SFO News

Astronomers Observe Changing Solar Radius

Solar astronomers are announcing today that they have detected a change in the solar radius over the last two solar cycles that appears to track sunspot cycle maxima and minima, becoming larger as solar activity increases. Authored by Drs. Gary Chapman, Jan Dobias, and Stephen Walton, and associate Angela Cookson of California State University, Northridge, the report is being presented to the American Astronomical Society meeting in Honolulu, Hawaii. The result is of special interest because it could have profound implications for the climate of Earth if the radius variability is a result of changing solar luminosity and if it ultimately affects the amount of energy from the Sun that strikes the Earth.

Determining the solar radius is not a trivial problem. The Sun is not a solid sphere, but rather an ever-changing plasma, making it difficult to define its edge. Over the years, researchers world-wide have used various techniques to observe solar size changes, with differing degrees of success. Most recently, studies of solar radius variations using data from the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO) satellite were done by Drs. M. Emilio, J. Kuhn, R. Bush, and P. Scherrer in 2000 and, again, in 2004.

The San Fernando Observatory, California State University, Northridge (SFO/CSUN), located in the San Fernando Valley near Los Angeles, has studied sunspots and other solar features since 1986. The instrument used in this study was the smaller of two Cartesian Full-Disk Telescopes (CFDT1), a small refractor with a 2.5-cm (1.0-inch) aperture, that produces full-disk digital photometric images of the Sun in three different wavelengths. Over 4000 images taken at the 672.3 nm wavelength were carefully corrected for seasonal and temperature variations. By measuring the solar radius on these images, this eighteen-year dataset was used to determine whether or not the size of the Sun changed during that period.

Illustration 1: Result of a least squares fit to the solar radius after all seasonal, temperature, and instrumental corrections have been made to the images for solar cycles 22 and 23. Also plotted is the sunspot area calculated from the same SFO images.

What SFO researchers found was a small, but statistically significant, change in the solar radius that tracked the sunspot maxima and minima of the last two solar cycles (Cycles 22 and 23) . The Sun completes one solar cycle approximately every eleven years. It rises from a minimum, a relatively quiet time when there is little or no sign of magnetic activity, to a maximum when the Sun is at its most active, producing great solar magnetic storms and peppering the Sun's surface with sunspots and other features associated with such violent activity.

What our thousands of images show us, says Dr. Gary Chapman, Director of the San Fernando Observatory and Professor of Astronomy at CSUN, is that as more and more sunspots appear, which is an indication of increasing solar activity, the Sun's diameter grows, until it reaches a maximum size at about the same time as the Sun reaches its peak solar activity. Then it shrinks as solar activity slowly falls to a minimum and the cycle begins again. The change we see in the radius is very small, but we do see it and it is statistically significant.

Why is this radius change important Climate change and global warming are on everyone's mind these days, but the role the Sun plays is still not well understood. Changes in the Sun's energy output (luminosity) could profoundly affect Earth's climate. Observations of Total Solar Irradiance (TSI) show that the amount of energy produced by the Sun that falls on the Earth during a solar cycle differs by about 1.3 Watts/m2 or ~0.1% between solar maximum and solar minimum. If changes in the Sun's radius are indicative of luminosity changes, studies of this type could lead to a fuller understanding of the underlying physical mechanisms that drive the activity cycle. Scenarios that produce a change in solar radius at the same time we see an increase in magnetic activity, says Dr. Chapman, may provide glimpses into the working interior of the Sun. He adds that further studies with higher resolution telescopes, long-term data sets, and novel approaches are needed if we hope to understand the intimate connections between the Sun and our planet.

Support for this work came from NASA and the National Science Foundation.

For more information, contact:
Dr. Gary Chapman (00-1-818-367-9333 or 00-1-818-677-2766)

Please direct questions or comments regarding site content to Angie Cookson.

Last updated: May 28, 2007