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A stellar active region is a localized, transient volume of a stellar atmosphere in which plages, starspots, faculae, flares, etc., may be observed. Active regions are the result of enhanced magnetic fields; they are bipolar and may be complex if the region contains two or more bipolar groups.

This image from the TRACE satellite shows numerous flares from a stellar active region. Credit: NASA.

A stellar active region on a star's surface can form a bright spot which intensifies and grows. An active region may have a coronal portion.

Most stellar flares and coronal mass ejections originate in magnetically active regions around visible sunspot groupings. Similar phenomena indirectly observed on stars are commonly called starspots and both light and dark spots have been measured.[1]

Theory of stellar active regions

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"MOST current literature on solar activity assumes that the planets do not affect it, and that conditions internal to the Sun are primarily responsible for the solar cycle. Bigg1, however, has shown that the period of Mercury's orbit appears in the sunspot data, and that the influence of Mercury depends on the phases of Venus, Earth, and Jupiter."[2]

"The tidal forces hypothesis for solar cycles has been proposed by Wood (1972) and others. Table 2 below shows the relative tidal forces of the planets on the sun. Jupiter, Venus, Earth and Mercury are called the "tidal planets" because they are the most significant. According to Wood, the especially good alignments of J-V-E with the sun which occur about every 11 years are the cause of the sunspot cycle. He has shown that the sunspot cycle is synchronous with the alignments, and J. Schove's data for 1500 year of sunspot maxima supports the 11.07 year J-V-E period average."[3]

"Both the 11.86 year Jupiter tropical period (time between perihelion's or closest approaches to the sun and the 9.93 year J-S alignment periods are found in sunspot spectral analysis. Unfortunately direct calculations of the tidal forces of all planets does not support the occurrence of the dominant 11.07 year cycle. Instead, the 11.86 year period of Jupiter's perihelion dominates the results. This has caused problems for several researchers in this field."[3]

Variations in the electron and ion currents to and from the Sun may have a greater effect on active regions than any other.

Spicules

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Main resources: Stars/Sun/Spicules and Spicules
 
Spicules, visible as dark tubes occur in solar active region 10380, June 2004. Credit: SST, Royal Swedish Academy of Sciences, and LMSAL.
File:Whiskery plasma jets.jpg
Whiskery plasma jets, known as spicules, on the sun appear as dark, threadlike structures in this image, acquired at the Goode Solar Telescope in Big Bear, Calif. Credit: T. Samanta, GST & SDO.{{fairuse}}

A spicule is a dynamic jet of about 500 km diameter in the chromosphere of a star. It moves upwards at about 20 km/s from the photosphere. Spicules last for about 15 minutes;[4] at [a star's limb] they appear elongated (if seen on the disk, they are known as "mottles" or "fibrils"). They are usually associated with regions of high magnetic flux; their mass flux is about 100 times that of the [stellar wind]. They rise at a rate of 20 km/s (or 72,000 km/h) and can reach several thousand kilometers in height before collapsing and fading away.

"Tendrils of plasma near the surface of the sun emerge from realignments of magnetic fields and [apparently] pump heat into the corona, the sun’s tenuous outer atmosphere."[5]

"Spicules undulate like a wind-whipped field of wheat in the chromosphere, the layer of hot gas atop the sun’s surface. These plasma filaments stretch for thousands of kilometers and last for just minutes, shuttling ionized gas into the corona."[5]

Thickets "of spicules frequently emerged within minutes after pockets of the local magnetic field reversed course and pointed in the opposite direction from the prevailing field in the area."[5]

"Counterpointing magnetic fields create a tension that gets resolved when the fields break and realign".[5]

"The magnetic field energy is converted to kinetic and thermal energy. The kinetic energy is in the form of fast plasma motion — jets, or spicules."[6]

A "glow from charged iron atoms [occurred in images from NASA’s orbiting Solar Dynamics Observatory] directly over the spicules. That glow [...] means the plasma reached roughly 1 million degrees Celsius."[6]

Faculae

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Main resources: Stars/Sun/Faculae and Faculae

Def. a bright spot or patch between starspots is called a facula.

Bright spots also occur at the magnetic poles of magnetic stars.

"Faculae and flares arise in the chromosphere. Faculae are bright luminous hydrogen clouds which form above regions where [starspots] are about to form."[7]

Coronal loops

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Main resources: Plasmas/Plasma objects/Coronal loops and Coronal loops
This movie show the evolution of active region 1520, including coronal loops. Credit: NASA/Goddard Space Flight Center.

The population of coronal loops can be directly linked with the solar cycle; it is for this reason coronal loops are often found with sunspots at their footpoints. Coronal loops project through the chromosphere and transition region, extending high into the corona.

Coronal loops have a wide variety of temperatures along their lengths. Loops existing at temperatures below 1 MK are generally known as cool loops, those existing at around 1 MK are known as warm loops, and those beyond 1 MK are known as hot loops. Naturally, these different categories radiate at different wavelengths.[8]

Coronal loops populate both active and quiet regions of the solar surface. Active regions on the solar surface take up small areas but produce the majority of activity and 82% of the total coronal heating energy.[9] The quiet Sun, although less active than active regions, is awash with dynamic processes and transient events (bright points, nanoflares and jets).[10] As a general rule, the quiet Sun exists in regions of closed magnetic structures, and active regions are highly dynamic sources of explosive events. It is important to note that observations suggest the whole corona is massively populated by open and closed magnetic fieldlines. A closed fieldline does not constitute a coronal loop; however, closed flux must be filled with plasma before it can be called a coronal loop.

Nanoflares

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Main resources: Plasmas/Plasma objects/Nanoflares/Stars and Stellar nanoflares

In order to heat a region of very high X-ray emission, over an area 1" x 1", a nanoflare of 1017 J should happen every 20 seconds, and 1000 nanoflares per second should occur in a large active region of 105 x 105 km2.

Flickerings, brightenings, small explosions, bright points, flares and mass eruptions are observed very frequently, especially in active regions.

Flares

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Main resources: Plasmas/Plasma objects/Flares and Flares

Def. a violent explosion in a star's atmosphere is called a flare.

"A flare star is a variable star that can undergo unpredictable dramatic increases in brightness for a few minutes. The brightness increase is across the spectrum, from X rays to radio waves.

"Flare stars are intrinsically faint, but have been found to distances of 1,000 light years from Earth.[11]

BY Draconis variables

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Main resources: Stars/Variables/BY Draconis and BY Draconis variables

BY Draconis variables are main sequence variable stars of late spectral types, usually K or M. The name comes from the archetype for this category of variable star system, BY Draconis. They exhibit variations in their luminosity due to rotation of the star coupled with star spots, and other chromospheric activity.[12] Resultant brightness fluctuations are generally less than 0.5 magnitudes[12] on timescales equivalent to the star's rotational period, typically from a fraction of a day to several months. Oddly enough, Procyon the 8th brightest night-time star which is an F5 sub-giant or dwarf has also been classified as a BY Draconis variable.[13]

Some of these stars may exhibit flares, resulting in additional variations of the UV Ceti type.[14] Likewise, the spectra of BY Dra variables (particularly in their H and K lines) are similar to RS CVn stars, which are another class of variable stars that have active chromospheres.[15]

RS Canum Venaticorum variables

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Main resources: Stars/Variables/RS Canum Venaticorum and RS Canum Venaticorum variables

RS Canum Venaticorum variables are a type of variable star. They are close binary stars[16] having active chromospheres which can cause large stellar spots. These spots are believed to cause variations in their observed luminosity. Systems can exhibit variations on timescales of years due to variation in the spot surface coverage fraction, as well as periodic variations which are, in general, close to the orbital period of the binary system. Some systems exhibit variations in luminosity due to their being eclipsing binaries. Typical brightness fluctuation is around 0.2 magnitudes.

