How often do solar flares occur?

Since superflares can occur on stars apparently equivalent in every way to the Sun, it is natural to ask if they can occur on the Sun itself. An estimate based on the original Kepler photometric studies suggested a frequency on solar-type stars (early G-type and rotation period more than 10 days) of once every 800 years for an energy of 10 erg and every 5000 years at 10 erg. One-minute sampling provided statistics for less energetic flares and gave a frequency of one flare of energy 10 erg every 5–600 years for a star rotating as slowly as the Sun; this would be rated as X100 on the solar flare scale. This is based on a straightforward comparison of the number of stars studied with the number of flares observed. An extrapolation of the empirical statistics for solar flares to an energy of 10 erg suggests a frequency of one in 10,000 years.

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 which 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 satellite, 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, up to 10,000 times as powerful as the strongest flares known on the Sun. 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. However, no such planet has showed 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 two promising events have been found in the carbon-14 records in tree rings, it is not possible to associate them definitely with a superflare event.

Solar superflares would have drastic effects, especially if they occurred as multiple events. Since they can occur on stars of the same age, mass and composition as the Sun this cannot be ruled out. However, 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.

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:
Essentially such stars may be regarded as solar analogues.
Originally nine superflare stars were found, some of them similar to the Sun.

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 to 2 × 10 ergs.

The Kepler spacecraft is a space observatory designed to find planets by the method of transits. A photometer continually monitors the brightness of 150,000 stars in a fixed area of the sky (in the constellations of Cygnus, Lyra and Draco) to detect changes in brightness caused by planets passing in front of the stellar disc. More than 90,000 are G-type stars (similar to the Sun) on or near the main sequence. The observed area corresponds to about 0.25% of the entire sky. The photometer is sensitive to wavelengths of 400–865 nm: the entire visible spectrum and part of the infrared. The photometric accuracy achieved by Kepler is typically 0.01% (0.1 mmag) for 30 minute integration times of 12th magnitude stars.

The high accuracy, the large number of stars observed and the long period of observation make Kepler ideal for detecting superflares. Studies published in 2012 and 2013 involved 83,000 stars over a period of 500 days (much of the data analysis was carried out with the help of five first-year undergraduates). The stars were selected from the Kepler Input Catalog to have T, the effective temperature, between 5100 and 6000K (the solar value is 5750K) to find stars of similar spectral class to the Sun, and the surface gravity log g > 4.0 to eliminate sub-giants and giants. The spectral classes range from F8 to G8. The integration time was 30 min in the original study. 1547 superflares were found on 279 solar-type stars.The most intense events increased the brightness of the stars by 30% and had an energy of 10 ergs. White-light flares on the Sun change the brightness by about 0.01%, and the strongest flares have a visible-light energy of about 10 ergs. (All energies quoted are in the optical bandpass and so are lower limits since some energy is emitted at other wavelengths.) Most events were much less energetic than this: flare amplitudes below 0.1% of the stellar value and energies of 2 × 10 ergs were detectable with the 30 minute integration. The flares had a rapid rise followed by an exponential decay on a time scale of 1–3 hours. The most powerful events corresponded to energies ten thousand greater than the largest flares observed on the Sun. Some stars flared very frequently: one star showed 57 events in 500 days, a rate of one every nine days. For the statistics of flares, the number of flares decreased with energy E roughly as E, a similar behaviour to solar flares. The duration of the flare increased with its energy, again in accordance with the solar behaviour.

Some Kepler data is taken at one minute sampling, though inevitably with lower accuracy. Using this data, on a smaller sample of stars, reveals flares that are too brief for reliable detection with 30-min integrations, allowing detection of events as low as 10 ergs, comparable with the brightest flares on the Sun. The occurrence frequency as a function of energy remains a power law E when extended to lower energies, with n around 1.5. At this time resolution some superflares show multiple peaks with separations of 100 to 1000 seconds, again comparable to the pulsations in solar flares. The star KIC 9655129 showed two periods, of 78 and 32 minutes, suggesting magnetohydrodynamic oscillations in the flaring region. These observations suggest that superflares are different only in scale and not in type to solar flares.

Superflare stars show a quasi-periodic brightness variation, which is interpreted as evidence of starspots carried around by solar rotation. This allows an estimate of the rotation period of the star; values range from less than one day up to tens of days (the value for the Sun is 25 days). On the Sun, radiometer monitoring from satellites shows that large sunspots can reduce the brightness by up to 0.2%. 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. In some cases the brightness variations can be modelled by only one or two large starspots, though not all cases are so simple. The starspots could be groups of smaller spots or single giant spots.

Flares are more common in stars with short periods. However, the energy of the largest flares is not related to the period of rotation. Stars with larger variations also have much more frequent flares; there is as well a tendency for them to have more energetic flares. Large variations can be found on even the most slowly rotating stars: one star had a rotation period of 22.7 days and variations implying spot coverage of 2.5% of the surface, over ten times greater than the maximum solar value. By estimating the size of the starspots from the amplitude variation, and assuming solar values for the magnetic fields in the spots (1000 G), it is possible to estimate the energy available: in all cases there is enough energy in the field to power even the largest flares observed. This suggests that superflares and solar flares have essentially the same mechanism.

