Chromosphere

The chromosphere (literally, "sphere of color") is the second of the three main layers in the Sun's atmosphere and is roughly 3,000 to 5,000 kilometers deep. The chromosphere's rosy red color is only apparent during eclipses. The chromosphere sits just above the photosphere and below the solar transition region. The layer of the chromosphere atop the photosphere is homogeneous. A forest of hairy-appearing spicules rise from the homogeneous layer, some of which extend 10,000 km into the corona above.

The density of the chromosphere is only 10−4 times that of the photosphere, the layer beneath, and 10−8 times that of the atmosphere of Earth at sea level. This makes the chromosphere normally invisible and it can be seen only during a total eclipse, where its reddish color is revealed. The color hues are anywhere between pink and red.[1] Without special equipment, the chromosphere cannot normally be seen due to the overwhelming brightness of the photosphere beneath.

The density of the chromosphere decreases with distance from the center of the Sun. This decreases logarithmically from 1017 particles per cubic centimeter, or approximately 2×10−4 kg/m3 to under 1.6×10−11 kg/m3 at the outer boundary.[2] The temperature decreases from the inner boundary at about 6,000 K[3] to a minimum of approximately 3,800 K,[4] before increasing to upwards of 35,000 K[3] at the outer boundary with the transition layer of the corona.

Chromospheres have been observed also in stars other than the Sun.[5] The Sun's chromosphere has been hard to examine and decipher, although observations continue with the help of the electromagnetic spectrum.[6]

HI6563 fulldisk
The Sun observed through a telescope with a Hydrogen-alpha filter
Sun Atmosphere Temperature and Density SkyLab
Skylab measured the temperature (solid curve) and density (dashed curve) of the chromosphere between the thinner transition region and the lower photosphere (darker orange). See here for meanings of extra lines in the graph.

Comparing chromosphere and photosphere

Whilst the photosphere has an absorption line spectrum, the chromosphere's spectrum is dominated by emission lines. In particular, one of its strongest lines is the Hα at a wavelength of 656.3 nm; this line is emitted by a hydrogen atom whenever its electron makes a transition from the n=3 to the n=2 energy level. A wavelength of 656.3 nm is in the red part of the spectrum, which causes the chromosphere to have its characteristic reddish colour.

By analysing the spectrum of the chromosphere, it was found that the temperature of this layer of the solar atmosphere increases with increasing height in the chromosphere itself. The temperature at the top of photosphere is only about 4,400 K, while at the top of chromosphere, some 2,000 km higher, it reaches 25,000 K.[1][7] This is however the opposite of what we find in the photosphere, where the temperature drops with increasing height. It is not yet fully understood what phenomenon causes the temperature of the chromosphere to paradoxically increase further from the Sun's interior. However, it seems likely to be explained, partially or totally, by magnetic reconnection.

Features

Many interesting phenomena can be observed in the chromosphere, which is very complex and dynamic:

  • Filaments (and prominences, which are filaments viewed from the side) underlie many coronal mass ejections and hence are important to the prediction of space weather. Solar prominences rise up through the chromosphere from the photosphere, sometimes reaching altitudes of 150,000 km. These gigantic plumes of gas are the most spectacular of solar phenomena, aside from the less frequent solar flares.
  • The most common feature is the presence of spicules, long thin fingers of luminous gas which appear like the blades of a huge field of fiery grass growing upwards from the photosphere below. Spicules rise to the top of the chromosphere and then sink back down again over the course of about 10 minutes. Similarly, there are horizontal wisps of gas called fibrils, which last about twice as long as spicules.
  • Images taken in typical chromospheric lines show the presence of brighter cells, usually called as network, while the surrounding darker regions are named internetwork. They look similar to granules commonly observed on the photosphere due to the heat convection.
  • Periodic oscillations have been found since the first observations with the instrument SUMER on board SOHO with a frequency from 3 mHz to 10 mHz, corresponding to a characteristic periodic time of three minutes.[8] Oscillations of the radial component of the plasma velocity are typical of the high chromosphere. Now we know that the photospheric granulation pattern has usually no oscillations above 20 mHz while higher frequency waves (100 mHz or a 10 s period) were detected in the solar atmosphere (at temperatures typical of the transition region and corona) by TRACE.[9]
  • Cool loops can be seen at the border of the solar disk. They are different from prominences because they look as concentric arches with maximum temperature of the order 0,1 MK (too low to be considered coronal features). These cool loops show an intense variability: they appear and disappear in some UV lines in a time less than an hour, or they rapidly expand in 10–20 minutes. Foukal [10] studied these cool loops in detail from the observations taken with the EUV spectrometer on Skylab in 1976. Otherwise, when the plasma temperature of these loops becomes coronal (above 1 MK), these features appear more stable and evolve on longer times.

