Measuring the Corona's Magnetic Field

by Jeffrey Kuhn

A contour map of the coronal magnetic field strength measured by Haosheng Lin and Jeffrey Kuhn plotted over an image of the corona taken by the Extreme Ultraviolet Imaging Telescope.

Since the late 1930s, scientists have known that the outer part of the Sun, its corona, is much hotter than its surface. The corona is important because it extends far into space and eventually influences Earth's environment. Measuring the corona's magnetic field is a major unsolved problem because it cannot be observed directly. Although magnetic fields have been measured on the visible surface of the Sun for several decades, until now scientists have been unable to make useful observations of them into the corona. Instead, we have depended on hypothetical models based on invisible magnetic fields to explain the corona. This is our "dark energy" problem, and solving it is important because the corona's magnetic fields are responsible for virtually all of the Sun's explosive and dynamic phenomena, and they play a major role in driving variations in the Sun's energy output.

My colleague Haosheng Lin and I recently produced a "coronal magnetogram"—a map of the magnetic field in the corona. To see these fields, we used the world's largest coronagraphic telescope, the Solar Observatory for Limb Active Regions and Coronae (SOLARC), on Haleakala and a new type of spectrograph built by Lin, an imaging infrared spectropolarimeter. After several hours of continuous observing, we achieved the sensitivity needed to detect the coronal magnetic field. The magnetic fields were measured about 62,000 miles (100,000 km) above the surface of the Sun, which is comparable in strength to what a compass needle feels on Earth's surface.

Obtaining magnetic maps of the Sun with such sensitivity is a critical first step to understanding the variations in the amount and kinds of solar radiation that affects Earth's magnetosphere, ionosphere, and ultimately its climate.

SOLARC and its spectrograph are new instrument concepts that will soon be expanded to a much larger scale. Recently, the National Solar Observatory announced plans to build the Advanced Technology Solar Telescope (ATST) on Haleakala. ATST's mirror diameter will be nearly 10 times larger than that of SOLARC, but its optical design follows the design concepts used in SOLARC. Its first-light instrumentation is also patterned after our Haleakala imaging spectropolarimeter.