In order to determine whether superflares can occur on the Sun, it is important to narrow the definition of Sun-like stars. When the temperature range is divided into stars with T above and below 5600K (early and late G-type stars), stars of lower temperature are about twice as likely to show superflare activity as those in the solar range and those that do so have more flares: the occurrence frequency of flares (number per star per year) is about five times as great in the late-type stars. It is well known that both the rotation rate and the magnetic activity of a star decrease with age in G-type stars. When flare stars are divided into fast and slow rotators, using the rotation period estimated from brightness variations, there is a general tendency for the fastest-rotating (and presumably youngest) stars to show a greater probability of activity: in particular, stars rotating in less than 10 days are 20-30 times more likely to have activity. Nevertheless, 44 superflares were found on 19 stars with similar temperatures to the Sun and periods greater than 10 days (out of 14000 such stars examined); four superflares with energies in the range 1-5 × 10 ergs were detected on stars rotating more slowly than the Sun (of about 5000 in the sample). The distribution of flares with energy has the same shape for all classes of star: although Sun-like stars are less likely to flare, they have the same proportion of very energetic flares as younger and cooler stars.

Kepler data have also been used to search for flares on stars of later spectral types than G. A sample of 23,253 stars with effective temperature T less than 5150K and surface gravity log g > 4.2, corresponding to main sequence stars later than K0V, was examined for flares over a time period of 33.5 days. 373 stars were identified as having obvious flares. Some stars had only one flare, while others showed as many as fifteen. The strongest events increased the brightness of the star by 7-8%. This is not radically different from the peak brightness of flares on G-type stars; however, since K and M stars are less luminous than type G, this suggests that flares on these stars are less energetic. Comparing the two classes of stars studied, it seems that M stars flare more frequently than K stars but the duration of each flare tends to be shorter. It is not possible to draw any conclusions about the relative proportion of G and K type stars showing superflares, or about the frequency of flares on those stars that do show such activity, since the flare detection algorithms and criteria in the two studies are quite different.

Most (though not all) of the K and M stars show the same quasi-periodic brightness variations as the G stars. There is a tendency for more energetic flares to occur on more variable stars; however flare frequency is only weakly related to variability.

When superflares were originally discovered on solar-type stars it was suggested that these eruptions may be produced by the interaction of the star's magnetic field with the magnetic field of a gas-giant planet orbiting so close to the primary that the magnetic fields were linked. Rotation or orbital motion would wind up the magnetic fields until a reconfiguration of the fields would cause an explosive release of energy. The RS Canum Venaticorum variables are close binaries, with orbital periods between 1 and 14 days, in which the primary is an F- or G-type main sequence star, and with strong chromospheric activity at all orbital phases. These systems have brightness variations attributed to large starspots on the primary; some show large flares thought to be caused by magnetic reconnection. The companion is close enough to spin up the star by tidal interactions.

A gas giant however would not be massive enough to do this, leaving the various measurable properties of the star (rotation speed, chromospheric activity) unchanged. If the giant and the primary were close enough for the magnetic fields to be linked, the orbit of the planet would wrap the field lines until the configuration became unstable followed by a violent release of energy in the form of a flare. Kepler discovered a number of closely orbiting gas giants, known as hot Jupiters; studies of two such systems showed periodic variations of the chromospheric activity of the primary synchronised to the period of the companion.

Not all planetary transits can be detected by Kepler, since the planetary orbit may be out of the line of sight to Earth. However, the hot Jupiters orbit so close to the primary that the chance of a transit is about 10%. If superflares were caused by close planets the 279 flare stars discovered should have about 28 transiting companions; none of them actually showed evidence of transits, effectively excluding this explanation.

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 5600K 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. However, 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.

See also Chromosphere#On other stars

Since stars apparently identical 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.

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 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. However, other events can produce nitrate spikes, including biomass burning which also produces enhanced ammonium concentrations. An examination of fourteen ice cores from Antarctic and Arctic regions showed large nitrate spikes: however, none of them were dated to 1859 (the closest was 1863). All such spikes were associated with ammonium and other chemical indicators of combustion. There is no evidence that nitrate concentrations can be used as indicators of historic solar activity.

When energetic protons enter the atmosphere they create isotopes by reactions with the major components; the most important of these is carbon-14 (C), which is created when secondary neutrons react with nitrogen. 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 C content is the basis of radiocarbon dating. If wood of known age is available the process can be reversed. Measuring the 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 C does indeed vary with time, due to solar activity. This is the basis of the carbon dating calibration curve. Clearly, 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 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 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 C 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.

Carbon-14 is not the only isotope that can be produced by energetic particles. Beryllium-10 (Be) is also formed from nitrogen and oxygen, and deposited in polar ice. However, Be deposition can be strongly related to local weather and shows extreme geographic variability; it is also more difficult to assign dates. Nevertheless, a 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 C results (within the uncertainty of dating for the Be data). Chlorine-36 (Cl) 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 which showed Be also provided increases of 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 C in tree rings, but at a lower intensity. This event also produced measurable increases in Be and Cl in Greenland ice cores.

If these events are presumed to be produced by fast 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 C record, 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 once it has entered the carbon cycle; the fraction of the created radiocarbon taken up by trees depends to some extent on that cycle. As an extra complication, the cosmogenic isotopes are preferentially created by energetic protons (several hundred MeV). 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 C increase. The most powerful flare which 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 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. A sunspot group 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 C measurement; however, 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 proposed as being consistent with all the details of the AD 774/5 event if it was sufficiently close. However, as known presently, this explanation is very unlikely.