See the flash spectrum of the solar chromosphere (Eclipse of March 7, 1970).

On other stars

A spectroscopic measure of chromospheric activity on other stars is the Mount Wilson S-index.[11] [12] See also Superflare#Spectroscopic observations of superflare stars.

See also

References

  1. ^ a b Freedman, R. A.; Kaufmann III, W. J. (2008). Universe. New York, USA: W. H. Freeman and Co. p. 762. ISBN 978-0-7167-8584-2.
  2. ^ Kontar, E. P.; Hannah, I. G.; Mackinnon, A. L. (2008), "Chromospheric magnetic field and density structure measurements using hard X-rays in a flaring coronal loop", Astronomy and Astrophysics, 489 (3): L57, arXiv:0808.3334, Bibcode:2008A&A...489L..57K, doi:10.1051/0004-6361:200810719
  3. ^ a b "SP-402 A New Sun: The Solar Results From Skylab". Archived from the original on 2004-11-18.
  4. ^ Avrett, E. H. (2003), "The Solar Temperature Minimum and Chromosphere", ASP Conference Series, 286: 419, Bibcode:2003ASPC..286..419A, ISBN 978-1-58381-129-0
  5. ^ "The Chromosphere".
  6. ^ Jess, D.B; Morton, RJ; Verth, G; Fedun, V; Grant, S.T.D; Gigiozis, I. (July 2015). "Multiwavelength Studies of MHD Waves in the Solar Chromosphere". Space Science Reviews. 190 (1–4): 103–161. arXiv:1503.01769. Bibcode:2015SSRv..190..103J. doi:10.1007/s11214-015-0141-3.
  7. ^ "World Book at NASA – Sun".
  8. ^ Carlsson, M.; Judge, P.; Wilhelm, K. (1997). "SUMER Observations Confirm the Dynamic Nature of the Quiet Solar Outer Atmosphere: The Internetwork Chromosphere". The Astrophysical Journal. 486 (1): L63. arXiv:astro-ph/9706226. Bibcode:1997ApJ...486L..63C. doi:10.1086/310836.
  9. ^ De Forest, C.E. (2004). "High-Frequency Waves Detected in the Solar Atmosphere". The Astrophysical Journal. 617 (1): L89. Bibcode:2004ApJ...617L..89D. doi:10.1086/427181.
  10. ^ Foukal, P.V. (1976). "The pressure and energy balance of the cool corona over sunspots". The Astrophysical Journal. 210: 575. Bibcode:1976ApJ...210..575F. doi:10.1086/154862.
  11. ^ Observational evidence for enhanced magnetic activity of superflare stars
  12. ^ A small survey of the magnetic fields of planet-hosting stars gives "Wright J. T., Marcy G. W., Butler R. P., Vogt S. S., 2004, ApJS, 152, 261" as a ref for s-index.

External links

12 Ophiuchi

12 Ophiuchi is a variable star in the constellation Ophiuchus. No companions have yet been detected in orbit around this star, and it remains uncertain whether or not it possesses a dust ring.

This star is categorized as a BY Draconis variable, with variable star designation V2133. The variability is attributed to large-scale magnetic activity on the chromosphere (in the form of starspots) combined with a rotational period that moved the active regions into (and out of) the line of sight. This results in low amplitude variability of 12 Ophiuchi's luminosity. The star also appears to display rapid variation in luminosity, possibly due to changes in the starspots. Measurements of the long-term variability show two overlapping cycles of starspot activity (compared to the Sun's single, 11-year cycle.) The periods of these two cycles are 4.0 and 17.4 years.