UV Ceti variable

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Main resources: Stars/Variables/UV Ceti and UV Ceti variables

The type star goes through fairly extreme changes of brightness: for instance, in 1952, its brightness increased by 75 times in only 20 seconds.

Plages

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Main resources: Plasmas/Plasma objects/Plages and Plages

A plage is a bright region in the chromosphere of [a star], typically found in regions of the chromosphere near [starspots]. The plage regions map closely to the faculae in the photosphere below, but the latter have much smaller spatial scales. Accordingly plage occurs most visibly near a starspot region.

"Plages are formed in the inner parts of flux loops emerging from below. ... In the early stages of active region growth the appearance of the group is symmetric, while a few days later the f spot may disappear, leaving an extensive plage."[17]

"[M]ajor changes in active regions only take place in the following ways:

  1. [starspot] formation and break up;
  2. flux outflow from [starspots];
  3. new flux emergence; and
  4. magnetic reconnection."[17]

"In general there is no proper motion at all in the plage or the surrounding plagettes except for the latter two."[17]

Coronal streamers

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Main resources: Plasmas/Plasma objects/Coronal streamers and Coronal streamers

The interconnections of active regions are arcs connecting zones of opposite magnetic field, in different active regions. Significant variations of these structures are often seen after a flare. Some other features of this kind are helmet streamers—large cap-like coronal structures with long pointed peaks that usually overlie sunspots and active regions. Coronal streamers are considered as sources of the slow solar wind.[18]

Prominences

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Main resources: Plasmas/Plasma objects/Prominences and Prominences
 
A major eruptive prominence is imaged by Skylab in 1973. Credit: Skylab, NASA.
 
This shows a detached Solar prominence. Credit: Brocken Inaglory.

A prominence is a large, bright feature extending outward from [a star's] surface, often in a loop shape. Prominences are anchored to [a star's] surface in the photosphere, and extend outwards into the [star's] corona. While the corona consists of extremely hot ionized gases, known as plasma, which [does] not emit much visible light, prominences contain much cooler plasma, similar in composition to that of the chromosphere. A prominence forms over timescales of about a day, and stable prominences may persist in the corona for several months. Some prominences break apart and give rise to coronal mass ejections.

A typical prominence extends over many thousands of kilometers; the largest on record was estimated at over 800,000 kilometres (500,000 mi) long[19] – roughly the radius of the Sun.

"When a prominence is viewed from a different perspective so that it is against the [star] instead of against space, it appears darker than the surrounding background. This formation is instead called a [stellar] filament.[19] It is possible for a projection to be both a filament and a prominence. Some prominences are so powerful that they throw out matter from the [star] into space at speeds ranging from 600 km/s to more than 1000 km/s. Other prominences form huge loops or arching columns of glowing gases over [starspots] that can reach heights of hundreds of thousands of kilometres. Prominences may last for a few days or even for a few months.[20] Flocculi (plural of flocculus) is another term for these filaments, and dark flocculi typically describes the appearance of [stellar] prominences when viewed against the [stellar] disk in certain wavelengths.

Starspots

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Main resources: Stars/Starspots and Starspots

Starspots are equivalent to sunspots but located on other stars. Spots the size of sunspots are very hard to detect since they are too small to cause fluctuations in brightness. Observed starspots are in general much larger than those on the Sun, up to about 30 % of the stellar surface may be covered, corresponding to sizes 100 times greater than those on the Sun.

In 1947, G. E. Kron proposed that starspots were the reason for periodic changes in brightness on red dwarfs.[1] Since the mid-1990s, starspot observations have been made using increasingly powerful techniques yielding more and more detail: photometry showed starspot growth and decay and showed cyclic behavior similar to the Sun's; spectroscopy examined the structure of starspot regions by analyzing variations in spectral line splitting due to the Zeeman Effect; Doppler imaging showed differential rotation of spots for several stars and distributions different from the Sun's; spectral line analysis measured the temperature range of spots and the stellar surfaces. The largest cool starspot ever seen rotating the giant K0 star XX Triangulum (HD 12545) with a temperature of 3,500 K (3,230 °C), together with a warm spot of 4,800 K (4,530 °C).[1][21]

Stars with sizable sunspots may show significant variations in brightness as they rotate, and brighter areas of the surface are brought into view. Bright spots also occur at the magnetic poles of magnetic stars. The surface of the star is not uniformly bright, but has darker and brighter areas (like the sun's solar spots). The star's chromosphere too may vary in brightness. As the star rotates we observe brightness variations of a few tenths of magnitudes.

Observed starspots have a temperature which is in general 500–2000 Kelvin cooler than the stellar photosphere. This temperature difference could give rise to a brightness variation up to 0.6 magnitudes between the spot and the surrounding surface. There also seems to be a relation between the spot temperature and the temperature for the stellar photosphere, indicating that starspots behave similarly for different types of stars (observed in G-K dwarfs).

The lifetime for a starspot depends on its size.

  • For small spots the lifetime is proportional to their size, similar to spots on the Sun.[22]
  • For large spots the sizes depend on the differential rotation of the star, but there are some indications that large spots which give rise to light variations can survive for many years even in stars with differential rotation.[22]

The distribution of starspots across the stellar surface varies analogous to the solar case, but differs for different types of stars, e.g., depending on whether the star is a binary or not. The same type of activity cycles that are found for the Sun can be seen for other stars, corresponding to the solar (2 times) 11-year cycle. Some stars have longer cycles, possibly analogous to the Maunder minima for the Sun.

Another activity cycle is the so called flip-flop cycle, which implies that the activity on either hemisphere shifts from one side to the other. The same phenomena can be seen on the Sun, with periods of 3.8 and 3.65 years for the northern and southern hemispheres.

Flip-flop phenomena are observed for both binary RS CVn stars and single stars although the extent of the cycles are different between binary and singular stars.

Lithiums

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Main resources: Chemicals/Lithiums and Lithiums

A number of lithium emission lines is observed in sunspot umbra. These are "the lithium (I) line doublets from Li(6) and Li(7) at 670.8 nm".[23]

"[A] lithium abundance [is] εLi = 1.02 ± 0.121 ... Some evidence for the existence of a small but notable amount of Li6 is found."[23]

Berylliums

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Main resources: Chemicals/Berylliums and Berylliums

"The isotopes 7Be, with a half-life of 53 days, and 10Be are both cosmogenic nuclides because they are made on a recent timescale in the solar system by spallation, like 14C. These two radioisotopes of beryllium in the atmosphere track the sun spot cycle and solar activity, since this affects the magnetic field that shields the Earth from cosmic rays. The rate at which the short-lived 7Be is transferred from the air to the ground is controlled in part by the weather. 7Be decay in the sun is one of the sources of solar neutrinos, and the first type ever detected using the Homestake experiment.