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 773/4 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. Even if the dates do not precisely conform to the C increase this might suggest a period of high solar activity. Zhou et al. add further details from the Chinese chronicles. On a date which they give as 17 January AD 775, there were 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 Xian; although geomagnetic latitudes change over time, this would correspond to the lower twenties.

There are a number of difficulties involved when trying to link the 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 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.

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 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 which 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.

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 makes interpretation difficult. 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 (Ti) 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 C and 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 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 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 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.

The effect of the sort of superflare apparently found on the original nine candidate stars would be catastrophic for the Earth and 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. Thomas Gold suggested that the glaze on the top surface of certain lunar rocks might be caused by a solar outburst involving a luminosity increase of over a hundred times for 10 to 100 seconds at some time in the last 30,000 years. Apart from the terrestrial effects, this would cause local ice melting followed by refreezing as far out as the moons of Jupiter. There is no evidence of superflares on this scale having occurred in the Solar System..

Even for much smaller superflares, at the lower end of the Kepler range, the effects would be serious. In 1859 the Carrington event caused failures in the telegraph system in Europe and North America. Possible consequences today would include:
It is evident that superflares often repeat rather than occurring as isolated events. The NO and other odd nitrogens created by flare particles catalyse the destruction of ozone without being consumed themselves, and have a long lifetime in the stratosphere. Flares at a frequency of one a year or even less would have a cumulative effect; the destruction of the ozone layer could be permanent and lead to at least a low-level extinction event.

Superflares have also been suggested as a solution to the Faint young Sun paradox.

However, this does not match the known properties of superflare stars. Such stars are extremely rare in the Kepler data; one study showed only 279 such stars in 31,457 studied, a proportion below 1%; for older stars this fell to 0.25%. Also, about half of the stars which were active showed repeating flares: one had as many as 57 events in 500 days. Concentrating on solar-type stars, the most active averaged one flare every 100 days; the frequency of superﬂare occurrence in the most active Sun-like stars is 1000 times larger than that of the general average for such stars. This suggests that such behaviour is not present throughout a star's lifetime, but is confined to episodes of extraordinary activity. This is also suggested by the clear relation between the magnetic activity of a star and its superflare activity; in particular, superflare stars are much more active (based on starspot area) than the Sun.

There is no evidence for any flare greater than the Carrington event (about 10 erg, or 1/10,000 of the largest superflares) in the last 200 years. Although larger events from the C record ca. 775 AD is unambiguously identified as a solar event, its association to the flare energy is unclear, and it is unlikely to exceed 10 erg.

The more energetic superflares seem to be ruled out by energetic considerations for our sun, which suggest it is not capable of a flare of more than 10 ergs. A calculation of the free energy in magnetic fields in active regions that could be released as flares gives a lower upper bound of around 3×10 erg suggesting the most energetic a super flare can be is three times that of the Carrington event.

Some stars have a magnetic field 5 times that of Earth and rotate much faster and these could theoretically have a flare of up to 10 ergs. This could explain some superflares at the lower end of the range. To go higher than this may require an anti-solar rotation curve - one in which the polar regions rotate faster than the equatorial regions.

Other large solar flares also occurred on April 2, 2001 (X20), October 28, 2003 (X17.2 and 10), September 7, 2005 (X17), February 17, 2011 (X2), August 9, 2011 (X6.9), March 7, 2012 (X5.4), July 6, 2012 (X1.1). On July 6, 2012, a solar storm hit just after midnight UK time, when an X1.1 solar flare fired out of the AR1515 sunspot. Another X1.4 solar flare from AR 1520 region of the Sun, second in the week, reached the Earth on July 15, 2012 with a geomagnetic storm of G1–G2 level. A X1.8-class flare was recorded on October 24, 2012. There has been major solar flare activity in early 2013, notably within a 48-hour period starting on May 12, 2013, a total of four X-class solar flares were emitted ranging from an X1.2 and upwards of an X3.2, the latter of which was one of the largest year 2013 flares. Departing sunspot complex AR2035-AR2046 erupted on April 25, 2014 at 0032 UT, producing a strong X1.3-class solar flare and an HF communications blackout on the day-side of Earth. NASA's Solar Dynamics Observatory recorded a flash of extreme ultraviolet radiation from the explosion. The Solar Dynamics Observatory recorded an X9.3-class flare at around 1200 UTC on September 6, 2017.

Solar flare

A solar flare is a sudden flash of increased brightness on the Sun, usually observed near its surface
and in close proximity to a sunspot group.
Powerful flares are often, but not always, accompanied by a coronal mass ejection. Even the most powerful flares are barely detectable in the total solar irradiance (the "solar constant").

Solar flares occur in a power-law spectrum of magnitudes; an energy release of typically 10 joules of energy suffices to produce a clearly observable event, while a major event can emit up to 10 joules.

Flares are closely associated with the ejection of plasmas and particles through the Sun's corona into outer space; flares also copiously emit radio waves.
If the ejection is in the direction of the Earth, particles associated with this disturbance can penetrate into the upper atmosphere (the ionosphere) and cause bright auroras, and may even disrupt long range radio communication.
It usually takes days for the solar plasma ejecta to reach Earth. Flares also occur on other stars, where the term stellar flare applies.
High-energy particles, which may be relativistic, can arrive almost simultaneously with the electromagnetic radiations.

On July 23, 2012, a massive, potentially damaging, solar storm (solar flare, coronal mass ejection and electromagnetic radiation) barely missed Earth. According to NASA, there may be as much as a 12% chance of a similar event occurring between 2012 and 2022.

