Temperature as a function of distance
http://138.238.143.191/astronomy/Chaisson/AT416/IMAGES/AACHCYV0.JPG
Why is the sun’s corona so hot?
http://www.iop.org/EJ/abstract/0741-3335/42/2/304
Abstract. It has been known for over 50 years that the outer
atmosphere of the Sun, the so-called corona, has a temperature of
10^6 K (~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 question is whether the field
energy is dissipated via numerous, small, random reconnections
(known as `nanoflares', i.e. small energy releases of about 10^16
J) or the damping of magnetohydrodynamic (MHD) waves propagating
up from the Sun's surface layers (the photosphere).
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. However, these new
findings do not preclude the possibility 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 increasingly sophisticated
equipment. Some very fast imaging systems are now available, and
a description is given of a charge-coupled device camera
experiment conducted successfully during the recent total solar
eclipse visible in Europe.
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