Calciums

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Main resources: Chemicals/Calciums and Calciums

During the limb flares of December 18, 1956, a coronal line at 569.4 nm, a yellow line, occurred at 1822 UTC, 1900 UTC, undiminished up to 20,000 km above the solar limb, and at 2226 UTC, is identified as Ca XV.[24]

Coronal mass ejections

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Main resources: Plasmas/Plasma objects/Coronal mass ejections and Coronal mass ejections

Most coronal mass ejections originate from active regions on a star's surface. Near a stellar maxima a star such as the Sun produces about three CMEs every day, whereas near stellar minima there is about one CME every five days.[25]

"The magnetic field carried away by the coronal mass ejections (CMEs) is twisted. ... [Helicity] is defined as

 

... Helicity [(H)] is a quantitative measure of the chiral properties of the structures observed in the solar atmosphere."[26]

Evolution

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Main resources: Stars/Sun/Sunspots/Evolution and Sunspot evolution

"The region (McMath 12510) arose on the back side of the Sun and first was visible on August 30, 1973, displaying a normal amount of plage. ... During its second transit the spot (now Mt. Wilson 12510, McMath 12542) was the largest on the disk but already naked ... The spot returned a third time (Mt. Wilson 19281, McMath 12585), greatly reduced in size."[27]

Solar spicules

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Main resources: Stars/Sun/Spicules and Solar spicules

There are about 300,000 active spicules at any one time on the Sun's chromosphere, amounting to about 1% of the Sun's surface.[4] At any one time there are around 60,000 to 70,000 active spicules on the Sun; an individual spicule typically reaches 3,000-10,000 km altitude above the photosphere.[28]

Solar microflares

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Main resources: Plasmas/Plasma objects/Microflares/Sun and Solar microflares

Ultraviolet telescopes such as TRACE and SOHO/EIT can observe individual [solar] micro-flares as small brightenings in extreme ultraviolet light.[29]

Coronal arcades

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Main resources: Plasmas/Plasma objects/Coronal arcades and Coronal arcades
 
This is a TRACE image of the coronal arcade structure in the flare on Bastille Day, 1998. Credit: NASA.

Def. a close collection of loops in a cylindrical structure is called an arcade.

The TRACE image at right "is from near flare maximum (11:00 UT) and has a width of 230,000 km [...] how in the world can the footpoints of the arcade have such a clearly-organized pattern whose scale greatly exceeds the known scales of the largest convective scales known in the photosphere?"[30]

"The most obvious coronal signatures of CMEs in the low corona are the arcades of bright loops that develop after the CME material has erupted [...] nearly all (92%) EIT post-eruptive arcades from 1997 – 2002 were associated with LASCO CMEs [...] The activity associated with halo CMEs includes the formation of dimming regions, long-lived loop arcades, flaring active regions, large-scale coronal waves and filament eruptions".[31]

Helmet streamers

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Main resources: Plasmas/Plasma objects/Helmet streamers and Helmet streamers
 
An abundance of helmet streamers is shown at solar maximum. Credit: NASA.
 
Helmet streamers are shown at solar minimum restricted to mid latitudes. Credit: NASA.

Helmet streamers are bright loop-like structures which develop over active regions on the sun. They are closed magnetic loops which connect regions of opposite magnetic polarity. Electrons are captured in these loops, and cause them to glow very brightly. The solar wind elongates these loops to pointy tips. They far extend above most prominences into the corona, and can be readily observed during a solar eclipse. Helmet streamers are usually confined to the "streamer belt" in the mid latitudes, and their distribution follows the movement of active regions during the solar cycle. Small blobs of plasma, or "plasmoids" are sometimes released from the tips of helmet streamers, and this is one source of the slow component of the solar wind. In contrast, formations with open magnetic field lines are called coronal holes, and these are darker and are a source of the fast solar wind. Helmet streamers can also create coronal mass ejections if a large volume of plasma becomes disconnected near the tip of the streamer.

Solar flares

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Main resources: Plasmas/Plasma objects/Flares/Sun and Solar flares
 
"This graph shows the neutrons detected by a neutron detector at the University of Oulu in Finland from May 16 through May 18, 2012. The peak on May 17 represents an increase in the number of neutrons detected, a phenomenon dubbed a ground level enhancement or GLE. This was the first GLE since December of 2006. Credit: University of Oulu/NASA's Integrated Space Weather Analysis System"[25].
 
RHESSI observes high-energy phenomena from a solar flare. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio.

Balfour Stewart recorded a super flare on the evening of 28 August 1859 and the morning of 2 September 1859, at the Kew Observatory, and presented his findings in a paper presented to the Royal Society on 21 November 1861.[32][33] He noted that while "magnetic disturbances of unusual violence and very wide extent" were recorded in various places around the world, the Kew Observatory had the benefit of self-recording magnetographs,[34] which allowed "the means of obtaining a continuous photographic register of the state of the three elements of the earth’s magnetic force—namely, the declination, and the horizontal and vertical intensity."

During the limb flares of December 18, 1956, a coronal line at 569.4 nm, a yellow line, occurred at 1822 UTC, 1900 UTC, undiminished up to 20,000 km above the solar limb, and at 2226 UTC, is identified as Ca XV.[24] "The coronal temperature was 4000000°."[24] "The December 18, 1956, flare appears to have been a violent condensation of material from a dense coronal cloud above an active region."[24]

"The first true astrophysical gamma-ray sources were solar flares, which revealed the strong 2.223 MeV line predicted by Morrison. This line results from the formation of deuterium via the union of a neutron and proton; in a solar flare the neutrons appear as secondaries from interactions of high-energy ions accelerated in the flare process. These first gamma-ray line observations were from OSO-3 [and] OSO-7

Fairly large fluxes of neutrons have been observed during solar flares such as that of November 12, 1960, with a flux of 30-70 neutrons per cm-2 s-1.[35]

The Bastille Day Flare or Bastille Day Event was a powerful solar flare on July 14, 2000, occurring near the peak of the solar maximum in solar cycle 23.[36][37] NOAA Active region 9077 produced an X5.7-class flare, which caused an S3 radiation storm on Earth fifteen minutes later as energetic protons bombarded the ionosphere.[36][38] It was the biggest solar radiation event since 1989.[38] The proton event was four times more intense than any previously recorded since the launches of SOHO in 1995 and ACE in 1997.[36] The flare was followed by a full-halo coronal mass ejection[36] and a geomagnetic super storm on July 15-16. The extreme level, G5, was peaked in late hours of July 15.

"The Bastille Day event was observed by Voyager I and Voyager II,[39] thus it is the farthest out observed solar storm."

