Solar corona

(astronomy) The upper, rarefied solar atmosphere which becomes visible around the darkened sun during a total solar eclipse. Also known as corona.

The outer atmosphere of the Sun. The corona is dominated by intense magnetic forces, which penetrate it from denser regions of the Sun. Coronal gas accumulates around these magnetized regions to produce the shapes seen during a solar eclipse, with a coronagraph, or in x-rays. These shapes include long streamers that penetrate interplanetary space, looplike tubes over the strongest fields, and vast regions of very low density called coronal holes. Coronal holes are the source of solar matter streaming into interplanetary space. The general magnetic field of the Sun, about 1 gauss (0.1 millitesla), is revealed near the north and south poles by polar rays that resemble the pattern formed by iron filings near a bar magnet.

From a minimum in the lower atmosphere (approximately 4000 K or 7000°F), the tempera­ture rises to values in the corona of 1–5 × 10⁶ K (2–9 × 10⁶ °F) caused by dissipation of mechanical or magnetic energy produced by turbulent flows in the lower atmosphere. The corona is hot enough to emit x-rays, and x-ray telescopes in space can form images of the corona. See also Sun.