![]() ![]() ![]() However, these new findings do not preclude the possibility of MHD wave motions also supplying significant amounts of energy. Some new evidence from the SOHO 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 problem - it appears that the energy spectrum of small flare-like phenomena may be such that nanoflares are sufficiently numerous to supply the corona's energy requirements. J) or the damping of magnetohydrodynamic (MHD) waves propagating up from the Sun's surface layers (the photosphere). The magnetic field that pervades the corona is certainly the source of the heating, but the question is whether the field energy is dissipated via numerous, small, random reconnections (known as `nanoflares', i.e. K (~0.1 keV) yet the exact reasons for this are still being debated. It has been known for over 50 years that the outer atmosphere of the Sun, the so-called corona, has a temperature of 106 This fact was first recognized in the 1940s, when unfamiliar spectral lines that had been ob served since the nine- teenth century were identified with those emitted by Beyond the chromosphere is the corona, which has a most surprising property: its temperature exceeds 1 mi l- lion K, often much more in localized areas assoc iated with sunspots on the photosphere (Figure 1). The part visible as a bright red crescent during eclipses, is the chromosphere. Above the visible surface of the sun is a tenuous atmosphere. There is a steady decrease of temperature from the core (15 million K) to the surface, also known as photosphere (6000 K). It derives from nuclear reactions deep in the sun's core, where the temperatures are many millions of Kelvin the energy leaks out very gradually throughout the sun's interior towards the vis i- ble surface where it escapes to space and in particular to the earth. This radiation has a vital role in that it is the energy that supports all life forms on earth. For most people, the sun might seem like a uniform ball of gas which shines constantly, day after day. Understanding the s o- lar corona will help us gain insight into whole family of stars. Over the past twenty years or so, it has become apparent that the sun is not unique in ha v- ing a hot corona: many other stars have coronae too, some showing the same characteristics as that of the sun, others being quite different. Its existence and its peculiar properties, above all its extremely high temperature, have long been intriguing to astronomers who, after a half century of study, are still uncertain about the source of its energy. This 'halo' is the solar corona, the Latin word for crown. However, when the sun is totally eclipsed, a white halo appears around the edge of the moon, stretching out to a distance of solar diameter (1,400,000 km) or more. UNDER ordinary circumstances, the flood of light from the bright disk of the sun overwhelms the weak emis- sion that comes from the solar atmosphere which is thus hidden from view. ![]()
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