Solar flares affect all layers of the solar atmosphere (photosphere, chromosphere, and corona). The plasma medium is heated to tens of millions of Kelvin, while electrons, protons, and heavier ions are accelerated to near the speed of light. Flares produce electromagnetic radiation across the electromagnetic spectrum at all wavelengths, from radio waves to gamma rays. Most of the energy is spread over frequencies outside the visual range and so the majority of the flares are not visible to the naked eye and must be observed with special instruments. Flares occur in active regions around sunspots, where intense magnetic fields penetrate the photosphere to link the corona to the solar interior.
Flares are powered by the sudden (timescales of minutes to tens of minutes) release of magnetic energy stored in the corona. The same energy releases may produce coronal mass ejections (CMEs), although the relationship between CMEs and flares is still not well understood.

X-rays and UV radiation emitted by solar flares can affect Earth's ionosphere and disrupt long-range radio communications. Direct radio emission at decimetric wavelengths may disturb the operation of radars and other devices that use those frequencies.

Solar flares were first observed on the Sun by Richard Christopher Carrington and independently by Richard Hodgson in 1859 as localized visible brightenings of small areas within a sunspot group. Stellar flares can be inferred by looking at the lightcurves produced from the telescope or satellite data of variety of other stars.

The frequency of occurrence of solar flares varies, from several per day when the Sun is particularly "active" to less than one every week when the Sun is "quiet", following the 11-year cycle (the solar cycle). Large flares are less frequent than smaller ones.

Flares occur when accelerated charged particles, mainly electrons, interact with the plasma medium. Evidence suggests that the phenomenon of magnetic reconnection leads to this copious acceleration of charged particles.
On the Sun, magnetic reconnection may happen on solar arcades – a series of closely occurring loops following magnetic lines of force. These lines of force quickly reconnect into a lower arcade of loops leaving a helix of magnetic field unconnected to the rest of the arcade. The sudden release of energy in this reconnection is the origin of the particle acceleration. The unconnected magnetic helical field and the material that it contains may violently expand outwards forming a coronal mass ejection. This also explains why solar flares typically erupt from active regions on the Sun where magnetic fields are much stronger.

Although there is a general agreement on the source of a flare's energy, the mechanisms involved are still not well understood. It is not clear how the magnetic energy is transformed into the kinetic energy of the particles, nor is it known how some particles can be accelerated to the GeV range (10 electron volt) and beyond. There are also some inconsistencies regarding the total number of accelerated particles, which sometimes seems to be greater than the total number in the coronal loop. Scientists are unable to forecast flares.

The classification system for solar flares uses the letters A, B, C, M or X, according to the peak flux in watts per square metre (W/m) of X-rays with wavelengths 100 to 800 picometre, as measured at the Earth by the GOES spacecraft.

The strength of an event within a class is noted by a numerical suffix ranging from 1 to 9, which is also the factor for that event within the class. Hence, an X2 flare is twice the strength of an X1 flare, an X3 flare is three times as powerful as an X1, and only 50% more powerful than an X2. An X2 is four times more powerful than an M5 flare.

An earlier flare classification was based on Hα spectral observations. The scheme uses both the intensity and emitting surface. The classification in intensity is qualitative, referring to the flares as: faint (f), normal (n) or brilliant (b). The emitting surface is measured in terms of "millionths" of the hemisphere and is described below. (The total hemisphere area "A" = 15.5 × 10 km.)

A flare then is classified taking S or a number that represents its size and a letter that represents its peak intensity, v.g.: Sn is a "normal sunflare".

Solar flares strongly influence the local space weather in the vicinity of the Earth. They can produce streams of highly energetic particles in the solar wind or stellar wind, known as a solar proton event. These particles can impact the Earth's magnetosphere (see main article at geomagnetic storm), and present radiation hazards to spacecraft and astronauts. Additionally, massive solar flares are sometimes accompanied by coronal mass ejections (CMEs) which can trigger geomagnetic storms that have been known to disable satellites and knock out terrestrial electric power grids for extended periods of time.

The soft X-ray flux of X class flares increases the ionization of the upper atmosphere, which can interfere with short-wave radio communication and can heat the outer atmosphere and thus increase the drag on low orbiting satellites, leading to orbital decay. Energetic particles in the magnetosphere contribute to the aurora borealis and aurora australis. Energy in the form of hard x-rays can be damaging to spacecraft electronics and are generally the result of large plasma ejection in the upper chromosphere.

The radiation risks posed by solar flares are a major concern in discussions of a manned mission to Mars, the Moon, or other planets. Energetic protons can pass through the human body, causing biochemical damage, presenting a hazard to astronauts during interplanetary travel. Some kind of physical or magnetic shielding would be required to protect the astronauts. Most proton storms take at least two hours from the time of visual detection to reach Earth's orbit. A solar flare on January 20, 2005 released the highest concentration of protons ever directly measured, giving astronauts as little as 15 minutes to reach shelter.

Flares produce radiation across the electromagnetic spectrum, although with different intensity. They are not very intense in visible light, but they can be very bright at particular atomic lines. They normally produce bremsstrahlung in X-rays and synchrotron radiation in radio.

