ejections (CMEs) are eruptions into interplanetary space of
as much as a few billion tons of
plasma and embedded
magnetic fields from the Sun's corona. In contrast to the
CMEs originate in regions where the
magnetic field is closed and result from the catastrophic
disruption of large-scale coronal magnetic structures, such as
coronal streamers. The exact processes involved in the
release of CMEs are not known. CMEs can occur at any time
during the solar cycle, but their occurrence rate increases
with increasing solar activity and peaks around solar
maximum. CMEs are propelled outward at speeds ranging
from <50 to ~2000 kilometers per second. Fast CMEs --those
traveling faster than the ambient solar wind-- are responsible
for triggering large, nonrecurrent
geomagnetic storms when
they encounter the Earth's magnetosphere. Such storms can
result from the passage either of the CME itself or of the
shock created by the fast CME's interaction with the
slower-moving solar wind. The majority of large and major
geomagnetic storms are generated by the encounter with
both the interplanetary shock and the CME that drives it.
The "geoeffectiveness" of CMEs --i. e., their ability to disturb
the Earth's magnetosphere-- is a function of their speed, the
strength of their magnetic field, and the presence of a strong
southward magnetic field component.
The image above shows a CME that occurred on January
The bright ring inside the dark disk indicates the location and
diameter of the
Sun. The outer edge of the disk lies at ~3.5 solar radii.
This white-light image was acquired with the C3 coronagraph
developed by the Naval Research Laboratory as part of the
Large-scale and spectrometic coronagraph (LASCO) experiment on SOHO.
(Courtesy of the SOHO/LASCO
consortium. SOHO is a project of internatinal
collaboration between ESA and NASA.
This figure, along with many other CME images, can be found on the Web
at the SOHO Gallery.)