"On May 17, 2012 an M-class flare exploded from the sun. The eruption also shot out a burst of solar particles traveling at nearly the speed of light that reached Earth about 20 minutes after the light from the flare. An M-class flare is considered a "moderate" flare, at least ten times less powerful than the largest X-class flares, but the particles sent out on May 17 were so fast and energetic that when they collided with atoms in Earth's atmosphere, they caused a shower of particles to cascade down toward Earth's surface. The shower created what's called a ground level enhancement (GLE)."[40]

"[O]n Saturday, May 5, ... a large sunspot rotated into view on the left side of the sun. ... [J]ust before [Active Region 1476] disappeared over the right side of the sun, it ... erupted with an M-class flare."[40]

The solar flare at Active Region 10039 on July 23, 2002, exhibits many exceptional high-energy phenomena including the 2.223 MeV neutron capture line and the 511 keV electron-positron (antimatter) annihilation line. In the image at right, the RHESSI low-energy channels (12-25 keV) are represented in red and appear predominantly in coronal loops. The high-energy flux appears as blue at the footpoints of the coronal loops. Violet is used to indicate the location and relative intensity of the 2.2 MeV emission.

During solar flares “[s]everal radioactive nuclei that emit positrons are also produced; [which] slow down and annihilate in flight with the emission of two 511 keV photons or form positronium with the emission of either a three gamma continuum (each photon < 511 keV) or two 511 keV photons."[41] The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) made the first high-resolution observation of the solar positron-electron annihilation line during the July 23, 2003 solar flare.[41] The observations are somewhat consistent with electron-positron annihilation in a quiet solar atmosphere via positronium as well as during flares.[41] Line-broadening is due to "the velocity of the positronium."[41] "The width of the annihilation line is also consistent ... with thermal broadening (Gaussian width of 8.1 ± 1.1 keV) in a plasma at 4-7 x 105 K. ... The RHESSI and all but two of the SMM measurements are consistent with densities ≤ 1012 H cm-3 [but] <10% of the p and α interactions producing positrons occur at these low densities. ... positrons produced by 3He interactions form higher in the solar atmosphere ... all observations are consistent with densities > 1012 H cm-3. But such densities require formation of a substantial mass of atmosphere at transition region temperatures."[41]

"[N]eutrino flux increases noted in Homestake results [coincide] with major solar flares [14]."[42]

"The correlation between a great solar flare and Homestake neutrino enhancement was tested in 1991. Six major flares occurred from May 25 to June 15 including the great June 4 flare associated with a coronal mass ejection and production of the strongest interplanetary shock wave ever recorded (later detected from spacecraft at 34, 35, 48, and 53 AU) [15]. It also caused the largest and most persistent (several months) signal ever detected by terrestrial cosmic ray neutron monitors in 30 years of operation [16]. The Homestake exposure (June 1–7) measured a mean 37Ar production rate of 3.2 ± 1.5 atoms/day (≈19 37Ar atoms produced in 6 days) [13]; about 5 times the rate of ≈ 0.65 day −1 for the preceding and following runs, > 6 times the long term mean of ≈ 0.5 day−1 and > 2 1/2 times the highest rates recorded in ∼ 25 operating years."[42]

Solar Moreton waves

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Main resources: Stars/Chromospheres/Tsunamis and Chromospheric tsunamis
File:MoretonWaveAnimation200612.gif
This is an animation of a Moreton wave which occurred on the Sun at December 6, 2006. Credit: National Solar Observatory (NSO)/AURA/NSF and USAF Research Laboratory.
 
This image shows a solar tsunami on May 19, 2007. Credit: NASA/STEREO/EUVI consortium.

A Moreton wave is the chromospheric signature of a large-scale solar coronal shock wave. Described as a kind of solar 'tsunami',[43] they are generated by solar flares[44][45][46].

The 1995 launch of the Solar and Heliospheric Observatory led to observation of coronal waves, which cause Moreton waves. (SOHO's EIT instrument discovered another, different wave type called 'EIT waves'.)[47] The reality of Moreton waves (aka 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/second, in conjunction with a big coronal mass ejection in February 2009.[48][49]

Moreton waves propagate at a speed of usually 500–1500 km/s. Yutaka Uchida interpreted Moreton waves as MHD fast mode shock waves propagating in the corona.[50] He links them to type II radio bursts, which are radio wave discharges created when coronal mass ejections accelerate shocks.[51]

Moreton waves can be observed primarily in the Hα band.[52]

Sunspots

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Main resources: Stars/Sun/Sunspots and Sunspots
File:Sunspot vtt1.jpg
A sunspot is a depression on the Sun's face that is slightly cooler and less luminous than the rest of the Sun. Credit: Vacuum Tower Telescope, NSO, NOAO.
 
A planet-sized sunspot showing for the first time dark cores of the filaments extending into the sunspot. These filaments are thousands of km long by about 100 km wide. Recorded on July 15, 2002, using the Swedish Solar Telescope (SST). Solar active region AR 10030. Credit: SST, Royal Swedish Academy of Sciences.

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. Like magnets, they also have two poles. Although they are at temperatures of roughly 3,000–4,500 K (2,727–4,227 °C), the contrast with the surrounding material at about 5,780 K leaves them clearly visible as dark spots, as the luminous intensity of a heated black body (closely approximated by the photosphere) is a function of temperature to the fourth power. If the sunspot were isolated from the surrounding photosphere it would be brighter than an electric arc. Sunspots expand and contract as they move across the surface of the Sun and can be as large as 80,000 kilometers (49,710 mi) in diameter, making the larger ones visible from Earth without the aid of a telescope.[53] They may also travel at relative speeds ("proper motions") of a few hundred m/s when they first emerge onto the solar photosphere.

Manifesting intense magnetic activity, sunspots host secondary phenomena such as coronal loops (prominences) and reconnection events. Most solar flares and coronal mass ejections originate in magnetically active regions around visible sunspot groupings. Similar phenomena indirectly observed on stars are commonly called starspots and both light and dark spots have been measured.[1]

Solar active region AR 10030 contained a group of sunspots including the largest one partially included in the image at the right. It is a planet-sized sunspot showing for the first time the dark cores of the filaments extending into the sunspot. These filaments are thousands of km long by about 100 km wide. The image is recorded on July 15, 2002, using the Swedish Solar Telescope (SST).

Solar CMEs

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Main resources: Plasmas/Plasma objects/Coronal mass ejections/Sun and Solar coronal mass ejections
File:LASCO20011001.gif
A coronal mass ejection in time-lapse imagery is obtained with the LASCO instrument. The Sun (center) is obscured by the coronagraph's mask. (September 30 – October 1, 2001). Credit: SOHO (ESA & NASA).

