Corona (sun)

The sun's corona during the solar eclipse in 1999 , just before the sunspot maximum. The visible rays run in all directions.

The corona during the solar eclipse in 2006 , just before the sunspot minimum. The rays almost only run in the equatorial plane.

The solar corona ( ancient Greek κορώνη korṓnē "curved", Latin corona "wreath, crown ") is the area of the sun's atmosphere that lies above the chromosphere and, compared to deeper layers, has significantly lower densities , but higher temperatures.

The solar corona should not be confused with the corona or the halo around the sun or moon, which are based on diffraction effects in the earth's atmosphere.

visibility

The weak glow of the corona is only visible to the naked eye during a total solar eclipse . The halo, which is mainly generated by Thomson scattering of electrons , extends outwards by around 1 to 3 solar radii, depending on solar activity, and continuously changes into the solar wind .

Independent of solar eclipses, the inner part of the corona can be observed with the help of coronographs or satellites that operate in other spectral ranges than the optical one.

In times of high solar activity, the visible corona can be seen up to a distance of several million kilometers or 2 to 3 solar diameters above the photosphere . Due to the arrangement of the coronal magnetic field, it shows a radial structure that changes globally over the course of the 11-year sunspot cycle . As a result of the different structure of the magnetic field in which the coronal plasma is enclosed, the visible rays usually run in all directions during an activity maximum, while at the sunspot minimum the clearest structures appear on the solar equator (see figures).

construction

Corona during the solar eclipse of July 2, 2019

The corona consists of an almost completely ionized plasma and, at typically several million Kelvin, is significantly hotter than the layers of the sun below, the chromosphere and the photosphere, which is the actual surface of the sun.

The causes and mechanisms of action that lead to this corona heating are not yet fully understood and represent a central subject of current research in solar physics .

Physical models

Possible explanatory models for heating the corona include

the dissipation of plasma waves
shock dominated dissipation of electrical currents
Shock waves
Reconnection of continuously restructured magnetic field configurations

and other processes.

Space probes such as SOHO , TRACE , RHESSI and CHANDRA contribute significantly to these investigations with their measurements. In the course of its orbit, the Parker Solar Probe space probe is supposed to approach the photosphere up to a distance of 8.5 solar radii and thus fly through the corona.

Logarithmic light profile of the corona (blue). The red curve represents the photosphere and the decrease in its brightness near the visible edge of the sun.

A particularly steep temperature gradient prevails in the lowest corona, where the density decreases rapidly with the distance from the surface (see diagram): within a few hundred kilometers the kinetic gas temperature rises by a million Kelvin. The high temperature and possibly additional acceleration mechanisms ultimately lead to coronal plasma escaping as solar wind.

The corona can be so hot only because of their extremely low density: the high temperature marks as in any gas or plasma, the kinetic energy of the gas particles. On the other hand, a solid would have a much lower temperature due to its higher particle density and the large number of degrees of freedom at the same height above the sun, because a completely different thermal equilibrium would be established. Considered clearly, the entire thermal energy is distributed over a few gas particles, so that each individual particle receives a relatively high amount of energy. The gas particles behave almost like an ideal gas and only have degrees of freedom of translation. All additional amounts of energy, in addition to the thermal radiation emanating from the sun's surface, therefore act as momentum transmission on the gas particles.

The following approximation formula describes the intensity of the corona radiation in the projection , normalized to the radiation in the center of the solar disk : ${\ displaystyle I (\ rho = 0)}$

{\ displaystyle {I (\ rho) \ over I (0)} = 10 ^ {- 6} \ left ({\ frac {3 {,} 670} {\ rho ^ {18}}} + {\ frac { 1 {,} 939} {\ rho ^ {7 {,} 8}}} + {\ frac {0 {,} 0551} {\ rho ^ {2 {,} 5}}} \ right)}

with the dimensionless distance from the center of the sun, where corresponds to the sun's edge. ${\ displaystyle \ rho> 1}$ ${\ displaystyle \ rho = 1}$

This approximation only represents a temporal and spatial mean , because the intensity of the corona radiation varies greatly with the heliographic latitude and the current solar activity. More precise conclusions on possible explanatory models for the heating of the corona can be drawn from detailed spectra . The intensity ratio described, however, illustrates the almost unsolvable problem of obtaining such spectroscopic data from the earth.

