since," says Solomon. Only with SNOE have there been enough data to construct a reasonable alternative.

Now that it's clear there are more soft X rays than previously thought, the spikes and drops in this part of the Sun's output may draw more research interest. Until now, says Solomon, "many people in the ionosphere field hadn't thought of the soft X rays as being all that important." He's now working on a paper that will outline the "sweeping implications" of what SNOE has verified. For instance, the dissociation of both nitrogen and oxygen in the ionosphere may be taking place at higher rates than previously believed. Techniques that use airglow emissions to infer the density and composition of the thermosphere will have to be reexamined.

All this is providing plenty of grist for students at CU and elsewhere. Aimee Merkel got involved in building SNOE five years ago as a CU senior. "I was one of three students on the instrument team. There were three instruments, so we each were able to work on a specific instrument," says Merkel. She worked with supervisors Scott Bailey and Richard Kohnert. Along with the instrument team, a large number of fellow students designed and built the successful SNOE spacecraft.

Merkel is now completing her doctoral dissertation in aerospace engineering at CU with data from the instrument she created. Her research is on the dynamical effects on polar mesospheric clouds (PMCs), more commonly known as noctilucent clouds. "I was at the end of my master's program when SNOE was launched. I knew I wanted to get a Ph.D, so I thought it would be pretty cool to analyze the data from the instrument that I built," she says. The ultraviolet spectrometer was designed to monitor nitric oxide, "but we also serendipitously obtain these PMC measurements. It's incredible data. We should be able to help answer some of the ongoing questions about these rare clouds."

The names of SNOE students, including Merkel's, are engraved on the satellite, now stationed 500 kilometers (310 miles) above Earth. Solomon is happy that the SNOE students are making their mark in the research world as well. "Our goal was not just to build a student satellite and not just to perform scientific research but to do both. We showed you can do true cutting-edge research on a small budget in a university-laboratory environment with tons of student involvement and excitement."

On the Web:
SNOE in the making, UCAR Highlights 1996

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UCAR > Communications > UCAR Quarterly > Winter 2001 Search

Edited by Bob Henson, bhenson@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Thu Dec 20 16:42:17 MST 2001