An explosive limb flare occurred above 30,000 km in the corona of the Sun.[54] "So the aftermath of the flare explosion, usually visible in disk pictures as extensive Hα brightening, but hidden from us in this case, was seen by the ionosphere as an intense flux of ionizing radiation from the coronal cloud created by the explosion."[54] "The November 20, 1960, event is very similar to that of February 10, 1956, which was observed at Sacramento Peak. A bright ball appears above the surface, grows in size and Hα brightness, and explodes upward and outward."[54] "The great breadth and intensity of the Hα emission from the suspended ball at 2013 U.T. testify to the large amount of energy stored there, as no corresponding macroscopic motion was observed until the explosion at 2023 U.T."[54] "[T]he great energy of the preflare cloud was released into the corona by the explosion of 2023 U.T., and Hα radiation disappeared by 2035 U.T."[54]

"On 16 June 1972, the Naval Research Laboratory's coronagraph aboard OSO-7 tracked a huge coronal cloud moving outward from the Sun."[55]

"Many CMEs have also been observed to be unassociated with any obvious solar surface activity ... The frequency of occurrence of CMEs observed in white light tends to follow the solar cycle in both phase and amplitude, which varies by an order of magnitude over the cycle ... The latitude distribution of the central position angles of CMEs tends to cluster about the equator around solar minimum but broadens over all latitudes near solar maximum. ... Many CMEs viewed at the solar limb also appear to arise from large-scale, pre-existing coronal streamers which often overlie active regions ... Many energetic CMEs actually involve the disruption (“blowout”) of such a structure, which can increase in brightness and size for days before erupting as a CME ... A streamer is a bright (dense) structure containing closed and open fields, which help guide denser, outward-flowing solar wind material. ... The absence of solar surface activity with observed CME activity is not a new observation ... the CME originated high enough up in the corona such that no surface signatures were evident. ... about a third of the CMEs were “stealth”, having no distinct surface association, and tending to be slow, i.e., < 300 km s–1. Faint coronal changes could be detected in about half of the stealth CMEs ... Based on the highest mass (1014 kg) and speed (∼ 3500 km s–1) observed one can estimate a maximum kinetic energy of ∼ 6 x 1034 erg. Assuming that only a fraction of the stored energy is released in a single episode and that the CME derives all of its energy from a single active region, we can set a limit of ∼ 1036 erg for the maximum free energy available in a solar active region. This is consistent with the size and magnetic field strengths in solar active regions"[31].

Solar ejections

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Main resources: Plasmas/Plasma objects/Coronal ejections/Sun and Solar coronal ejections

Magnetic clouds represent about one third of ejecta observed by satellites at Earth. Other types of ejecta are multiple-magnetic cloud events (a single structure with multiple subclouds distinguishable)[56][57] and complex ejecta, which can be the result of the interaction of multiple CMEs.

Active region designations

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Main resources: Stars/Sun/Active regions/Designations and Solar active region designations

"75% of the naked sunspots represented the return of large dominant p spots which had been part of large active regions during previous rotations."[27]

Greenwich numbering

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Designations for specific sunspot groups.[58]
Date Kodaikanal number Greenwich group number
1/29/68-2/2/68 13105 21482
2/20-26/69 13483 21894
3/21/69 13510 21936
8/2-3/69 13621 22064
8/2/69 13625 22068
9/26/69 13640 22086
10/8-10/69 13683 22138
11/2/69 13696 22152
11/24/69 13713 22176
12/26/69 13743 22210
1/17/70 13776 22247?
1/25-30/70 13778 22251
1/26/70 13783 22255
1/29/70 13784 22261
2/9-12/70 13791 22272
2/7/70 13792 22274
2/21-25/70 13811 22291
4/9-13/70 13859 22349?
4/24-25/70 13870 22362
4/25/70 13875 22370?
5/7-8/70 13881 22379
5/15-16/70 13891 22392
5/30/70 13901 22411
6/13-16/70 13916 22433
6/27-30/70 13932 22448
6/30/70-7/1/70 13937 22454
8/6/70 13973 22495
8/24/70 13980 22508
9/27-29/70 14021 22556
11/13-14/70 14064 22608
12/1/70 14108 22664
1/21-5/71 14120 22679
1/31/71-2/3/71 14128 22686
2/16/71 14144 22710
3/21/71 14175 22738
4/11/71 14184 22755

In a 1904 article, Maunder was to describe the storm as a "very intense and long-continued disturbance", which in total, lasted between November 11 and 26. He pointed out that this synchronised "with the entire passage across the visible disc" of sunspot group 885 (Greenwich numbering).[59] This group originally had formed on the disc on October 20, passed off at the west limb on October 28, passed again east-west between November 12–25, and returned at the east limb on December 10, before finally disappearing on the disc on December 20.[60]

Mt. Wilson designations

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Notation: let the symbol CMP stand for central meridian passage.

"The centre of activity which is under study contains a widely-separated bipolar sunspot group, Mt. WILSON region #13333, and associated plage region, MCMATH HULBERT #4622. The strength of the magnetic field is 2600 gauss."[61]

Mt. Wilson spot numbers usually correspond with Hale, McMath, or Big Bear region numbers.

Designations for specific naked sunspots.[27]
Month/CMP/Year Hale, McMath or Big Bear number Mt. Wilson number
9/3.8/73 12543 19270
2/25.1/79 15838 20464
4/23.3/79 15955 20562
7/27.6/79 16167 20765
8/21.2/79 16232 20817
9/22.3/79 16300 20898
12/8.9/79 16479 21105
3/28.2/80 16733 21337
4/4.1/80 16744 21350
5/31.7/80 16868 21479
6/3.3/80 16873 21483
6/4.2/80 16875 21488
7/11.9/80 16970 21577
8/11.4/80 17040 21640
9/26.3/80 17163 21764
9/29.5/80 17169 21770
10/8.0/80 17179 21793
10/15.1/80 17199 21819
3/5.6/81 17494 22102
5/4.1/81 17619 22246
5/14.2/81 17641 22272
7/22.1/81 17750 22406
1/8.9/82 18123 22906

Hale designations

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"The spotless flare occurred in Hale plage region No. 15521 during its first rotation. The region survived four rotations and was most flare productive in the second rotation (Hale No. 15570) when it produced 58 subflares, 3 importance 1 flares, and one importance 2 flare which was also associated with a metric type IV event."[62]

"HELlOS-1 was in an ideal position to study the solar events on June 7, 1980, which occurred in Hale plage region 16886."[63]

"Hale regions 16918 and the northern portions of 16923 ... [comprise] NOAA active regions 2516, 2517, and 2519 ... whose disk passage occurred during rotation 1696 in the latter half of 1980 June. ... [T]his area [is] simply [designated] as 16918. ... Region 16918 was the decaying remnant of NOAA regions 2466, 2469, and 2470 crossing the disk the previous rotation, 1695. During this earlier disk passage the combined sunspot area reached a maximum of 2600 millionths, and the region was even more subdued in rotation 1697."[64]

"Hale region 17244 (NOAA region 2776) ... produced several m and x class flares 1980 November 5-7, while the region was near central meridian. This region, located at 10° north latitude, reached maximum sunspot area of 1440 millionths on November 4 and maximum sunspot number of 95 3 days later. During both the subsequent and previous disk passages, the region was much subdued."[65]

McMath designations

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"[T]he McMath region Number 8461 ... passed over the solar disk during the 1966 Proton Flare Project period, from August 21 to September 4, and produced two important solar particle events on August 28 and September 2."[66]

"A time sequence of magnetograms and velocity-grams in the Hα and Fe I 6569 Å lines has been made at a rate of 12 h-1 of McMath Region 10385 from 26 to 29 October, 1969."[67]

"Region C corresponds to McMath region 10607, where a flare occurred approximately 7 hr before the X-ray photograph was taken."[68]

"Region B corresponds to McMath regions 10618 I and 10618 II, in which no flare occurred on March 7, 1970."[68]

"McMath region 12379 [corresponds to] NOAA 131 [on] June 15, 1973 ... [or] AR 131 in the NOAA nomenclature ... NOAA region 131".[68]