There are indications that the cause of the high temperatures in the corona is to be found in nanoflares , which have much lower energy than normal solar flares (factor ), but reach extremely high temperatures ( degrees Kelvin) and occur millions of times per second on the sun. The EUNIS (Extreme Ultraviolet Normal Incidence Spectrograph) missile probe mission in 2013 found evidence of extremely hot plasma in areas of the sun in the extreme UV and NuSTAR in the X-ray range, which otherwise had no major solar flare activities. The heating of the corona with such nanoflares was proposed by Thomas Gold (1964) and Eugene Parker (1972). The Parker Solar Probe is supposed to provide definitive information about their existence . ${\ displaystyle 10 ^ {- 9}}$ ${\ displaystyle 10 ^ {7}}$

Overall brightness

If the distance from 1 (edge of the sun) to infinity is integrated in the radiation formula, the total brightness of the corona is obtained under ideal measurement conditions, as they are approximately in a total solar eclipse . It is about 1.6 · 10 ^{−6 of} the total brightness of the sun, which corresponds to an apparent brightness of −12.3 ^m . This relatively weak glow is comparable to the apparent brightness of the full moon , which is why the corona can be observed during a total solar eclipse without eye protection . But as soon as the edge of the sun reappears behind the moon as a narrow, blinding crescent , the corona disappears for our eyes within a very short time.

Spectroscopic composition

Different scattering processes shape the corona. The names go back to historical characterizations:

F-corona ( Fraunhofer corona ): Dust scatters sunlight. Apart from a preference for the forward scattering direction, the radiation remains unchanged. That is why Fraunhofer lines for primary solar radiation can be detected there.
K-Corona ( Continuous Corona ): Free electrons scatter light ( Rayleigh scattering ). Since the free electrons move at different speeds, the wavelengths of the light are shifted by the Doppler effect in such a way that all Fraunhofer lines are "smeared" into a continuum . In addition, the radiation is scattered depending on its polarization .
L-corona or E-corona ( line corona / emission corona ): The corona gas emits characteristic spectral lines .
T-corona ( thermal corona ): heated particles emit as temperature radiators in a continuous spectrum .

literature

A.Berroth, W. Hofmann : Cosmic Geodesy . Verlag G.Baun, Karlsruhe 1960, chapter eclipse observations.
Helmut Scheffler, Hans Elsässer : Physics of the stars and the sun. BI, Mannheim 1990, ISBN 3-411-14172-7 .

Web links

Commons : Korona (Sun) - Collection of images, videos and audio files

Solar corona heating region observed directly for the first time

Individual evidence

↑ According to Stowasser, Latin-German School Dictionary , 1894, corona is a loan word in Latin from the Greek κορώνη , which means something crooked.
↑ November, LJ; Koutchmy, S .: White-Light Coronal Dark Threads and Density Fine Structure . In: Astrophysical Journal . tape 466 , July 1996, p. 512 ff .
↑ Physicists present solution to the riddle of coronal heating , Astropage, May 1, 2015

[1] According to Stowasser, Latin-German School Dictionary , 1894, corona is a loan word in Latin from the Greek κορώνη , which means something crooked.

[Koutchmy-2] November, LJ; Koutchmy, S .: White-Light Coronal Dark Threads and Density Fine Structure . In: Astrophysical Journal . tape 466 , July 1996, p. 512 ff .

[3] Physicists present solution to the riddle of coronal heating , Astropage, May 1, 2015