The Large Angle Spectrometric Coronagraph (LASCO) was developed for flight in 1995 on the Solar and Heliospheric Observatory (SOHO). LASCO is a wide-field white light and spectrometric coronagraph. It consists of three optical systems with nested fields of view that together observe the solar corona from 1.1 to 32 solar radii. One solar radius is about 420,000 miles or 16 arc minutes. A coronagraph is a telescope that is designed to block light coming from the solar disk in order to see the extremely faint emission from the region around the sun, called the corona.
On December 23, 1996, the LASCO experiment on the SOHO satellite recorded images of a large "coronal mass ejection." The bright ring in the center of the image represents the diameter of the visible sun. The dark circle surrounding it is an image of the occulting disk, which blocks the glaring light of the sun from the entrance lens of the telescope. A comet is shown on the lower left of the image. Its path curves toward the sun, and on December 23 it disappears behind the occulting mask of the coronagraph.
Coronal mass ejections are the hurricanes of space weather. SOHO is ideally placed and instrumented to report and even anticipate their origins in the sun's atmosphere. Although the sun is supposedly very quiet at present, being close to the minimum count of sunspots, LASCO observes so many outbursts large and small, "roughly one a day," that scientists are having to think again about how to define a coronal mass ejection.