This star is among the top 100 target stars for NASA's planned Terrestrial Planet Finder mission [1]. However, the mission is now postponed indefinitely.

Its abundance of heavy elements (elements heavier than helium) is nearly identical to that of the Sun. The surface gravity is equal to , which is somewhat higher than the Sun's. The space velocity is 30 km/s relative to the solar system. The high rotation period and active chromosphere are indicative of a relatively young star.

Baily's beads

The Baily's beads effect, or diamond ring effect, is a feature of total and annular solar eclipses. As the Moon covers the Sun during a solar eclipse, the rugged topography of the lunar limb allows beads of sunlight to shine through in some places while not in others. The effect is named after Francis Baily, who explained the phenomenon in 1836. The diamond ring effect is seen when only one bead is left, appearing as a shining "diamond" set in a bright ring around the lunar silhouette.Lunar topography has considerable relief because of the presence of mountains, craters, valleys, and other topographical features. The irregularities of the lunar limb profile (the "edge" of the Moon, as seen from a distance) are known accurately from observations of grazing occultations of stars. Astronomers thus have a fairly good idea which mountains and valleys will cause the beads to appear in advance of the eclipse. While Baily's beads are seen briefly for a few seconds at the center of the eclipse path, their duration is maximized near the edges of the path of the umbra, lasting 1–2 minutes.

After the diamond ring effect has diminished, the subsequent Baily's beads effect and totality phase are safe to view without the solar filters used during the partial phases. By then, less than 0.001% of the Sun's photosphere is visible.

Observers in the path of totality of a solar eclipse see first a gradual covering of the Sun by the lunar silhouette for over an hour, followed by the diamond ring effect (visible without filters) as the last bit of photosphere disappears. As the burst of light from the ring fades, Bailey's beads appear as the last bits of the bright photosphere shine through valleys aligned at the edge of the Moon. As the Baily's beads disappear behind the advancing lunar edge (the beads also reappear at the end of totality), a thin reddish edge called the chromosphere (the Greek chrōma meaning "color") appears. Though the reddish hydrogen radiation is most visible to the unaided eye, the chromosphere also emits thousands of additional spectral lines.

Corona

A corona (meaning 'crown' in Latin derived from Ancient Greek 'κορώνη' (korōnè, “garland, wreath”)) is an aura of plasma that surrounds the Sun and other stars. The Sun's corona extends millions of kilometres into outer space and is most easily seen during a total solar eclipse, but it is also observable with a coronagraph.

Spectroscopy measurements indicate strong ionization in the corona and a plasma temperature in excess of 1000000 kelvin, much hotter than the surface of the Sun.

Light from the corona comes from three primary sources, from the same volume of space. The K-corona (K for kontinuierlich, "continuous" in German) is created by sunlight scattering off free electrons; Doppler broadening of the reflected photospheric absorption lines spreads them so greatly as to completely obscure them, giving the spectral appearance of a continuum with no absorption lines. The F-corona (F for Fraunhofer) is created by sunlight bouncing off dust particles, and is observable because its light contains the Fraunhofer absorption lines that are seen in raw sunlight; the F-corona extends to very high elongation angles from the Sun, where it is called the zodiacal light. The E-corona (E for emission) is due to spectral emission lines produced by ions that are present in the coronal plasma; it may be observed in broad or forbidden or hot spectral emission lines and is the main source of information about the corona's composition.

Coronal radiative losses

In astronomy and in astrophysics, for radiative losses of the solar corona, it is meant the energy flux radiated from the external atmosphere of the Sun (traditionally divided into chromosphere, transition region and corona), and, in particular, the processes of production of the radiation coming from the solar corona and transition region, where the plasma is optically-thin. On the contrary, in the chromosphere, where the temperature decreases from the photospheric value of 6000 K to the minimum of 4400 K, the optical depth is about 1, and the radiation is thermal.

The corona extends much further than a solar radius from the photosphere and looks very complex and inhomogeneous in the X-rays images taken by satellites (see the figure on the right taken by the XRT on board Hinode).

The structure and dynamics of the corona are dominated by the solar magnetic field. There are strong evidences that even the heating mechanism, responsible for its high temperature of million degrees, is linked to the magnetic field of the Sun.

