The Extreme Ultraviolet Imaging Telescope (EIT) is a small but powerful telescope aboard the SOHO spacecraft that has been operational since January 2, 1996. EIT uses specially coated, normal incidence reflective optics and filters to image the sun and its unimaginably hot outer atmosphere with high precision and cadence.

EIT image of the Sun

EIT image of the Sun.

The EIT telescope was designed and built by an international consortium of scientists associated with many institutes. NRL developed the CCD camera and the entire electronic support package. Using EIT as a keystone for many other experiments onboard SOHO, scientists are striving to understand the nature of the mechanisms that heat the corona to temperatures far higher than the sun's surface gases, and that drive the solar wind outward at speeds of more than a million miles per hour. The solar atmosphere is layered, like the skins of an onion, with the layers marked by higher and higher temperatures as one looks at higher and higher altitudes in the solar "sky" of searing heat and blinding light. Unlike an onion though, the sun's surface (or photosphere, at about 6,000 degrees Celsius) and the atmospheric layers above it (the chromosphere and corona) are by no means smooth and symmetrical. On the contrary, they are pierced through and through with a lacy forest of magnetic fields that rise in giant plumes and arches, surging in constant motion, waxing and waning, and sometimes thrashing like whips or erupting with explosive violence.

EIT has four optical "channels," each providing images of the sun in a different temperature regime, and therefore a different "layer" of the sun's outer atmosphere.

Seen here is the chromosphere, a layer just above the white photospheric disk visible to our eyes. Its temperatures range from about 20,000 to 100,000 degrees Celsius, making it too hot to be seen in visible light. It is imaged here in the ultraviolet emission of singly ionized helium, at a wavelength of 304 A. The large arch surging above the limb at upper right is a prominence, or condensation of this "cool" helium (compared to the million-degree corona surrounding it!) held up against gravity by magnetic forces. This frame shows the prominence in the process of erupting, subsequently to be thrown into outer space, where it will eventually become part of the interplanetary solar wind.