Superflare

Superflares are very strong explosions observed on stars with energies up to ten thousand times that of typical solar flares. The stars in this class satisfy conditions that should make them solar analogues, and would be expected to be stable over very long time scales. The original nine candidates were detected by a variety of methods. No systematic study was possible until the launch of the Kepler space telescope, which monitored a very large number of solar-type stars with very high accuracy for an extended period. This showed that a small proportion of stars had violent outbursts. In many cases there were multiple events on the same star. Younger stars were more likely to flare than old ones, but strong events were seen on stars as old as the Sun. The flares were initially explained by postulating giant planets in very close orbits, such that the magnetic fields of the star and planet were linked. The orbit of the planet would warp the field lines until the instability released magnetic field energy as a flare. No such planet has shown up as a Kepler transit and this theory has been abandoned. All superflare stars show quasi-periodic brightness variations interpreted as very large starspots carried round by rotation. Spectroscopic studies found spectral lines that were clear indicators of chromospheric activity associated with strong and extensive magnetic fields.…

]] Superflares are very strong explosions observed on stars with energies up to ten thousand times that of typical solar flares. The stars in this class satisfy conditions that should make them solar analogues, and would be expected to be stable over very long time scales. The original nine candidates were detected by a variety of methods. No systematic study was possible until the launch of the Kepler space telescope, which monitored a very large number of solar-type stars with very high accuracy for an extended period. This showed that a small proportion of stars had violent outbursts. In many cases there were multiple events on the same star. Younger stars were more likely to flare than old ones, but strong events were seen on stars as old as the Sun. The flares were initially explained by postulating giant planets in very close orbits, such that the magnetic fields of the star and planet were linked. The orbit of the planet would warp the field lines until the instability released magnetic field energy as a flare. No such planet has shown up as a Kepler transit and this theory has been abandoned. All superflare stars show quasi-periodic brightness variations interpreted as very large starspots carried round by rotation. Spectroscopic studies found spectral lines that were clear indicators of chromospheric activity associated with strong and extensive magnetic fields. This suggests that superflares only differ in scale from solar flares. Attempts have been made to detect past solar superflares from nitrate concentrations in polar ice, from historical observations of auroras, and from those radioactive isotopes that can be produced by solar energetic particles. Although three events and a few candidates have been found in the carbon-14 records in tree rings, it is not possible to associate them definitely with superflare events. Solar superflares would have drastic effects, especially if they occurred as multiple events. Because they can occur on stars of the same age, mass and composition as the Sun, this cannot be ruled out, but no indication of solar superflares have been found for the past ten millennia. Solar-type superflare stars are very rare and are magnetically much more active than the Sun; if solar superflares do occur, it may be in well-defined episodes that occupy a small fraction of its time.

Superflare stars

A superflare star is not the same as a flare star, which usually refers to a very late spectral type red dwarf. The term is restricted to large transient events on stars that satisfy the following conditions: The star is in spectral class F8 to G8 It is on or near the main sequence It is single or part of a very wide binary It is not a rapid rotator It is not exceedingly young Essentially such stars may be regarded as solar analogues. Originally nine superflare stars were found, some of them similar to the Sun.

Original superflare candidates

The original paper identified nine candidate objects from a literature search: Type gives the spectral classification including spectral type and luminosity class. V (mag) means the normal apparent visual magnitude of the star. EW(He) is the equivalent width of the 5875.6Å He I D3 line seen in emission. The observations vary for each object. Some are X-ray measurements, others are visual, photographic, spectroscopic or photometric. The energies for the events vary from 2 × 10 33 to 2 × 10 38 ergs.

