The Evershed effect, named after the British

astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the scope of Earth. They observe astronomical objects such as stars, planets, natural satellite, moons, comets and galaxy, g ...

John Evershed John Evershed CIE FRS FRAS (26 February 1864 – 17 November 1956) was an English astronomer. He was the first to observe radial motions in sunspots, a phenomenon now known as the Evershed effect. Biography Evershed was born in Gomshall ...

, is the radial flow of gas across the

photospheric The photosphere is a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...

surface of the

penumbra The umbra, penumbra and antumbra are three distinct parts of a shadow, created by any light source after impinging on an opaque object. Assuming no diffraction, for a collimated beam (such as a point source) of light, only the umbra is cast. Th ...

sunspot Sunspots are phenomena on the Sun's photosphere that appear as temporary spots that are darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic flux that inhibit convection. Sun ...

s from the inner border with the

umbra The umbra, penumbra and antumbra are three distinct parts of a shadow, created by any light source after impinging on an opaque object. Assuming no diffraction, for a collimated beam (such as a point source) of light, only the umbra is cast. Th ...

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 The Kodaikanal Solar Observatory is a solar observatory owned and operated by the Indian Institute of Astrophysics. It is on the southern tip of the Palani Hills from Kodaikanal town. The Evershed effect was first detected at this observato ...

in India, when he found that the

spectral line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...

s of sunspots showed

doppler shift The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who d ...

. Afterwards, measurements of the spectral

emission lines A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...

emitted in the

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...

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 A chromosphere ("sphere of color") is the second layer of a star's atmosphere, located above the photosphere and below the solar transition region and corona. The term usually refers to the Sun's chromosphere, but not exclusively. In the Su ...

. The observed velocity is about 5 km/s. Of course, this is impossible, since if it were true, the

corona Corona (from the Latin for 'crown') most commonly refers to: * Stellar corona, the outer atmosphere of the Sun or another star * Corona (beer), a Mexican beer * Corona, informal term for the coronavirus SARS-CoV-2, which causes the COVID-19 di ...

would disappear in a short time instead of being suspended over the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

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 loop In solar physics, a coronal loop is a well-defined arch-like structure in the Sun's atmosphere made up of relatively dense plasma confined and isolated from the surrounding medium by magnetic flux tubes. Coronal loops begin and end at two footp ...

s driven by a pressure difference, * different cross-sections of the coronal loops footpoints, * the return of

spicules Spicules are any of various small needle-like anatomical structures occurring in organisms Spicule may also refer to: *Spicule (sponge), small skeletal elements of sea sponges *Spicule (nematode), reproductive structures found in male nematodes ( ...

, * multiple flows, * nanoflares, and * thermal instabilities during ''chromospheric condensation''. The effect was commemorated in a postage stamp issued in India on 2 December 2008.

References

Solar phenomena Emission spectroscopy Plasma physics {{Spectroscopy-stub

See also

References