Richard Carrington observed a flare for the first time on 1 September 1859 projecting the image produced by an optical telescope through a broad-band filter. It was an extraordinarily intense "white light flare". Since flares produce copious amounts of radiation at Hα, adding a narrow ( ≈1 Å) passband filter centered at this wavelength to the optical telescope, allows the observation of not very bright flares with small telescopes. For years Hα was the main, if not the only, source of information about solar flares. Other passband filters are also used.

During World War II, on February 25 and 26, 1942, British radar operators observed radiation that Stanley Hey interpreted as solar emission. Their discovery did not go public until the end of the conflict. The same year Southworth also observed the Sun in radio, but as with Hey, his observations were only known after 1945. In 1943 Grote Reber was the first to report radioastronomical observations of the Sun at 160 MHz. The fast development of radioastronomy revealed new peculiarities of the solar activity like "storms" and "bursts" related to the flares. Today ground-based radiotelescopes observe the Sun from c. 15 MHz up to 400 GHz.

Since the beginning of space exploration, telescopes have been sent to space, where they work at wavelengths shorter than UV, which are completely absorbed by the atmosphere, and where flares may be very bright. Since the 1970s, the GOES series of satellites observe the Sun in "soft" X-rays, and their observations became the "standard measure" of flares, diminishing the importance of the Hα classification. "Hard" X-rays were observed by many different instruments, the most important today being the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Nonetheless, UV observations are today the "stars" of solar imaging with their incredible fine details that reveal the complexity of the solar corona. Spacecraft may also bring radio detectors at extremely long wavelengths (as long as a few kilometers) that cannot propagate through the ionosphere.
The following spacecraft missions have flares as their main observation target.

In addition to these solar observing facilities, many non-solar astronomical satellites observe flares either intentionally (e.g., NuSTAR), or simply because the penetrating hard radiations coming from a flare can
readily penetrate most forms of shielding.

The most powerful flare ever observed was the first one to be observed, on September 1, 1859, and was reported by British astronomer Richard Carrington and independently by an observer named Richard Hodgson. The event is named the Solar storm of 1859, or the "Carrington event". The flare was visible to a naked eye (in "white light"), and produced stunning auroras down to tropical latitudes such as Cuba or Hawaii, and set telegraph systems on fire. The flare left a trace in Greenland ice in the form of nitrates and beryllium-10, which allow its strength to be measured today. Cliver and Svalgaard reconstructed the effects of this flare and compared with other events of the last 150 years. In their words: "While the 1859 event has close rivals or superiors in each of the above categories of space weather activity, it is the only documented event of the last ∼150 years that appears at or near the top of all of the lists."

The ultra-fast coronal mass ejection of August 1972 is suspected of triggering magnetic fuses on naval mines during the Vietnam War, and would have been a life-threatening event to Apollo astronauts if it had occurred during a mission to the Moon.

In modern times, the largest solar flare measured with instruments occurred on November 4, 2003. This event saturated the GOES detectors, and because of this its classification is only approximate. Initially, extrapolating the GOES curve, it was estimated to be X28. Later analysis of the ionospheric effects suggested increasing this estimate to X45. This event produced the first clear evidence of a new spectral component above 100 GHz.

Flare sprays are a type of eruption associated with solar flares. They involve faster ejections of material than eruptive prominences, and reach velocities of 20 to 2000 kilometers per second.

Current methods of flare prediction are problematic, and there is no certain indication that an active region on the Sun will produce a flare. However, many properties of sunspots and active regions correlate with flaring. For example, magnetically complex regions (based on line-of-sight magnetic field) called delta spots produce the largest flares. A simple scheme of sunspot classification due to McIntosh, or related to fractal complexity. is commonly used as a starting point for flare prediction. Predictions are usually stated in terms of probabilities for occurrence of flares above M or X GOES class within 24 or 48 hours. The U.S. National Oceanic and Atmospheric Administration (NOAA) issues forecasts of this kind.

Sources

A solar flare is a phenomenon where the Sun suddenly releases a great amount of solar radiation, much more than normal. Solar flares are unlikely to cause any direct injury, but can destroy electrical equipment. The potential of solar storms to cause disaster was seen during the 1859 Carrington event, which disrupted the telegraph network, and the March 1989 geomagnetic storm which blacked out Quebec. Some major known solar flares include the X20 event on August 16, 1989, and a similar flare on April 2, 2001. The most powerful flare ever recorded occurred on November 4, 2003 (estimated at between X40 and X45).

Natural disaster

A natural disaster is a major adverse event resulting from natural processes of the Earth; examples are floods, hurricanes, tornadoes, volcanic eruptions, earthquakes, tsunamis, and other geologic processes. A natural disaster can cause loss of life or damage property, and typically leaves some economic damage in its wake, the severity of which depends on the affected population's resilience, or ability to recover and also on the infrastructure available.

An adverse event will not rise to the level of a disaster if it occurs in an area without vulnerable population. In a vulnerable area, however, such as Nepal during the 2015 earthquake, an earthquake can have disastrous consequences and leave lasting damage, which can require years to repair.

A landslide is described as an outward and downward slope movement of an abundance of slope-forming materials including rock, soil, artificial, or even a combination of these things.

During World War I, an estimated 40,000 to 80,000 soldiers died as a result of avalanches during the mountain campaign in the Alps at the Austrian-Italian front. Many of the avalanches were caused by artillery fire.

