Why is the Sun's corona so hot? Views from SOHO, Yohkoh and solar eclipses

It has been known for over 50 years that the outer atmosphere of the Sun, t he so-called corona, has a temperature of 10(6) K (similar to 0.1 keV) yet the exact reasons for this are still being debated. The magnetic field that pervades the corona is certainly the source of the heating, but the questi on is whether the field energy is dissipated via numerous, small, random re connections (known as 'nanoflares', i.e. small energy releases of about 10( 16) J) or the damping of magnetohydrodynamic (MHD) waves propagating up fro m the Sun's surface layers (the photosphere). Some new evidence from the SO HO and Yohkoh spacecraft, with instruments on board which image the Sun in the extreme ultraviolet and soft x-rays, are shedding new light on the prob lem-it appears that the energy spectrum of small flare-like phenomena may b e such that nanoflares are sufficiently numerous to supply the corona's ene rgy requirements. However, these new findings do not preclude the possibili ty of MHD wave motions also supplying significant amounts of energy. Theory suggests very short period (<1 s) MHD waves are implicated in the heating process if so. Searches during total solar eclipses for modulations of the white-light coronal intensity that may result are being made with increasin gly sophisticated equipment. Some very fast imaging systems are now availab le, and a description is given of a charge-coupled device camera experiment conducted successfully during the recent total solar eclipse visible in Eu rope.