The Corona:



During total solar eclipses, as the Moon completely obscures the dazzling light of the photosphere, it briefly becomes possible to see the outer solar atmosphere, which extends for several solar radii from the disc of the Sun: the corona. The corona reaches from just above the chromosphere far out into interplanetary space. Some indication of its great extent is given by observations from satellites equipped with coronagraphs; results in X-ray wavelengths, particularly, from such spacecraft as Yohkoh and SOHO, clearly show the corona to be an active, dynamic environment.

Most of the corona consists of great arches of hot, ionized gas (plasma): smaller arches within active regions and larger arches between active regions. The corona is shaped by the extended solar magnetic field. Closed magnetic field loops above active regions give rise to bright structures, described as “helmets”. Regions of open magnetic field, where only one end of the field line is embedded in the Sun, give rise to long “streamers” extending radially away from the Sun.

The shape of the corona changes over the sunspot cycle. At sunspot maximum, when active regions are abundant, the corona consists mainly of evenly distributed closed loops; at minimum, long streamers extend to either side of the Sun, mainly from its equatorial regions. Around sunspot maximum, when flare activity is common, the corona in X-ray wavelengths is frequently seen to be disturbed by outward-travelling shock waves. These coronal mass ejections (CMEs) have become recognized as an important source of turbulence in the solar wind. CMEs directed towards Earth can cause magnetic storms. A primary aim of the SOHO satellite mission is to observe CMEs with a view to forecasting such disruption. In 1999 X-ray observations made by Yohkoh linked CMEs to the appearance of sigmoids, S-shaped formations on the photosphere (inverted in the Sun's northern hemisphere) some 160,000 km (100,000 mi) long, which may indicate the magnetic field twisting back on itself. The data indicated a strong statistical correlation between the appearance of sigmoids and the subsequent eruption of CMEs.

In the 1940s the corona was discovered to be much hotter than either the photosphere or the chromosphere, with a temperature of over 1 million K (1.8 million degrees F). Finding the mechanism by which this energy reaches the corona is one of the classic problems of astrophysics. Early ideas to account for coronal heating included the dissipation of acoustic waves produced by the motion of the turbulent solar granules. Through further analysis, it became apparent that such waves would give up their energy before reaching coronal heights. Propagation of gravitational waves was rejected for similar reasons. The most widely accepted theory suggests that the corona is heated by energy carried by magnetic loops emerging from the deep solar interior. The Yohkoh and SOHO spacecraft have provided ample observational evidence to support the idea that considerable magnetic energy is transferred to the corona via CMEs and other transient phenomena.