An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. At the Earth's surface, earthquakes manifest themselves by vibration, shaking, and sometimes displacement of the ground. Earthquakes are caused by slippage within geological faults. The underground point of origin of the earthquake is called the "seismic focus". The point directly above the focus on the surface is called the "epicenter". Earthquakes by themselves rarely kill people or wildlife. It is usually the secondary events that they trigger such as building collapse, fires, tsunamis (seismic sea waves) and volcanoes. Many of these could possibly be avoided by better construction, safety systems, early warning and planning.

When natural erosion, human mining or underground excavation makes the ground too weak to support the structures built on it, the ground can collapse and produce a sinkhole. For example, the 2010 Guatemala City sinkhole which killed fifteen people was caused when heavy rain from Tropical Storm Agatha, diverted by leaking pipes into a pumice bedrock, led to the sudden collapse of the ground beneath a factory building.

Volcanoes can cause widespread destruction and consequent disaster in several ways. The effects include the volcanic eruption itself that may cause harm following the explosion of the volcano or falling rocks. Secondly, lava may be produced during the eruption of a volcano, and so as it leaves the volcano the lava destroys many buildings, plants and animals due to its extreme heat. Thirdly, volcanic ash, generally meaning the cooled ash, may form a cloud, and settle thickly in nearby locations. When mixed with water this forms a concrete-like material. In sufficient quantities, ash may cause roofs to collapse under its weight but even small quantities will harm humans if inhaled. Since the ash has the consistency of ground glass, it causes abrasion damage to moving parts such as engines. The main killer of humans in the immediate surroundings of a volcanic eruption is the pyroclastic flows, which consist of a cloud of hot volcanic ash which builds up in the air above the volcano and rushes down the slopes when the eruption no longer supports the lifting of the gases. It is believed that Pompeii was destroyed by a pyroclastic flow. A lahar is a volcanic mudflow or landslide. The 1953 Tangiwai disaster was caused by a lahar, as was the 1985 Armero tragedy in which the town of Armero was buried and an estimated 23,000 people were killed.

Volcanoes rated at 8 (the highest level) on the Volcanic Explosivity Index are known as supervolcanoes. According to the Toba catastrophe theory, 75,000 to 80,000 years ago a supervolcanic eruption at what is now Lake Toba in Sumatra reduced the human population to 10,000 or even 1,000 breeding pairs, creating a bottleneck in human evolution, and killed three-quarters of all plant life in the northern hemisphere. However, there is considerable debate regarding the veracity of this theory. The main danger from a supervolcano is the immense cloud of ash, which has a disastrous global effect on climate and temperature for many years.

A violent, sudden and destructive change either in the quality of Earth's water or in the distribution or movement of water on land below the surface or in the atmosphere.

A flood is an overflow of water that'submerges' land. The EU Floods Directive defines a flood as a temporary covering the land with water which is usually not covered by water. In the sense of 'flowing water', the word may also be applied to the inflow of the tides. Flooding may result from the volume of water within a body of water, such as a river or lake, which overflows, causing some of the water to escape its usual boundaries. While the size of a lake or other body of water will vary with seasonal changes in precipitation and snow melt, it is not a significant flood unless the water covers land used by man, like a village, city or other inhabited area, roads, expanses of farmland, etc.

A tsunami (plural: tsunamis or tsunami; from Japanese: 津波, lit. "harbour wave"; English pronunciation: /tsuːˈnɑːmi/), also known as a seismic sea wave or as a tidal wave, is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Tsunamis can be caused by undersea earthquakes such as the 2004 Boxing Day tsunami, or by landslides such as the one in 1958 at Lituya Bay, Alaska, or by volcanic eruptions such as the ancient eruption of Santorini. On March 11, 2011, a tsunami occurred near Fukushima, Japan and spread through the Pacific Ocean.

A limnic eruption occurs when a gas, usually CO, suddenly erupts from deep lake water, posing the threat of suffocating wildlife, livestock and humans. Such an eruption may also cause tsunamis in the lake as the rising gas displaces water. Scientists believe landslides, volcanic activity, or explosions can trigger such an eruption. To date, only two limnic eruptions have been observed and recorded. In 1984, in Cameroon, a limnic eruption in Lake Monoun caused the deaths of 37 nearby residents, and at nearby Lake Nyos in 1986 a much larger eruption killed between 1,700 and 1,800 people by asphyxiation.

Cyclone, tropical cyclone, hurricane, and typhoon are different names for the same phenomenon, which is a cyclonic storm system that forms over the oceans. The determining factor on which term is used is based on where they originate. In the Atlantic and Northeast Pacific, the term "hurricane" is used; in the Northwest Pacific it is referred to as a "typhoon" and "cyclones" occur in the South Pacific and Indian Ocean.

The deadliest hurricane ever was the 1970 Bhola cyclone; the deadliest Atlantic hurricane was the Great Hurricane of 1780 which devastated Martinique, St. Eustatius and Barbados. Another notable hurricane is Hurricane Katrina, which devastated the Gulf Coast of the United States in 2005.

Blizzards are severe winter storms characterized by heavy snow and strong winds. When high winds stir up snow that has already fallen, it is known as a ground blizzard. Blizzards can impact local economic activities, especially in regions where snowfall is rare. The Great Blizzard of 1888 affected the United States, when many tons of wheat crops were destroyed, and in Asia, 2008 Afghanistan blizzard and the 1972 Iran blizzard were also significant events. The 1993 Superstorm originated in the Gulf of Mexico and traveled north, causing damage in 26 states as well as Canada and leading to more than 300 deaths.