McMath 12417 is observed on July 4, 1973.[17]

"Region A corresponds to McMath region 19622, a small region which, in its short life (approximately three days), developed no flare activity and shows the typical X-ray structures which have been found to overlay relatively simple bipolar magnetic configurations."[68]

"[T]he 1972 August flares [occurred] in the active region McMath 11976."[69]

Boulder designations

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"During a period in early June 1980 a series of particle events was produced by Boulder Region 2495 which was situated between W67 and the west limb."[70]

"[A] sequence of very high quality filtergrams of Boulder Region No. 2490 (heliographic coordinates 15°S, 36°E) photographed ... on 1980 June 6."[71]

NOAA designations

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This sequence of still photos shows solar active region NOAA 875. Credit: Bruno Sánchez-Andrade Nuño, Klaus Gerhard Puschmann, Franz Kneer, Julián Blanco Rodríguez & Nazaret Bello González.
This sequence of stills is of active region NOAA 875 in hydrogen alpha. Credit: Bruno Sánchez-Andrade Nuño, Klaus Gerhard Puschmann, Franz Kneer, Julián Blanco Rodríguez & Nazaret Bello González.

The National Oceanic and Atmospheric Administration of the USA keeps track of stellar active regions on the Sun. Each new region that appears for the first time on the surface of the Sun receives a sequentially assigned number.

Solar Active Region NOAA 2372 was observed extensively by the Solar Maximum Mission (SMM) satellite and several ground-based observatories during 1980 April 4–13 in the Solar Maximum Year. After its birth around April 4, it underwent a rapid growth and produced a reported 84 flares in the course of its disc passage. NOAA 2372 first appeared as an Η-alpha plage (Hale Plage Region No. 16747) on April 4 in the NE-quadrant of the solar disc near an old region, Hale No. 16752".[69]

"NOAA Active Region 9165 [September 14-17, 2000]

  1. ... is the site of both new flux emergence and intense horizontal shearing flows;
  2. ... shows rapid development and rapid decay, and for a few days ... is the site of violent activity;
  3. the horizontal motions occur when it is close to disk center ...; and
  4. observations of a magnetic cloud associated with one of the CMEs linked to the active region are available."[26]

Proxima Centauri

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Main resources: Stars/Proxima Centauri and Proxima Centauri

Although it has a very low average luminosity, Proxima is a flare star that undergoes random dramatic increases in brightness because of magnetic activity.[72]

The resulting flare activity generates a total X-ray emission similar to that produced by the Sun.[73]>

In 1951, American astronomer Harlow Shapley announced that Proxima Centauri is a flare star. Examination of past photographic records showed that the star displayed a measurable increase in magnitude on about 8% of the images, making it the most active flare star then known.[74] [75] The proximity of the star allows for detailed observation of its flare activity. In 1980, the Einstein Observatory produced a detailed X-ray energy curve of a stellar flare on Proxima Centauri. Further observations of flare activity were made with the EXOSAT and ROSAT satellites, and the X-ray emissions of smaller, solar-like flares were observed by the Japanese ASCA satellite in 1995.[76] Proxima Centauri has since been the subject of study by most X-ray observatories, including XMM-Newton and Chandra.[77]

These flares can grow as large as the star and reach temperatures measured as high as 27 million K[77]—hot enough to radiate X-rays.[78] Indeed, the quiescent X-ray luminosity of this star, approximately (4–16) x 1026 erg/s ((4–16) x 1019 W), is roughly equal to that of the much larger Sun. The peak X-ray luminosity of the largest flares can reach 1028 erg/s (1021 W.)[77]

The chromosphere of this star is active, and its spectrum displays a strong emission line of singly ionized magnesium at a wavelength of 280 nm.[79] About 88% of the surface of Proxima Centauri may be active, a percentage that is much higher than that of the Sun even at the peak of the solar cycle. Even during quiescent periods with few or no flares, this activity increases the corona temperature of Proxima Centauri to 3.5 million K, compared to the 2 million K of the Sun's corona.[80] However, the overall activity level of this star is considered low compared to other M-class dwarfs.[73]

"[S]imultaneous Einstein and IUE observations of the dM5e flare star Proxima Centauri ... On 1979 March 6 ... [and] ... Again on 1980 August 20 [captured] X-ray [flares] on Proxima [Centauri] ... The X-ray characteristics of this flare event are strongly suggestive of a solar analog: the two-ribbon flare. ... [This] less common flare class, the long decay X-ray events, are characterized by large and diffuse systems of X-ray loops and are associated with prominence eruptions generally away from active regions."[81]

Alpha Centauri B

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Main resources: Stars/Alpha Centauri B and Alpha Centauri B

"The light curve of B varies on a short time scale and there has been at least one observed flare.[82]

Barnard's star

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Main resources: Stars/Barnard's and Barnard's star

In 1998, astronomers observed an intense stellar flare, surprisingly showing that Barnard's Star is a flare star.[83]

A "cold super-Earth, three times the size of our planet, [has been detected] orbiting [Barnard's] star every 233 days."[84]

"Measurements from high-precision instruments, including the High-Resolution Echelle Spectrometer (HIRES) at W. M. Keck Observatory unearthed the planet – after 16 years of observations."[84]

"Barnard’s star is among the nearby red dwarfs that represents an ideal target to search for exoplanets that could someday actually be reached by future interstellar spacecraft."[85]

"We knew we would have to be patient. We followed Barnard’s star for 16 long years at Keck, amassing some 260 radial velocities of Barnard’s star by 2013."[85]

"Fortunately, our long-running Keck planet search program gave us the years we needed to gather enough precision radial velocity data with HIRES to begin to sense the presence of a planet."[85]

"It is the most common type of star in the galaxy — over 70 percent of Milky Way stars are like this dim, M dwarf star. Though it is extremely close, Barnard’s star is too faint to be seen with the naked eye."[85]

Wolf 359

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Main resources: Stars/Wolf 359 and Wolf 359

Wolf 359 is a flare star that can undergo sudden increases in luminosity for several minutes. These flares emit strong bursts of X-ray and gamma ray radiation that have been observed by space telescopes.