Spectroscopic observations of superflare stars

Spectroscopic studies of superflares allow their properties to be determined in more detail, in the hope of detecting the cause of the flares. The first studies were made using the high dispersion spectrograph on the Subaru Telescope in Hawaii. Some 50 apparently solar-type stars, known from the Kepler observations to show superflare activity, have been examined in detail. Of these, only 16 showed evidence of being visual or spectroscopic binaries; these were excluded since close binaries are frequently active, while in the case of visual binaries there is the chance of activity taking place on the companion. Spectroscopy allows accurate determinations of the effective temperature, the surface gravity and the abundance of elements beyond helium ('metallicity'); most of the 34 single stars proved to be main sequence stars of spectral type G and similar composition to the Sun. Since properties such as temperature and surface gravity change over the lifetime of a star, stellar evolution theory allows an estimate of the age of a star: in most cases the age appeared to be above several hundred million years. This is important since very young stars are known to be much more active. Nine of the stars conformed to the narrower definition of solar-type given above, with temperatures greater than 5600 K and rotation periods longer than 10 days; some had periods above 20 or even 30 days. Only five of the 34 could be described as fast rotators. Observations from LAMOST have been used to measure chromospheric activity of 5,648 solar-like stars in the Kepler field, including 48 superflare stars. These observations show that superflare stars are generally characterized by larger chromospheric emissions than other stars, including the Sun. Superflare stars with activity levels lower than, or comparable to, the Sun do exist, suggesting that solar flares and superflares most likely share the same origin. The very large ensemble of solar-like stars included in this study enables detailed and robust estimates of the relation between chromospheric activity and the occurrence of superflares. All the stars showed the quasi-periodic brightness variations, ranging from 0.1% to nearly 10%, interpreted as the rotation of large starspots. When large spots exist on a star, the activity level of the chromosphere becomes high; in particular, large chromospheric plages form around sunspot groups. The intensities of certain solar and stellar lines generated in the chromosphere, particularly the lines of ionised calcium (Ca II) and the Hα line of hydrogen, are known to be indicators of magnetic activity. Observations of the Ca lines in stars of similar age to the Sun even show cyclic variations reminiscent of the 11-year solar cycle. By observing certain infrared lines of Ca II for the 34 superflare stars it was possible to estimate their chromospheric activity. Measurements of the same lines at points within an active region on the Sun, together with simultaneous measurements of the local magnetic field, show that there is a general relation between field and activity. Although the stars show a clear correlation between rotational speed and activity, this does not exclude activity on slowly rotating stars: even stars as slow as the Sun can have high activity. All the superflare stars observed had more activity than the Sun, implying larger magnetic fields. There is also a correlation between the activity of a star and its brightness variations (and therefore the starspot coverage): all stars with large amplitude variations showed high activity. Knowing the approximate area covered by starspots from the size of the variations, and the field strength estimated from the chromospheric activity, allows an estimate of the total energy stored in the magnetic field; in all cases there was enough energy stored in the field to account for even the largest superflares. Both the photometric and the spectroscopic observations are consistent with the theory that superflares are different only in scale from solar flares, and can be accounted for by the release of magnetic energy in active regions very much larger than those on the Sun. Nevertheless, these regions can appear on stars with masses, temperatures, compositions, rotation speeds and ages similar to the Sun.

Detecting past superflares on the Sun

Since stars apparently similar to the Sun can produce superflares it is natural to ask if the Sun itself can do so, and to try to find evidence that it has done in the past. Large flares are invariably accompanied by energetic particles, and these particles produce effects if they reach the Earth. The Carrington Event of 1859, the largest flare of which we have direct observation, produced global auroral displays extending close to the equator. Energetic particles can produce chemical changes in the atmosphere, which can be permanently recorded in the polar ice. Fast protons generate distinctive isotopes, particularly carbon-14, which can be taken up and preserved by living creatures.

Nitrate concentrations in polar ice

When solar energetic particles reach the Earth's atmosphere they cause ionisation that creates nitric oxide (NO) and other reactive nitrogen species, which then precipitate out in the form of nitrates. Since all energetic charged particles are deflected to a greater or lesser extent by the geomagnetic field, they enter preferentially at the polar latitudes; since high latitudes also contain permanent ice, it is natural to look for the nitrate signature of particle events in ice cores. A study of a Greenland ice core extending back to 1561 AD achieved resolutions of 10 or 20 samples a year, allowing in principle the detection of single events. Precise dates (within one or two years) can be achieved by counting annual layers in the cores, checked by identification of deposits associated with known volcanic eruptions. The core contained an annual variation of nitrate concentration, accompanied by a number of 'spikes' of different amplitudes. The strongest of these in the entire record was dated to within a few weeks of the Carrington event of 1859. Other events can produce nitrate spikes, however, including biomass burning that also produces enhanced ammonium concentrations. An examination of fourteen ice cores from Antarctic and Arctic regions showed large nitrate spikes: none of them were dated to 1859 other than the one already mentioned, and that one seems to be too soon after the Carrington event and too short to be explained by it. All such spikes were associated with ammonium and other chemical indicators of combustion. The conclusion is that nitrate concentrations cannot be used as indicators of historic solar activity.

