Solar Flares, and the Sun-Earth Connection

Solar Flares, and the
Sun-Earth Connection

The above image shows two examples of a coronal mass ejection (CME). The black disk blocks out the bright light from the Sun, creating an artificial eclipse so that the dim light from the CME can be observed. (The disk blocks out light from a region that is 1.6 times the diameter of the Sun.) Each row shows the evolution of a CME with time. Coronal mass ejections expand away from the Sun at speeds as high as 2000 km per second. They carry up to ten billion tons (10^16 grams) of plasma away from the Sun.

Coronal mass ejections were once thought to be initiated by solar flares. Although most are accompanied by flares, it is now understood that flares and CMEs are related phenomena, but one does not cause the other. This has important implications for understanding and predicting the effects of solar activity on the Earth and in space. If a CME collides with the Earth, it can excite a geomagnetic storm. Large geomagnetic storms have, among other things, caused electrical power outages and damaged communications satellites. In space CMEs typically drive shock waves that produce energetic particles that can be damaging to both electronic equipment and astronauts that venture outside the protection of the Earth's magnetic field. Solar flares, on the other hand, directly affect the ionosphere and radio communications at the Earth, and also release energetic particles into space. Therefore, to understand and predict "space weather" and the effect of solar activity on the Earth, an understanding of both CMEs and flares is required.

A recent coronal mass ejection and subsequent geomagnetic storm have been observed in unprecedented detail thanks to the International Solar-Terrestrial Physics (ISTP) Program. No solar flare was observed in association with this CME. An AT&T communications satellite, Telstar 401, malfunctioned during the geomagnetic storm. The following Web site is devoted to the study of this event.

http://hesperia.gsfc.nasa.gov/sftheory/cme.htm