In 1969, a brief flare in the luminosity of Wolf 359 was observed, linking it to the class of variable stars known as flare stars.[86]

In 2001, Wolf 359 became the first star other than the Sun to have the spectrum of its corona observed from a ground-based telescope. The spectrum showed emission lines of Fe XIII, which is heavily ionized iron that has been stripped of twelve of its electrons.[87] The strength of this line can vary over a time period of several hours, which may be evidence of microflare heating.[88]

Wolf 359 has a relatively high flare rate. Observations with the Hubble Space Telescope detected 32 flare events within a two hour period, with energies of 1027 ergs (1020 joules) and higher.[89]

During flare activity, Wolf 359 has been observed emitting X-rays and gamma rays.[90][91]

Van Biesbroeck's star

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Main resources: Stars/Van Biesbroeck's and Van Biesbroeck's star

VB 10 is a variable star and is identified in the General Catalogue of Variable Stars as V1298 Aquilae. It is classified as a UV Ceti type variable star and is known to be subject to frequent flare events.[92] Its dynamics were studied from the Hubble Space Telescope in the mid-1990s and although the star has a normally cool surface temperature of 2600 K it was found to produce violent flares of up to 100,000 K.[93]

AD Leonis

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Main resources: Stars/AD Leonis and AD Leonis

AD Leonis (Gliese 388) is a red dwarf star. It is located relatively near the Sun, at a distance of about 16 light years, in the constellation Leo. AD Leonis is a main sequence star with a spectral classification of M3.5V.[94] It is a flare star that undergoes random increases in luminosity.[95]

AD Leonis is one of the most active flare stars known, and the emissions from the flares have been detected across the electromagnetic spectrum as high as the X-ray band.[96][97]

Besides star spots, about 73% of the surface is covered by magnetically active regions.[98] Examination of the corona in X-ray shows compact loop structures that span up to 30% of the size of the star.[99]

GJ 1214

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Main resources: Stars/GJ 1214 and GJ 1214

GJ 1214 is a dim M4.5[100] red dwarf in the constellation Ophiuchus with an apparent magnitude of 14.7.[101] It is located at a distance of approximately 40 light years from Earth. It is about one-fifth as large as the Sun[102] with a surface temperature estimated to be 3,000 K (2,730 °C; 4,940 °F).[102] Its luminosity is only 0.003% that of the Sun.[102] The estimate for the stellar radius is 15% larger than predicted by theoretical models.[103] It also shows a 1% intrinsic variability in the near-infrared probably caused by stellar spots.[101]

BY Draconis variables

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Main resources: Stars/Variables/BY Draconis and BY Draconis variables

BY Draconis stars are of spectral class K or M and vary by less than 0.5 magnitudes (70% change in luminosity).

FK Comae Berenices variables

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Main resources: Stars/Variables/FK Comae Berenices and FK Comae Berenices variables

These stars rotate extremely fast (~100 km/s at the equator); hence they are ellipsoidal in shape. They are (apparently) single giant stars with spectral types G and K and show strong chromospheric emission lines. Examples are FK Com, HD 199178 and UZ Lib.

T Tauri variables

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Main resources: Stars/Variables/T Tauri and T Tauri variables

Variability of T Tauri stars is due to spots on the stellar surface and gas-dust clumps, orbiting in the circumstellar disks.

Big Bear Solar Observatory

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The old dome on the main BBSO building is viewed from Big Bear Lake. Credit: .

The Big Bear Solar Observatory (BBSO) is a solar observatory located on the north side of Big Bear Lake in the San Bernardino Mountains of southwestern San Bernardino County, California (USA), approximately 120 kilometers (75 mi) east of downtown Los Angeles.

Kodaikanal Solar Observatory

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This image shows the Kodaikanal Solar Observatory. Credit: Marcus334.
 
Solar Tunnel Telescope is imaged at Kodaikanal. Credit: Indian Institute of Astrophysics.

The Kodaikanal Solar Observatory is a 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, Tamil Nadu state, South India.

A Grubb Parson 60 cm diameter two-mirror fused quartz coelostat mounted on 11 m tower platform directs sunlight via a flat mirror into a 60 m long underground horizontal 'tunnel'. A 38 cm aperture f/90 achromat forms a 34 cm diameter solar image at the focal plane. The telescope has an option to mount a 20 cm achromat, which provides an f/90 beam to form a 17 cm image.

A Littrow-type spectrograph is the main instrument of the telescope. A 20 cm diameter, 18 m focal length achromat in conjunction with a 600 lines/mm grating gives 9 mm/A dispersion in the fifth order of the grating. Together with the 5.5 arcsec/mm spatial resolution of the image, it forms a high resolution set up for solar spectroscopy. Recording of the spectrum can be done photographically or with a Photometrix 1k x 1k CCD system. A large format CCD system is being procured to enhance the coverage of spectrum especially for the broad resonance lines and the nearby continuum.

Lomnický štít station

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The astronomical and meteorological station on the Lomnický štít (working since 1960) is the third highest summit of Slovakia. Credit: 23am.com.

The astronomical station on Lomnický štít (2632 m) has been active since 1960. It is equipped with a 20 cm coronograph and a spectrograph.

McMath-Pierce Solar Telescope

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The McMath-Pierce Solar Telescope in Tucson, Arizona, is pictured. Credit: John Owens.

The McMath-Pierce Solar Telescope is a 1.6-m f/54 reflecting solar telescope at Kitt Peak National Observatory in Arizona, USA. It is the largest telescope of its kind in the world and is named for astronomers Robert McMath and Keith Pierce.

"The telescope contains a heliostat at the top of its main tower which focuses the sun's light down a long shaft. The distinctive diagonal shaft continues underground, where the telescope's primary mirror is located. The theoretical resolution of this main telescope 0.07 arcsec, although this is never reached because atmospheric distortions degrade the image quality severely. The image scale is 2.50 arcsec/mm at the image plane.

Solar active regions detected using the McMath-Pierce Solar Telescope receive a numerical designation as soon as they are observed on the solar surface.

Norikura Solar Observatory

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The Norikura Solar Observatory is pictured on Mount Norikura, Nagano, Japan. Credit: 松岡明芳.

"[I]n June 1991 large solar flares were seen repeatedly in the sunspot region 6659. Among those flares, the new solar neutron detector located at Mt. Norikura observatory, Japan (2770 m) which is made of plastic scintillators, succeeded in catching the signal of neutrons even though the size of the neutron telescope is only 1 m2 [23] and [24]."[104]

"[T]he roots of the [coronal] streamers seem to coincide best with the quiescent type prominences. ... the intensive regions of the green corona coincide best with the center of activity".[105]

"The peaks in the eleven-year component are found in the declining phase of activity, 2–4 years after the sunspot number maximum."[106]

Sayan Solar Observatory

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This image shows the Sayan Solar Observatory in Buryatia, Russia. Credit: Utopiya87.

"A set of three subsequent vector magnetograms of the leading spot of the active region NOAA 4216 (Solnechnye Dannye 163/83) was made about 10 hours after the spot's central meridian passage by the vector magnetrographs of the Sayan Observatory (Irkutsk) and the Solar Observatory "Einsteinturm" (Potsdam) within a cooperative observational programme."[107]

Hypotheses

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Main resource: Hypotheses
  1. Stellar active regions are initiated by a specific conjunction of Venus and Jupiter.

An initial control group for a star in general may be a uniformly luminous sphere of finite and constant size, similar to the bulb of a light bulb minus the socket attachment.

The simplest stellar active region may be any region on the surface of the sphere undergoing variation.

