The above image from February 27, 1998 ( also see previous days' images ) shows the solar corona as it appears in ultraviolet light emitted by electrically charged atoms in the ultra-hot solar corona. The image is in two parts separated by a black ring that covers a region that is not observed by either instrument.
The part inside the ring is from SOHO's Extreme Ultraviolet Imaging Telescope (EIT) and is made from light emitted by electrically charged iron with 14 of its 26 electrons removed by collisions with other hot particles. The color of the image indicates the intensity of the light with the lowest intensity being black and the highest being white. Regions with the highest intensity tend to have the highest temperatures.
The part of the image outside the black ring is from the Ultraviolet Coronagraph Spectrometer (UVCS) and is made from light emitted by electrically charged oxygen with 5 of its 8 electrons removed by collisions with other hot particles.
UVCS observes the faint ultraviolet light from the extended solar corona. That is the part of the solar atmosphere that can only be seen with the naked eye during a natural solar eclipse. Even during an eclipse, only the visible light can be observed from the ground because the ultraviolet light can not penetrate the earth's atmosphere. The UVCS optical system produces an artificial eclipse in ultraviolet light.
Although the UVCS data can be used to produce images like the one above, its primary use is to study the changes in the intensity of the ultraviolet light from one color (that is wavelengh) to the next. This powerful technique is called atomic spectroscopy. Such information can be used to determine the temperatures and densities of the particles emitting the light. It can also be used to measure the speed of particles as they stream away from the Sun to form the solar wind.
The solar wind is a hot electrically charged gas that flows out of the sun and fills the solar system. The solar wind causes comets to have tails, and it produces the northern and southern lights. When these charged particles enter the earth's outer atmosphere, they can alter the earth's magnetic field causing geomagentic storms which, in turn, cause communications disruptions and even power outages. The solar wind can also affect the operation of satellites. The primary goal of the UVCS observations is to determine how coronal particles are accelerated out of the sun to form the solar wind. A long term goal of this work is to develop a capability to predict geomagnetic storms so that satellites can take protective measures and communications disruptions can be anticipated.
During the week of the natural eclipse, UVCS will provide a daily snapshot of the solar corona. It will also provide special observations containing a wealth of scientific information including measurements of coronal temperatures, and determination of ion densities from many of the elements participating in the sun's fusion reaction including hydrogen, oxygen, silicon, sulpher and iron. Most of the UVCS observations during the week of the eclipse will be in support of the NSF, SAO, GSFC Eclipse Expedition whose goal is to determine densities and temperatures of electrons and electrically charged atoms in regions of the corona above the equator of the sun and near the poles of the sun.
During and immediately after the eclipse, UVCS is planning to coordinate observations of its visible light channel with ground based measurements. Careful measurement of polarized light enables scientists to calculate the electron density of the corona. For this series of observations, UVCS will be coordinating their efforts with SOHO's LASCO instrument and scientists from the Smithsonian Center for Astrophysics, Universita di Firenza and the Goddard Space Flight Center.