The solar corona

A Swedish scientist from Lund, Bengt Edlén, was the first (1940) to establish that coronal emission lines arise from normal elements that are ionised by an extremely hot corona ( > 106K degrees). Stanford scientists have very recently found out why the corona is so hot (see Figure 1).

Figure 1 illustrates how magnetic energy is tranferred upward from the Sun's surface toward the corona above. There is more than enough energy coming up from the loops of the "magnetic carpet" to heat the corona. These dramatic new observations have been made by the Solar Oscillations Investigation (SOI) group at Lockheed-Martin Solar and Astrophysics Laboratory and at Stanford University.

The interaction between the plasma and the magnetic field in the solar corona determines what kind of phenomena will occur in the corona. An unbalanced magnetic flux causes the magnetic field lines to open and a so called coronal hole (CH) is formed. From the CHs a fast stream of plasma (the solar wind) expands into the interplanetary space. From the so called coronal streamers (CSs) a slow stream of plasma is thought to expand into the interplanetary space. Plumes, outside the coronal holes, have also recently been suggested as sources of the slow solar wind. The fast solar wind catches up with the slow solar wind and an interaction region is produced.

Figure 2 shows a coronal hole at the Sun's north pole. The X-ray picture was taken on May 8, 1992 with the Soft X-Ray Telescope (SXT) aboard Yohkoh satellite.

Figure 3 shows a coronal hole at the Sun's north pole. Jets of gas (polar plumes) periodically spurt from coronal holes. "Courtesy of SOHO consortium. SOHO is a project of international cooperation between ESA and NASA."

Figure 4 shows a coronal hole at the Sun's north pole. The "zoomed-in" region shows a Doppler velocity map of million degree plasma, where the solar wind originates. Blue represents blue shifts or outflows and red represents red shifts or downflows. Superposed are the edges of "honey-comb" shaped patterns of magnetic fields at the photospheric surface of the Sun, where the strongest flows (dark blue) occur. "Courtesy of SOHO consortium. SOHO is a project of international cooperation between ESA and NASA."

Observations with the use of instruments aboard NASA's Spartan 2001 spacecraft and SOHO suggest a new acceleration mechanism of the solar wind. The electrical charges of the solar wind spiral out along the magnetic field in the corona (especially coronal holes). When the magnetic field lines vibrate, as they do in a magnetic wave, the spiraling particles accelerate out and away from the Sun into the interplanetary space.

Figure 5 shows how the solar wind particles are accelerated as the particle spiraling out of from the corona along vibrating magnetic field lines. (Courtesy of NASA Spartan and ESA/SOHO).

The closed magnetic field loops, located below the CSs, can start to expand into, or be explosively ejected into interplanetary space. A so called transient coronal mass ejection (CME) has occurred, which produces an enormous plasma cloud in the interplanetary space. These clouds reach Earth already within 1-2 days. They can involve 1016 g (ten billions tons) gas thas is suddenly ejected at speed up to 2000 km/s with kinetic energy of 1032 ergs. The most intense effects on earth are associated with these fast plasma clouds. They cause interplanetary shocks, large solar particle events (SEP) and non-recurrent geomagnetic storms. In the article Solar energetic particles: A paradigm shift D. V. Reames describes how proton events and CME-Driven shocks are related.

Figure 6 and 7 show a very fast solar coronal mass ejection, observed with the High Altitude Observatory's coronagraph aboard NASA's Solar Maximum Mission spacecraft on October 24 18:09 and 18:19, 1989.

Figure 8 shows the famous January 6, 1997 halo CME. Halo events are most often geoeffective. To see our prediction of the geomagnetic storm and the satellite anomaly please click here.

Figure 9 shows the halo CME of July 14, 2000 observed with LASCO onboard SOHO. The CME caused the third largest proton event, and all kind of space weather effects..

Figure 10 shows a tall gyrating storm far larger and faster than tornadoes on the Earth. Tornadoes on Earth blow at about 400-500 km/h. The solar tornadoes blow with speeds of about 500 000 km/h! They occur most frequently near north and south poles of the Sun and are almost as wide as the Earth. British scientists discovered the tornadoes, using observations with the CDS instrument aboard SOHO.

Figure 11 and 12 show a solar flare observed with the Yohkoh mission.

It is believed that solar flares are a result of a sudden (a few minutes to few tens of minutes) release of energy (up to 1032 ergs with temperatures of 10-20 million degrees) stored in the magnetic fields that thread the solar corona in active regions around sunspots. Solar flares produce radiation from longest wavelength radio waves to shortest gamma rays. Solar flares affect ionosphere and radio communication at earth, and also release energetic particles.

Figure 13 shows a solar quake produced by a solar flare. MDI data were used by scientists at Stanford and Glasgow. The flare-generated solar quake contained about 40 000 times the energy released in the great earthquake that devastated San Francisco in 1906.

Figure 14 shows the solar eclipse of 18 March 1988 in white light (Courtesy of HAO).

For further information about eclipses:

Stanford Solar Center.