The energy flux irradiated from the corona changes in active regions, in the quiet Sun and in coronal holes; actually, part of the energy is irradiated outwards, but approximately the same amount of the energy flux is conducted back towards the chromosphere, through the steep transition region. In active regions the energy flux is about 107 erg cm−2sec−1, in the quiet Sun it is roughly 8 105 – 106 erg cm−2sec−1, and in coronal holes 5 105 - 8 105 erg cm−2sec−1, including the losses due to the solar wind.

The required power is a small fraction of the total flux irradiated from the Sun, but this energy is enough to maintain the plasma at the temperature of million degrees, since the density is very low and the processes of radiation are different from those occurring in the photosphere, as it is shown in detail in the next section.

H-alpha

H-alpha (Hα) is a specific deep-red visible spectral line in the Balmer series with a wavelength of 656.28 nm in air; it occurs when a hydrogen electron falls from its third to second lowest energy level. H-alpha light is important to astronomers as it is emitted by many emission nebulae and can be used to observe features in the Sun's atmosphere, including solar prominences and the chromosphere.

HD 13931

HD 13931 is G-type star with an apparent visual magnitude of 7.60, located approximately 155 light years away in the constellation Andromeda. This star is slightly larger, hotter, brighter, and more massive than our Sun. Also its metal content is about 8% greater than the Sun, and has a quiet chromosphere.In 2009, a very long-period giant planet, more massive than Jupiter, was found in orbit around the star by measuring changes in the star's radial velocity.

According to a 2018 research, HD 13931 is the most promising Solar System analogue existing, since it has a star similar to the Sun and a planet with mass and semimajor axis similar to Jupiter. Those characteristics yelds a probability greater than 50% for the existence of a dynamically stable habitable zone, where an Earth-like planet may exist and sustain life.

HD 189245

HD 189245 is the Henry Draper catalogue designation for a solitary star in the southern constellation of Sagittarius. It has an apparent visual magnitude of 5.66, which means it is faintly visible to the naked eye. Parallax measurements from the Hipparcos satellite indicate a distance of around 69 light years from the Sun.The stellar classification of this star is F8.5 V Fe−0.6 CH−0.5, indicating that it is an F-type main sequence star with a spectrum that shows deficiencies in iron (Fe) and methylidyne (CH) in its outer atmosphere. It is a variable star with an active chromosphere and is a source of X-ray emission. HD 189245 is spinning rapidly with a projected rotational velocity of 72.6 km/s. Gyrochronology indicates this is a young star with an estimated age of 500 million years. However, the amount of X-ray emission suggests an even younger star that is roughly 100 million years old.The velocity components of HD 189245 indicate that it is a likely member of the AB Doradus moving group of stars, which share a common motion through space. This group has an age of around 50 million years and is centered at a point 98 ly (30 pc) from the Sun.

HR 6806

HR 6806 is a solitary, orange, main sequence (K2 V) star located thirty-six light-years away, in the constellation Hercules. The star is smaller than the Sun, with around 79% of the solar mass and radius, and 35% of the solar luminosity. It appears to be rotating slowly with an estimated period of 42 days. The star has an inactive chromosphere, with a surface magnetic field strength of 1,500 G.There is a nearby brown dwarf, WISE J180901.07+383805.4, at an angular separation of 769″, which would correspond to a projected separation of 8460 AU at the distance of HR 6806. However, this is most likely a typical T7 dwarf, which would place it at a distance of 91 ly (28 pc)—ruling out a physical association. This is confirmed by the differing proper motion of the star and this object.

Interface Region Imaging Spectrograph

The Interface Region Imaging Spectrograph (IRIS), also called Explorer 94, is a NASA solar observation satellite. The mission was funded through the Small Explorer program to investigate the physical conditions of the solar limb, particularly the chromosphere of the Sun. The spacecraft consists of a satellite bus and spectrometer built by the Lockheed Martin Solar and Astrophysics Laboratory (LMSAL), and a telescope provided by the Smithsonian Astrophysical Observatory. IRIS is operated by LMSAL and NASA's Ames Research Center.

The satellite's instrument is a high-frame-rate ultraviolet imaging spectrometer, providing one image per second at 0.3 arcsecond angular resolution and sub-ångström spectral resolution.

