A helmet streamer is a beautiful structure, seen during a solar
eclipse, in which million-degree plasma in the Sun's corona is trapped by closed
magnetic field lines. Prominences are
often situated beneath helmet streamers, and active
regions occur beneath streamers near the equator (sometimes called active
region streamers). In some regions, the coronal magnetic field cannot confine
the plasma, and the plasma expands outward, reaching supersonic velocities.
Regions on the Sun with these open magnetic field lines (which stretch far
out into the Solar System) correspond to coronal
holes and are the source of the solar
wind. The electrons in the coronal hole plasma are typically cooler
and less dense than streamers, and so they show up as dark regions in both
X-rays and white light.