Hailstorms are precipitation in the form of ice, with the ice not melting before it hits the ground. Hailstones usually measure between 0.2 inch (5 millimetres) and 6 inches (15 centimetres) in diameter. A particularly damaging hailstorm hit Munich, Germany, on July 12, 1984, causing about $2 billion in insurance claims.

An ice storm is a type of winter storm characterized by freezing rain. The U.S. National Weather Service defines an ice storm as a storm which results in the accumulation of at least 0.25 inch (6.4 mm) of ice on exposed surfaces.

A cold wave (known in some regions as a cold snap or cold spell) is a weather phenomenon that is distinguished by a cooling of the air. Specifically, as used by the U.S. National Weather Service, a cold wave is a rapid fall in temperature within a 24-hour period requiring substantially increased protection to agriculture, industry, commerce, and social activities. The precise criterion for a cold wave is determined by the rate at which the temperature falls, and the minimum to which it falls. This minimum temperature is dependent on the geographical region and time of year.

A heat wave is a period of unusually and excessively hot weather. The worst heat wave in recent history was the European Heat Wave of 2003. A summer heat wave in Victoria, Australia, created conditions which fuelled the massive bushfires in 2009. Melbourne experienced three days in a row of temperatures exceeding 40 °C (104 °F) with some regional areas sweltering through much higher temperatures. The bushfires, collectively known as "Black Saturday", were partly the act of arsonists. The 2010 Northern Hemisphere summer resulted in severe heat waves, which killed over 2,000 people. It resulted in hundreds of wildfires which caused widespread air pollution, and burned thousands of square miles of forest.

Drought is the unusual dryness of soil caused by levels of rainfall significantly below average over a prolonged period. Hot dry winds, shortage of water, high temperatures and consequent evaporation of moisture from the ground can also contribute to conditions of drought. Droughts result in crop failure and shortages of water.

Well-known historical droughts include the 1997–2009 Millennium Drought in Australia led to a water supply crisis across much of the country. As a result, many desalination plants were built for the first time (see list). In 2011, the State of Texas lived under a drought emergency declaration for the entire calendar year and severe economic losses. The drought caused the Bastrop fires.

Severe storms, dust clouds, and volcanic eruptions can generate lightning. Apart from the damage typically associated with storms, such as winds, hail, and flooding, the lightning itself can damage buildings, ignite fires and kill by direct contact. Especially deadly lightning incidents include a 2007 strike in Ushari Dara, a remote mountain village in northwestern Pakistan, that killed 30 people, the crash of LANSA Flight 508 which killed 91 people, and a fuel explosion in Dronka, Egypt caused by lightning in 1994 which killed 469. Most lightning deaths occur in the poor countries of America and Asia, where lightning is common and adobe mud brick housing provides little protection.

A tornado is a violent and dangerous rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud, or the base of a cumulus cloud in rare cases. It is also referred to as a "twister" or a "cyclone", although the word cyclone is used in meteorology in a wider sense, to refer to any closed low pressure circulation. Tornadoes come in many shapes and sizes, but are typically in the form of a visible condensation funnel, whose narrow end touches the Earth and is often encircled by a cloud of debris and dust. Most tornadoes have wind speeds less than, are approximately across, and travel a few miles (several kilometers) before dissipating. The most extreme tornadoes can attain wind speeds of more than 300 mph (480 km/h), stretch more than two miles (3 km) across, and stay on the ground for dozens of miles (perhaps more than 100 km).

Wildfires are large fires which often start in wildland areas. Common causes include lightning and drought but wildfires may also be started by human negligence or arson. They can spread to populated areas and can thus be a threat to humans and property, as well as wildlife. Notable cases of wildfires were the 1871 Peshtigo Fire in the United States, which killed at least 1700 people, and the 2009 Victorian bushfires in Australia.

Asteroids that impact the Earth have led to several major extinction events, including one which created the Chicxulub crater 64.9 million years ago and which is associated with the demise of the dinosaurs. Scientists estimate that the likelihood of death for a living human from a global impact event is comparable to the probability of death from an airliner crash.

No human death has been definitively attributed to an impact event, but the 1490 Ch'ing-yang event in which over 10,000 people may have died has been linked to a meteor shower. Even asteroids and comets that burn up in the atmosphere can cause significant destruction on the ground due to the air burst explosion: notable air bursts include the Tunguska event in June 1908, which devastated large areas of Siberian countryside, and the Chelyabinsk meteor on 15 February 2013, which caused widespread property damage in the city of Chelyabinsk and injured 1,491.

International law, for example Geneva Conventions defines International Red Cross and Red Crescent Movement the Convention on the Rights of Persons with Disabilities, requires that "States shall take, in accordance with their obligations under international law, including international humanitarian law and international human rights law, all necessary measures to ensure the protection and safety of persons with disabilities in situations of risk, including the occurrence of natural disaster." And further United Nations Office for the Coordination of Humanitarian Affairs is formed by General Assembly Resolution 44/182. People displaced due to natural disasters are currently protected under international law (Guiding Principles of International Displacement, Campala Convention of 2009).

According to the UN, Asia-Pacific is the world's most disaster prone region. According to ReliefWeb, a person in Asia-Pacific is five times more likely to be hit by a natural disaster than someone living in other regions.

