|Helmet Streamers and the Magnetic Structure of the Corona|
Magnetic field lines from a computer simulation performed in advance of the October 24, 1995 solar eclipse, to predict the structure of the solar corona. Field line colors are arbitrary, colors on the Sun's surface show the strength of the magnetic field (yellow is largest). Click on image for full size ( 14K GIF).
The gas in the solar corona is at very high temperatures (typically 1-2 million degrees Kelvin in most regions) so it is almost completely in a plasma state (made up of charged particles, mostly protons and electrons). Strong magnetic fields thread through the corona. Where these magnetic lines of force are closed, the magnetic field is strong enough to trap the solar plasma and keep it from escaping. Plasma accumulates in these regions and forms the beautiful structures called helmet streamers seen during solar eclipses. Prominences are often situated beneath helmet streamers, and active regions occur beneath streamers near the equator (sometimes called active regions 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, which accelerates outward from the Sun and fills interplanetary space. 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.