Visible coronal radiation

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The brightness of the brighter inner part of the corona in visible radiation is indeed only 10~6th of the solar disc's brightness. This faint radiation is produced by scattering of sunlight in the Earth's atmosphere and barely reaches 10~4th of the sky luminosity; hence, the corona cannot be seen from Earth under normal conditions. However, by a happy accident, the Sun and the Moon are seen from the Earth with virtually the same angular size, so that the Moon occults precisely the whole solar disc during total eclipses. Since this occultation occurs above the Earth's atmosphere, the sky brightness is also lowered, so that the corona can be seen on these occasions, even by the unaided eye. Total eclipses are rare, and other techniques have been developed to study the corona and the chromosphere, in particular by devising artificial

Figure 1.12 The solar corona after solar activity minimum (left panel) and near solar activity maximum (right panel). The images are composites of eclipse photographs taken respectively in Guadeloupe on 26 February 1998 (left, obs. C. Viladrich) and in Angola on 21 June 2001 (right, obs. J. Mouette), and nearly simultaneously from the LASCO-C2 coronagraph on the spacecraft SoHO (ESA and NASA). (Composites by Institut d'Astrophysique de Paris - CNRS; by courtesy of S. Koutchmy.)

Figure 1.12 The solar corona after solar activity minimum (left panel) and near solar activity maximum (right panel). The images are composites of eclipse photographs taken respectively in Guadeloupe on 26 February 1998 (left, obs. C. Viladrich) and in Angola on 21 June 2001 (right, obs. J. Mouette), and nearly simultaneously from the LASCO-C2 coronagraph on the spacecraft SoHO (ESA and NASA). (Composites by Institut d'Astrophysique de Paris - CNRS; by courtesy of S. Koutchmy.)

eclipses inside optical instruments. Figure 1.12 is a composite obtained with both techniques, at different epochs of solar activity. The inner part shows the visible appearance of the corona observed from Earth during an eclipse; the outer part is obtained by making an artificial eclipse aboard a spacecraft on the same days.13

These images raise a number of questions. In the first place, what produces the observed radiation?

This question can be addressed, up to a point, in a very simple way. At T ^ 106 K, the mean kinetic energy of a particle, 3kBT/2, amounts to 130 eV -roughly ten times more than the binding energy of the hydrogen atom. Hence not only do ambient electrons have enough energy for ionising hydrogen atoms by collisions and knocking out the outer electron of heavier atoms, but they can also knock out a large part of the more strongly bound inner electrons. The corona is thus a mixture of ions, including several-times ionised ones, and free electrons: a plasma. These free electrons are subjected to the solar radiation, are accelerated by the wave electric field, and in turn radiate at the same frequency; this is called Thomson scattering. Thomson scattering of sunlight is responsible for the visible radiation of the corona at altitudes up to a few solar radii.

From analysis of this radiation, one can deduce the density of electrons in the corona. Let us perform a simple estimate. The lower part of the corona is

13An opaque disc is put into the telescope to mask out the bright central emitting region.

close to hydrostatic equilibrium, hence the density is expected to fall roughly in an exponential way with a scale height given by (1.10); H is in fact twice larger because the medium is essentially made of protons and electrons, so that the mean mass per particle is ^ ^ (mp + me) /2 ^ mp/2 instead of mp; we will return to this point later. With T ^ 106 K, we find H ^ 0.1 x Rq . With such a small scale height, most of the scattered radiation comes from the electrons lying low in the corona, at distances from the Sun's centre close to Rq, so that the flux of solar radiation they receive is about Lq/4^R2 .

The ratio of the power radiated by one electron to the incident flux of radiation is given by Thomson's cross-section aT = 8nr2/3 (1.11)

where the so-called classical electron radius is e

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