Superflares are very strong explosions observed on stars with energies up to ten thousand times that of typical solar flares.
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.
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.
Since 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.
The most powerful events corresponded to energies ten thousand times greater than the largest flares observed on the Sun.
At this time resolution some superflares show multiple peaks with separations of 100 to 1000 seconds, again comparable to the pulsations in solar flares.
Superflare stars show a quasi-periodic brightness variation, which is interpreted as evidence of starspots carried around by stellar rotation.
In superflare stars the most common brightness variations are 1–2%, though they can be as great as 7–8%, suggesting that the area of the starspots can be very much larger than anything found on the Sun.
Similarly, a search for superflares at radio wavelengths that may be caused by hot Jupiters interacting magnetically with their stars failed to detect any such flares.
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.
Observations of the Ca lines in stars of similar age to the Sun even show cyclic variations reminiscent of the 11-year solar cycle.
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.
The Carrington Event of 1859, the largest flare of which we have direct observation, produced global auroral displays extending close to the equator.
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.
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.
An examination of fourteen ice cores from Antarctic and Arctic regions showed large nitrate spikes: however, 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.
[16] 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.
The result was confirmed by studies of German oak, bristlecone pine from California, Siberian larch, and Kauri wood from New Zealand.
In addition, measurements of coral skeletons from the South China Sea showed substantial variations in 14C over a few months around the same time; however, the date could only be established to within a period of ±14 years around 783 AD.
However, 10Be deposition can be strongly related to local weather and shows extreme geographic variability; it is also more difficult to assign dates.
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.
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.
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.
As an additional check, it is possible to recover the isotope Titanium-44 (44Ti, 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.
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.
A more detailed scrutiny of the period AD 731 to 825, combining several 14C 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.
The effect of the sort of superflare apparently found on the original nine candidate stars would be catastrophic for the Earth and would cause serious damage to the atmosphere and to life.
[35] It also would leave traces on the Solar System; the event on S Fornacis for example involved an increase in the stars' luminosity by a factor of about twenty.