See also

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References

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  1. ↑ 1.0 1.1 1.2 1.3 press release 990610, K. G. Strassmeier, 1999-06-10, University of Vienna, "starspots vary on the same (short) time scales as Sunspots do", "HD 12545 had a warm spot (350 K above photospheric temperature; the white area in the picture)"
  2. ↑ K. D. Wood (November 10, 1972). "Physical Sciences: Sunspots and Planets". Nature 240 (5376): 91-3. doi:10.1038/240091a0. http://www.nature.com/nature/journal/v240/n5376/abs/240091a0.html. Retrieved 2013-07-07. 
  3. ↑ 3.0 3.1 Ray Tomes (February 1990). Towards a Unified Theory of Cycles. Cycles Research Institute. pp. 21. http://cyclesresearchinstitute.org/cycles-general/tomes_unified_cycles.pdf. Retrieved 2013-07-07. 
  4. ↑ 4.0 4.1 Freedman, Roger A.; Kaufmann III, William J. (2008). Universe. New York, USA: W. H. Freeman and Company. pp. 762. ISBN 978-0-7167-8584-2. 
  5. ↑ 5.0 5.1 5.2 5.3 Christopher Crockett (14 November 2019). "Realigning magnetic fields may drive the sun's spiky plasma tendrils". Science News. Retrieved 18 November 2019.
  6. ↑ 6.0 6.1 Hui Tian (14 November 2019). "Realigning magnetic fields may drive the sun's spiky plasma tendrils". Science News. Retrieved 18 November 2019.
  7. ↑ Calvin J. Hamilton (2009). Sun. http://www.solarviews.com/eng/sun.htm. 
  8. ↑ A. Vourlidas, J. A. Klimchuk, C. M. Korendyke, T. D. Tarbell, B. N. Handy (2001). "On the correlation between coronal and lower transition region structures at arcsecond scales". The Astrophysical Journal 563 (1): 374–80. doi:10.1086/323835. 
  9. ↑ M. J. Aschwanden (2001). "An evaluation of coronal heating models for Active Regions based on Yohkoh, SOHO, and TRACE observations". The Astrophysical Journal 560 (2): 1035–44. doi:10.1086/323064. 
  10. ↑ M. J. Aschwanden (2004). Physics of the Solar Corona. An Introduction. Praxis Publishing Ltd.. ISBN 3-540-22321-5. 
  11. ↑ Kulkarni SR, Rau A (2006). "The Nature of the Deep Lens Survey Fast Transients". The Astrophysical Journal 644 (1): L63. doi:10.1086/505423. 
  12. ↑ 12.0 12.1 Lopez-Morales, Mercedes; Morrell, N. I.; Butler, R. P.; Seager, S. (2006), "Limits to Transits of the Neptune-mass planet orbiting Gl 581", Publications of the Astronomical Society of the Pacific, 118, arXiv:astro-ph/0609255, Bibcode:2006PASP..118.1506L, doi:10.1086/508904, BY Draconis variable. This type of variable is characterized by quasiperiodic photometric variations over time scales from less than a day to months, and amplitudes ranging from a few hundredths of a magnitude to 0.5 mags.
  13. ↑ Schaaf, The Brightest Stars, Wiley, 2008.
  14. ↑ Schrijver, Carolus J.; Zwaan, Cornelis (2000), Solar and stellar magnetic activity, Cambridge astrophysics series, vol. 34, Cambridge University Press, p. 343, ISBN 0-521-58286-5
  15. ↑ Jaschek, Carlos; Jaschek, Mercedes (1990), The Classification of Stars, Cambridge University Press, p. 374, ISBN 0-521-38996-8
  16. ↑ Berdyugina 2.4 RS CVn stars
  17. ↑ 17.0 17.1 17.2 17.3 H. Zirin (1974). R. Grant Athay. ed. The Magnetic Structure of Plages, In: Chromospheric Fine Structure. Dordrecht: International Astronomical Union. pp. 161-75. Bibcode: 1974IAUS...56..161Z. 
  18. ↑ Ofman, Leon (2000). "Source regions of the slow solar wind in coronal streamers". Geophysical Research Letters 27 (18): 2885–8. doi:10.1029/2000GL000097. 
  19. ↑ 19.0 19.1 Nancy Atkinson (August 6, 2012). Huge Solar Filament Stretches Across the Sun. http://www.universetoday.com/96649/huge-solar-filament-stretches-across-the-sun/. Retrieved August 11, 2012. 
  20. ↑ About Filaments and Prominences. http://solar.physics.montana.edu/ypop/Program/hfilament.html. Retrieved 2010-01-02. 
  21. ↑ derived images showing rotation of cool and warm starspots
  22. ↑ 22.0 22.1 Berdyugina 5.3 Lifetimes
  23. ↑ 23.0 23.1 S. Ritzenhoff, E. H. Schroter, W. Schmidt (December 1997). "The lithium abundance in sunspots". Astronomy and Astrophysics 328 (12): 695-701. PMID p. 
  24. ↑ 24.0 24.1 24.2 24.3 Harold Zirin (March 1959). "Physical Conditions in Limb Flares and Active Prominences. II. a Remarkable Limb Flare, December 18, 1956". Astrophysical Journal 129 (3): 414-23. doi:10.1086/146633. 
  25. ↑ 25.0 25.1 Nicky Fox. Coronal Mass Ejections. Goddard Space Flight Center @ NASA. http://www-istp.gsfc.nasa.gov/istp/nicky/cme-chase.html. Retrieved 2011-04-06. 
  26. ↑ 26.0 26.1 A. Nindos and H. Zhang (July 10, 2002). "Photospheric Motions and Coronal Mass Ejection Productivity". The Astrophysical Journal 573 (2): L133-6. doi:10.1086/341937. http://iopscience.iop.org/1538-4357/573/2/L133. Retrieved 2012-11-24. 
  27. ↑ 27.0 27.1 27.2 Margaret Liggett and Harold Zirin (April 1983). "Naked Sunspots". Solar Physics 84 (04): 3-11. doi:10.1007/BF00157438. 
  28. ↑ Paul Lorrain and Serge Koutchmy (April 1996). "Two Dynamical Models for Solar Spicules". Solar Physics 165 (1): 115–37. doi:10.1007/BF00149093. 
  29. ↑ S. Patsourakos, J.-C. Vial (2002). "Intermittent behavior in the transition region and the low corona of the quiet Sun". Astronomy and Astrophysics 385: 1073–1077. doi:10.1051/0004-6361:20020151. 
  30. ↑ Brian Handy, Hugh Hudson (July 14, 2000). Super Regions. Helena, Montana, USA: University of Montana. http://solar.physics.montana.edu/nuggets/2000/000714/000714.html. Retrieved 2012-11-09. 
  31. ↑ 31.0 31.1 David F. Webb, Timothy A. Howard (2012). "Coronal Mass Ejections: Observations". Living Reviews in Solar Physics 9: 3. http://www.boulder.swri.edu/~howard/Papers/2012_lrsp.pdf. Retrieved 2012-11-11. 
  32. ↑ On the Great Magnetic Disturbance of 28 August to 7 September 1859, as Recorded by Photography at the Kew Observatory. (Abstract) by Balfour Stewart, Proceedings of the Royal Society of London, Vol. 11, (1860 - 1862), pp. 407-410
  33. ↑ On the Great Magnetic Disturbance Which Extended from 28 August to 7 September 1859, as Recorded by Photography at the Kew Observatory by Balfour Stewart, Philosophical Transactions of the Royal Society of London, Vol. 151, (1861), pp. 423-430
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Further reading

edit
  • Manuel Güdel (September 2004). "X-ray astronomy of stellar coronae". The Astronomy and Astrophysics Review 12 (2-3): 71–237. doi:10.1007/s00159-004-0023-2. 
edit

{{Charge ontology}}{{Principles of radiation astronomy}}

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