NASA announced on 19 June 2009 that IRIS was selected from six Small Explorer mission candidates for further study, along with the Gravity and Extreme Magnetism (GEMS) space observatory.The spacecraft arrived at Vandenberg Air Force Base, California, on 16 April 2013 and was successfully launched on 27 June 2013 by a Pegasus-XL rocket.

Mauna Loa Solar Observatory

Mauna Loa Solar Observatory (MLSO) is a solar observatory located on the slopes of Mauna Loa on the island of Hawaii in the U.S. state of Hawaii. It is operated by the High Altitude Observatory (HAO), a laboratory within the National Center for Atmospheric Research (NCAR). The MLSO sits on property managed by the Mauna Loa Observatory (MLO), which is part of the U.S. Department of Commerce National Oceanic and Atmospheric Administration (NOAA). MLSO was built in 1965.The MLSO is tasked with monitoring the solar atmosphere and recording data on plasmic and energetic emissions from the chromosphere and corona. Studies of coronal mass ejections (CMEs) are also conducted at MLSO. A number of non-solar astronomical observatories are located at the site. The MLSO instruments record images of the solar disk and limb every 3 minutes for 3–10 hours daily starting at 17:00 UT, weather permitting.

Moreton wave

A Moreton wave or Moreton-Ramsey wave is the chromospheric signature of a large-scale solar coronal shock wave. Described as a kind of solar "tsunami", they are generated by solar flares. They are named for American astronomer Gail Moreton, an observer at the Lockheed Solar Observatory in Burbank, and Harry E. Ramsey, an observer who spotted them in 1959 at The Sacramento Peak Observatory. He discovered them in time-lapse photography of the chromosphere in the light of the Balmer alpha transition.

There were few follow-up studies for decades. Then the 1995 launch of the Solar and Heliospheric Observatory led to observation of coronal waves, which cause Moreton waves. Moreton waves were a research topic again. (SOHO's EIT instrument discovered another, different wave type called "EIT waves".)

The reality of Moreton waves (a.k.a. fast-mode MHD waves) has also been confirmed by the two STEREO spacecraft. They observed a 100,000-km-high wave of hot plasma and magnetism, moving at 250 km/s, in conjunction with a big coronal mass ejection in February 2009.

Moreton measured the waves propagating at a speed of 500–1500 km/s. Yutaka Uchida interpreted Moreton waves as MHD fast mode shock waves propagating in the corona. He links them to type II radio bursts, which are radio-wave discharges created when coronal mass ejections accelerate shocks.Moreton waves can be observed primarily in the Hα band.

Photosphere

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 being a spherical surface that is perceived to emit light. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately 2/3, or equivalently, a depth from which 50% of light will escape without being scattered.

In other words, a photosphere is the deepest region of a luminous object, usually a star, that is transparent to photons of certain wavelengths.

Pierre Janssen

Pierre Jules César Janssen (22 February 1824 – 23 December 1907), also known as Jules Janssen, was a French astronomer who, along with English scientist Joseph Norman Lockyer, is credited with discovering the gaseous nature of the solar chromosphere, and with some justification the element helium.

Plage (astronomy)

A plage is a bright region in the chromosphere of the Sun, typically found in regions of the chromosphere near sunspots.

The term itself is poetically taken from the French word for "beach". The plage regions map closely to the bright spots (faculae) in the photosphere below, but the latter have much smaller spatial scales. Accordingly, plage occurs most visibly near a sunspot region. Faculae have a strong influence on the

solar constant, and the more readily detectable (because chromospheric) plage areas traditionally are used to monitor this influence. In this context, "active network" consists of plage-like brightenings extending away from active regions as their magnetism appears to diffuse into the quiet Sun, but constrained to follow the network boundaries.

Because we can explain faculae with the strictly photospheric "hot wall" model, what the actual physical relationship between plage and faculae may be is not clear.

Psi1 Lupi

For other star systems with this Bayer designation, see ψ Lupi.Psi1 Lupi (ψ1 Lupi) is a red clump giant star located 205 light years away from the Sun, in the Lupus constellation. The star is surrounded by a cold circumstellar envelope, hinted at by the anomaly of the small observed power of the doublet Mg II emission at 2800 angstrom. The absorption cores on the peaks of the emission profiles Mg II k and h are mainly of interstellar origin and only partly due to self-absorption in the star's chromosphere.