Single events from cosmogenic isotopes

When energetic protons enter the atmosphere they create isotopes by reactions with the major components; the most important of these is carbon-14 ( 14 C), which is created when secondary neutrons react with nitrogen. 14 C, which has a half-life of 5,730 years, reacts with oxygen to form carbon dioxide, which is taken up by plants; dating wood by its 14 C content was the original basis of radiocarbon dating. If wood of known age is available the process can be reversed. Measuring the 14 C content and using the half-life allows estimation of the content when the wood was formed. The growth rings of trees show patterns, caused by various environmental factors: dendrochronology uses these growth rings of trees, compared across overlapping sequences, to establish accurate dates. Applying this method shows that atmospheric 14 C does indeed vary with time, due to solar activity. This is the basis of the carbon dating calibration curve. It can also be used to detect any peaks in production caused by solar flares, if those flares create enough energetic particles to produce a measurable increase in 14 C. An examination of the calibration curve, which has a time resolution of five years, showed three intervals in the last 3,000 years in which 14 C increased significantly. On the basis of this two Japanese cedar trees were examined with a resolution of a single year, and showed an increase of 1.2% in AD 774, some twenty times larger than anything expected from the normal solar variation. This peak steadily diminished over the next few years. The result was confirmed by studies of German oak, bristlecone pine from California, Siberian larch, and Kauri wood from New Zealand. All determinations agreed on both the time and amplitude of the effect. In addition, measurements of coral skeletons from the South China Sea showed substantial variations in 14 C over a few months around the same time; the date could only be established to within a period of ±14 years around 783 AD. Carbon-14 is not the only isotope that can be produced by energetic particles. Beryllium-10 ( 10 Be, half-life 1.4 million years) is also formed from nitrogen and oxygen, and deposited in polar ice. Nonetheless, 10 Be deposition can be strongly related to local weather and shows extreme geographic variability; it is also more difficult to assign dates. Nevertheless, a 10 Be increase during the 770s was found in an ice core from the Antarctic, though the signal was less striking because of the lower time resolution (several years); another smaller increase was seen in Greenland. When data from two sites in North Greenland and one in the West Antarctic, all taken with a one-year resolution, were compared they all showed a strong signal: the time profile also matched well with the 14 C results (within the uncertainty of dating for the 10 Be data). Chlorine-36 ( 36 Cl, half-life 301 thousand years) can be produced from argon and deposited in polar ice; because argon is a minor atmospheric constituent the abundance is low. The same ice cores that showed 10 Be also provided increases of 36 Cl, though with a resolution of five years a detailed match was impossible. A second event in AD 993/4 has also been found from 14 C in tree rings, but at a lower intensity, and another event was found for 660 BC This event also produced measurable increases in 10 Be and 36 Cl in Greenland ice cores. If these events are presumed to be produced by energetic particles from large flares, it is not easy to estimate the particle energy in the flare or compare it with known events. The Carrington event does not appear in the cosmogenic records, and neither did any other large particle event that has been directly observed. The flux of particles must be estimated by calculating production rates of radiocarbon, and then modelling the behaviour of the CO 2 once it has entered the carbon cycle; the fraction of the created radiocarbon taken up by trees depends to some extent on that cycle. The energetic particle spectrum of a solar flare varies considerably between events; one with a 'hard' spectrum, with more high-energy protons, will be more efficient at producing a 14 C increase. The most powerful flare that also had a hard spectrum that has been observed instrumentally took place in February 1956 (the beginning of nuclear testing obscures any possible effects in the 14 C record); it has been estimated that if a single flare were responsible for the AD 774/5 event it would need to be 25–50 times more powerful than this. One active region on the Sun may produce several flares over its lifetime, and the effects of such a sequence would be aggregated over the one-year period covered by a single 14 C measurement; the total effect would still be ten times greater than anything observed in a similar period in modern times. Solar flares are not the only possibility for producing the cosmogenic isotopes. A long or short gamma-ray burst has been initially proposed as a possible cause of the AD 774/5 event. This explanation turned out to be very unlikely, and the current paradigm is that these events are caused by extreme solar particle events.

