Topics : Expeditions

Capturing a Solar Eclipse

Understanding the Sun is a key to understanding Earth's weather and climate. For years, the only way solar scientists could get a clear view of the vast, faint atmosphere surrounding the Sun was during the fleeting moments of a total solar eclipse. Even though Sun-monitoring satellites are now stationed in space, eclipses remain a vital window for research on the solar atmosphere.

Scientists from NCAR’s High Altitude Observatory—which predates the center itself—have taken instruments to the far corners of the globe to capture eclipse data. Out of their 16 eclipse expeditions between 1952 and 1998, only three were thwarted by cloudiness.

The corona, or outer atmosphere of the Sun, is a million times dimmer than the solar disk. Scientists can observe the corona at any time using a coronagraph—an instrument that blacks out the disk—but sunlight scattered by Earth's atmosphere masks the very faint coronal light. A real eclipse gives much better results, because the moon blocks sunlight before it reaches Earth's atmosphere. An eclipse also allows scientists to look at the lower layers of the corona, much closer to the Sun itself than is possible using coronagraphs.

In 1966, a coronal camera designed at NCAR was deployed in Bolivia for its first eclipse expedition. With three plates to block interference from the solar disk, the instrument opened a new chapter in eclipse observing. Researchers kept it in use for almost 30 years to periodically sample the Sun's corona. Coronagraphs at the High Altitude Observatory and elsewhere filled in between eclipses. Some coronagraphs were deployed by balloon and others sent into space aboard Skylab in 1973.

Eclipse expeditions have become more sophisticated over time. The eclipse of February 28, 1998, was one of the most heavily studied on record. Scientists observed the eclipse from a ground station on the Caribbean island of Curaçao, a research aircraft flying out of Panama, and the Solar and Heliospheric Observatory (SOHO) satellite operated by NASA. The aircraft data confirmed a new technique—the most sensitive yet found—to measure the corona's strength.

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