Solar prominence

A prominence is a large, bright, gaseous feature extending outward from the Sun's surface, often in a loop shape. Prominences are anchored to the Sun's surface in the photosphere, and extend outwards into the Sun's corona. While the corona consists of extremely hot ionized gases, known as plasma, which do not emit much visible light, prominences contain much cooler plasma, similar in composition to that of the chromosphere. The prominence plasma is typically a hundred times more luminous and denser than the coronal plasma.

A prominence forms over timescales of about a day and may persist in the corona for several weeks or months, looping hundreds of thousands of miles into space. Some prominences break apart and may then give rise to coronal mass ejections. Scientists are currently researching how and why prominences are formed.

The red-glowing looped material is plasma, a hot gas composed of electrically charged hydrogen and helium. The prominence plasma flows along a tangled and twisted structure of magnetic fields generated by the Sun's internal dynamo. An erupting prominence occurs when such a structure becomes unstable and bursts outward, releasing the plasma.

A typical prominence extends over many thousands of kilometers; the largest on record was estimated at over 800,000 km (500,000 mi) long, roughly a solar radius.

Solar transition region

The solar transition region is a region of the Sun's atmosphere, between the chromosphere and corona.

It is visible from space using telescopes that can sense ultraviolet. It is important because it is the site of several unrelated but important transitions in the physics of the solar atmosphere:

Below, gravity tends to dominate the shape of most features, so that the Sun may often be described in terms of layers and horizontal features (like sunspots); above, dynamic forces dominate the shape of most features, so that the transition region itself is not a well-defined layer at a particular altitude.

Below, most of the helium is not fully ionized, so that it radiates energy very effectively; above, it becomes fully ionized. This has a profound effect on the equilibrium temperature (see below).

Below, the material is opaque to the particular colors associated with spectral lines, so that most spectral lines formed below the transition region are absorption lines in infrared, visible light, and near ultraviolet, while most lines formed at or above the transition region are emission lines in the far ultraviolet (FUV) and X-rays. This makes radiative transfer of energy within the transition region very complicated.

Below, gas pressure and fluid dynamics usually dominate the motion and shape of structures; above, magnetic forces dominate the motion and shape of structures, giving rise to different simplifications of magnetohydrodynamics. The transition region itself is not well studied in part because of the computational cost, uniqueness, and complexity of Navier–Stokes combined with electrodynamics.Helium ionization is important because it is a critical part of the formation of the corona: when solar material is cool enough that the helium within it is only partially ionized (i.e. retains one of its two electrons), the material cools by radiation very effectively via both black-body radiation and direct coupling to the helium Lyman continuum. This condition holds at the top of the chromosphere, where the equilibrium temperature is a few tens of thousands of kelvins.

Applying slightly more heat causes the helium to ionize fully, at which point it ceases to couple well to the Lyman continuum and does not radiate nearly as effectively. The temperature jumps up rapidly to nearly one million kelvin, the temperature of the solar corona. This phenomenon is called the temperature catastrophe and is a phase transition analogous to boiling water to make steam; in fact, solar physicists refer to the process as evaporation by analogy to the more familiar process with water. Likewise, if the amount of heat being applied to coronal material is slightly reduced, the material very rapidly cools down past the temperature catastrophe to around one hundred thousand kelvin, and is said to have condensed. The transition region consists of material at or around this temperature catastrophe.

The transition region is visible in far-ultraviolet (FUV) images from the TRACE spacecraft, as a faint nimbus above the dark (in FUV) surface of the Sun and the corona. The nimbus also surrounds FUV-dark features such as solar prominences, which consist of condensed material that is suspended at coronal altitudes by the magnetic field.

Spicule (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. They were discovered in 1877 by Father Angelo Secchi of the Observatory of Roman Collegium in Rome.

Stellar atmosphere

The stellar atmosphere is the outer region of the volume of a star, lying above the stellar core, radiation zone and convection zone.

Internal structure
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Heliosphere
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