The Corona

The corona is the very tenuous and very hot, outer atmosphere of the Sun, with temperatures up to 1 to 2,000,000 K. Its light emission is very faint—about as bright as the full moon or one millionth as bright as the solar disk. Therefore the corona is visible only during eclipses (natural or artificial). Magnetic phenomena are apparently responsible for conveying energy from the turbulent photosphere into the material of the corona. This energy shared amongst the very few atoms in the corona produces the high temperatures.

Prominences (when seen near the solar limb) and plages (when seen superimposed on the solar disk) are corona regions that appear very bright in the visible part of the spectrum. These features often appear as streamers or filaments, suggesting a structure related to magnetic fields. The conditions of these regions produce excess emission of light and do not necessarily indicate a flow of matter in the corona.

At million degree temperatures, emission of extremely short wavelength electromagnetic radiation—Xrays—becomes important. Viewed in Xrays, the luminosity of the corona is far from uniform, with regions of intense emission ( coronal loops) and other areas that are dark ( coronal holes).

The corona grades smoothly into the solar wind, an outward flow of ionized gas that has achieved escape velocity. The solar wind bypasses Earth's magnetic field at a velocity of 400 km/s and causes a bow shock; variations in the solar wind (for example, excess particles ejected by flares) may disturb Earth's global magnetic field and can disrupt long‐distance radio communication. Solar wind particles trapped by the Earth's magnetic field are funneled into the polar regions producing the aurora. Airglow is also a result of solar wind exciting atmospheric molecules to emit light. The night sky emits twice as much light as is received from stars.