Evershed effect
Encyclopedia
The Evershed effect, named after the British astronomer
John Evershed
, is the radial flow of gas across the photospheric surface of the penumbra of sunspot
s from the inner border with the umbra
towards the outer edge.
The speed varies from around 1 km/s at the border between the umbra
and the penumbra to a maximum of around double this in the middle of the penumbra and falls off to zero at the outer edge of the penumbra.
Evershed first detected this phenomenon in January 1909, whilst working at the Kodaikanal Solar Observatory
in India
, when he found that the spectral line
s of sunspots showed doppler shift.
Afterwards, measurements of the spectral emission lines emitted in the ultraviolet
wavelengths have shown a systematic red-shift. The Evershed effect is common to every spectral line formed at a temperature below 105 K; this fact would imply a constant downflow from the transition region towards the chromosphere
. The observed velocity is about 5 km/sec. Of course, this is impossible, since if it were true, the corona would disappear in a short time instead of being suspended over the Sun at temperatures of million degrees over distances much larger than a solar radius.
Many theories have been proposed to explain this redshift in line profiles of the transition region, but the problem is still unsolved, since a coherent theory should take into account all the physical observations: UV line profiles are redshifted on average, but they show back and forth velocity oscillations at the same time.
In synthesis, the proposed mechanisms are:
Astronomer
An astronomer is a scientist who studies celestial bodies such as planets, stars and galaxies.Historically, astronomy was more concerned with the classification and description of phenomena in the sky, while astrophysics attempted to explain these phenomena and the differences between them using...
John Evershed
John Evershed
John Evershed was an English astronomer. He was the first to observe radial motions in sunspots, a phenomenon known as the Evershed effect....
, is the radial flow of gas across the photospheric surface of the penumbra of sunspot
Sunspot
Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature....
s from the inner border with the umbra
Umbra
The umbra, penumbra and antumbra are the names given to three distinct parts of a shadow, created by any light source. For a point source only the umbra is cast.These names are most often used to refer to the shadows cast by celestial bodies....
towards the outer edge.
The speed varies from around 1 km/s at the border between the umbra
Umbra
The umbra, penumbra and antumbra are the names given to three distinct parts of a shadow, created by any light source. For a point source only the umbra is cast.These names are most often used to refer to the shadows cast by celestial bodies....
and the penumbra to a maximum of around double this in the middle of the penumbra and falls off to zero at the outer edge of the penumbra.
Evershed first detected this phenomenon in January 1909, whilst working at the Kodaikanal Solar Observatory
Kodaikanal Solar Observatory
The Kodaikanal Solar Observatory is an solar observatory owned and operated by Indian Institute of Astrophysics. It is located on the southern tip of the Palni Hills 4 km from Kodaikanal town, Dindigul district, Tamilnadu state, South India....
in India
India
India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...
, when he found that the spectral line
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
s of sunspots showed doppler shift.
Afterwards, measurements of the spectral emission lines emitted in the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
wavelengths have shown a systematic red-shift. The Evershed effect is common to every spectral line formed at a temperature below 105 K; this fact would imply a constant downflow from the transition region towards the chromosphere
Chromosphere
The chromosphere is a thin layer of the Sun's atmosphere just above the photosphere, roughly 2,000 kilometers deep....
. The observed velocity is about 5 km/sec. Of course, this is impossible, since if it were true, the corona would disappear in a short time instead of being suspended over the Sun at temperatures of million degrees over distances much larger than a solar radius.
Many theories have been proposed to explain this redshift in line profiles of the transition region, but the problem is still unsolved, since a coherent theory should take into account all the physical observations: UV line profiles are redshifted on average, but they show back and forth velocity oscillations at the same time.
In synthesis, the proposed mechanisms are:
- siphon flows in coronal loopCoronal loopCoronal loops form the basic structure of the lower corona and transition region of the Sun. These highly structured and elegant loops are a direct consequence of the twisted solar magnetic flux within the solar body. The population of coronal loops can be directly linked with the solar cycle; it...
s driven by a pressure difference; - different cross-sections of the coronal loopCoronal loopCoronal loops form the basic structure of the lower corona and transition region of the Sun. These highly structured and elegant loops are a direct consequence of the twisted solar magnetic flux within the solar body. The population of coronal loops can be directly linked with the solar cycle; it...
s footpoints; - the return of spiculesSpicule (solar physics)In solar physics, a spicule is a dynamic jet of about 500 km diameter in the chromosphere of the Sun. It moves upwards at about 20 km/s from the photosphere...
; - multiple flows;
- nanoflaresNanoflaresA nanoflare is a very small solar flare which happens in the corona, the external atmosphere of the Sun.The hypothesis of "microflares" as a possible explanation of the coronal heating was first suggested by Gold and then later developed by Eugene Parker....
; - thermal instabilities during chromospheric condensation.
Other related articles
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- transition region
- sunspots
- photospherePhotosphereThe photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...
- chromosphereChromosphereThe chromosphere is a thin layer of the Sun's atmosphere just above the photosphere, roughly 2,000 kilometers deep....
- coronaCoronaA corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...
- nanoflaresNanoflaresA nanoflare is a very small solar flare which happens in the corona, the external atmosphere of the Sun.The hypothesis of "microflares" as a possible explanation of the coronal heating was first suggested by Gold and then later developed by Eugene Parker....
- spiculesSpicule (solar physics)In solar physics, a spicule is a dynamic jet of about 500 km diameter in the chromosphere of the Sun. It moves upwards at about 20 km/s from the photosphere...
- coronal loopCoronal loopCoronal loops form the basic structure of the lower corona and transition region of the Sun. These highly structured and elegant loops are a direct consequence of the twisted solar magnetic flux within the solar body. The population of coronal loops can be directly linked with the solar cycle; it...
s - Spectral lines
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