At an intensely
hot one to two million K, the corona is warm enough to glow in x-rays.
It is nearly a billion times less dense than the photosphere. For comparison,
air is about a thousand times less dense than water. Due to the extremely
low density, the movement of plasma in the corona is ruled by the ever-shifting
magnetic field. In some regions (most commonly the poles) the magnetic
fields are open and plasma travels out along the field lines creating the
solar wind. In active regions, the magnetic field is highly concentrated
and forms closed loops. Prominences and filaments are the outlined by cooler
plasma suspended within the confines of an arched magnetic field. Occasionally,
a large amount of plasma is violently expelled from an active region. This
event is known as a coronal mass ejection, or CME.
Coronal Mass Ejections
The few CMEs that intercept earth are known as halo CMEs. Charged particles
from the CME follow the lines of the earth's magnetic field, concentrating
at the poles and sometimes causing beautiful auroras. If conditions are
right the magnetic field of the halo CME will interact in a more destructive
way with the earth's magnetosphere. In some cases the protective magnetosphere
is pushed back, leaving satellites and astronauts exposed to a potentially
fatal rain of highly energetic ions. Processes are set in motion that can
lead to induced currents in pipe and power lines. Eventually the damage
from these currents can lead to failures of flow rate monitors, and even
large portions of power grids. The ability to predict CMEs would be useful
in light of our heavy dependence on electric power and satellites.
Sigmoids
Often an S or inverse S shape, known as a sigmoid, appears at active regions
that produce CMEs. This distinctive shape is easily seen in x-ray images
of the sun. A deeper understanding of the processes that contribute to
the formation of sigmoids will give greater insights into the mechanics
of coronal mass ejections.