In the aftermath of natural disasters, 10% of the care delivered by rescue works concerns the needs of women and, in particular, pregnant women. While the entire population, women and men alike, face similar risks of health consequence related to blunt injury, malnutrition and infections, women face a unique set of challenges. Due to the social, political and cultural context of many places throughout the world, women are often disproportionately affected by disaster. In settings where women and children are likely to remain at home, natural disasters, such as earthquakes, can result in greater morbidity and mortality among women. For example, during the 1993 earthquake in Maharastra, India, more women died than men as they were more likely to be in the home, due to their role as caregivers. In the 2004 Indian Ocean tsunami, more women died than men, partly due to the fact that fewer women knew how to swim. As another example, three times as many women died in the 2005 Indonesian tsunami than men due to their role as child caretakers.

During and after a natural disaster, women are at increased risk of being affected by gender based violence and are increasingly vulnerable to sexual violence. Disrupted police enforcement, lax regulations, and displacement all contribute to increased risk of gender based violence and sexual assault. As food, water, and shelter becomes scarce, women may be forced into sexual relations as a bargain for providing essential resources. Furthermore, health care during times of disaster often focuses on life saving & critical care. However, as a result, many health care workers are not adequately trained to respond to sexual violence, screen for appropriate complications and treating non-life/limb threatening emergencies. As a result, women who have been affected by sexual violence are at a significantly increased risk of sexually transmitted infections, unique physical injuries and long term psychological consequences. All of these long-term health outcomes can prevent successful reintegration into society after the disaster recovery period.

During and after natural disasters, routine health behaviors become interrupted. Women who were taking contraceptives may forget or may no longer have access to these medications. In addition, health care systems may have broken down as a result of the disaster, further reducing access to contraceptives. Unprotected intercourse during this time can lead to increased rates of childbirth, unintended pregnancies and sexually transmitted infections (STIs). Methods used to prevent STIs (such as condom use) are often forgotten or not accessible during times surrounding a disaster. Lack of health care infrastructure and medical shortages hinder the ability to treat individuals once they acquire an STI. In addition, health efforts to prevent, monitor or treat HIV/AIDS are often disrupted, leading to increased rates of HIV complications and increased transmission of the virus through the population.

Furthermore, as a result of inadequate access to clean water, women may be unable to properly wash menstrual cloths or rags. Displacement often results in lack of privacy or places to hang menstrual cloths to dry. As a result, women are at increased risk of perineal rashes and urinary tract infections.

Pregnant women are one of the groups disproportionately affected by natural disasters. Inadequate nutrition, little access to clean water, lack of health-care services and psychological stress in the aftermath of the disaster can lead to significant increase in maternal morbidity and mortality. One of the major causes of maternal mortality in the context of emotional or physical trauma from natural disaster is placental abruption. Other factors contributing to increased maternal mortality risk include maternal hemorrhage (combined with inadequate blood supply) and infections. Furthermore, shortage of healthcare resources during this time can convert even routine obstetric complications into emergencies.

During and after a disaster, women’s prenatal, peri-natal and postpartum care can become disrupted. After disasters, there is often a significant increase in the number of women who receive late or no prenatal care. Among women affected by natural disaster, there are significantly higher rates of low birth weight infants, preterm infants and infants with low head circumference. Separation of mothers and babies as a result of poor infrastructure and displacement practices can interfere with breastfeeding and cause significant emotional stress for mom and baby. It can also lead to negative long-term health outcome mother and especially babies. In addition, it can be particularly difficult to find clean water for sterilizing bottles for breast milk or pre-made formula. These factors can further hinder breastfeeding practices and adequate infant nutrition, resulting in long term health consequences for the baby.

Natural disasters can also affect political relations with countries and vice versa. Violent conflicts within states can exacerbate the impact of natural disasters by weakening the ability of states, communities and individuals to provide disaster relief. Natural disasters can also worsen ongoing conflicts within states by weakening the capacity of states to fight rebels. In developed countries like the US, studies find that incumbents lose votes when the electorate perceives them as responsible for a poor disaster response. In Chinese and Japanese history, it has been routine for era names or capital cities and palaces of emperors to be changed after a major natural disaster, chiefly for political reasons such as association with hardships by the populace and fear of upheaveal. (i.e. in East asian government chronicles, such fears were recorded in a low profile way as an unlucky name or place requiring change.) Disasters and responses can dictate political careers; the once popular President Benigno Aquino III of Philippines, following a weak and confused response to Typhoon Yolanda which killed over 6,000 people and survivors were largely left to fend for themselves, this widely accepted sentiment carried over and the President never recovered his popularity, his hand picked successor Mar Roxas lost the subsequent election to a rival party in a landslide vote. Post-disaster mishandling can spread despair as bad news travels fast and far, and contribute to the appeal of electing a strongman out of sheer desperation.

Between 1995 and 2015, according to the UN’s disaster-monitoring system, the greatest number of natural disasters occurred in America, China and India.

In 2012, there were 905 natural disasters worldwide, 93% of which were weather-related disasters. Overall costs were US$170 billion and insured losses $70 billion. 2012 was a moderate year. 45% were meteorological (storms), 36% were hydrological (floods), 12% were climatological (heat waves, cold waves, droughts, wildfires) and 7% were geophysical events (earthquakes and volcanic eruptions). Between 1980 and 2011 geophysical events accounted for 14% of all natural catastrophes.

Studies on natural events require complete historical records and strategies related to obtaining and storing reliable records, allowing for both critical interpretation and validation of the sources. Under this point of view the irreplaceable role of traditional repositories (archives) can be supplemented by the use of such web sources as eBay.