Historical records

A number of attempts have been made to find additional evidence supporting the superflare interpretation of the isotope peak around AD 774/5 by studying historical records. The Carrington event produced auroral displays as far south as Caribbean and Hawaii, corresponding to geomagnetic latitude of about 22°; if the event of 774/5 corresponded to an even more energetic flare there should have been a global auroral event. Usoskin et al. cited references to aurorae in Chinese chronicles for AD 770 (twice), 773 and 775. They also quote a "red cross" in the sky in AD 773, 774, or 776 from the Anglo-Saxon Chronicle; "inflamed shields" or "shields burning with a red colour" seen in the sky over Germany in AD 776 recorded in the Royal Frankish Annals; "fire in heaven" seen in Ireland in AD 772; and an apparition in Germany in AD 773 interpreted as riders on white horses. The enhanced solar activity around the 14 C increase is confirmed by the Chinese auroral record on AD 776 January 12, as detailed by Stephenson et al. The Chinese records describe more than ten bands of white lights "like the spread silk" stretching across eight Chinese constellations; the display lasted for several hours. The observations, made during the Tang dynasty, were made from the capital Chang'an. Nevertheless, there are a number of difficulties involved when trying to link the 14 C results to historical chronicles. Tree ring dates may be in error because there is no discernible ring for a year (unusually cold weather), or two rings (a second growth during a warm autumn). If the cold weather were global, following a large volcanic eruption, it is conceivable that the effects could also be global: the apparent 14 C date may not always match the chronicles. For the isotope peak in AD 993/994 studied by Hayakawa et al. surveyed contemporary historical documents show clustering auroral observations in late 992, while their relationship with the isotope peak is still under discussion.

General solar activity in the past

Superflares seem to be associated with a general high level of magnetic activity. As well as looking for individual events, it is possible to examine the isotope records to find the activity level in the past and identify periods when it may have been much higher than now. Lunar rocks provide a record unaffected by geomagnetic shielding and transport processes. Both non-solar cosmic rays and solar particle events can create isotopes in rocks, and both are affected by solar activity. The cosmic rays are much more energetic and penetrate more deeply, and can be distinguished from the solar particles that affect the outer layers. Several different radioisotopes can be produced with very different half-lives; the concentration of each may be regarded as representing an average of particle flux over its half-life. Since fluxes must be converted into isotope concentrations by simulations there is a certain model-dependence here. The data are consistent with the view that the flux of energetic solar particles with energies above a few tens of MeV has not changed over periods ranging from five thousand to five million years. Of course, a period of intense activity over a time scale short with respect to the half-life would not be detected. 14 C measurements, even with low time resolution, can indicate the state of solar activity over the last 11,000 years until about 1900. Although radiocarbon dating has been applied as far back as 50,000 years, during the deglaciations at the start of the Holocene the biosphere and its carbon uptake changed dramatically making estimation before this impractical; after about 1900 the Suess effect and nuclear bomb-tests makes interpretation difficult. 10 Be concentrations in stratified polar ice cores provide an independent measure of activity. Both measures agree reasonably with each other and with the Zurich sunspot number of the last two centuries. As an additional check, it is possible to recover the isotope Titanium-44 ( 44 Ti, half-life 60 years) from meteorites; this provides a measurement of activity that is not affected by changes in transport process or the geomagnetic field. Although it is limited to about the last two centuries, it is consistent with all but one of the 14 C and 10 Be reconstructions and confirms their validity. The energetic flare events discussed above are rare; on long time scales (significantly more than a year), the radiogenic particle flux is dominated by cosmic rays. The inner Solar System is shielded by the general magnetic field of the Sun, which is strongly dependent on the time within a cycle and the strength of the cycle. The result is that times of powerful activity show up as decreases in the concentrations of all these isotopes. Because cosmic rays are also influenced by the geomagnetic field, difficulties in reconstructing this field set a limit to the accuracy of the reconstructions. The 14 C reconstruction of activity over the last 11,000 years shows no period significantly higher than the present; in fact, the general level of activity in the second half of the 20th century was the highest since 9000 BC. In particular, the activity in the period around the AD 774 14 C event (averaged over decades) was somewhat lower than the long-term average, while the AD 993 event coincided with a small minimum. A more detailed scrutiny of the period AD 731 to 825, combining several 14 C datasets of one- and two-year resolution with auroral and sunspot accounts does show a general increase in solar activity (from a low level) after about AD 733, reaching its highest level after 757 and remaining high in the 760s and 770s; there were several aurorae around this time, and even a low-latitude aurora in China.

See also

Flare star Solar flare Stellar magnetic field

References

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