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I read that solar flares are customarily viewed in H-alpha light, as a temporary brightening of a small portion of chromosphere.

What all can be interpreted from this? Is it because, energy of the radiation contained by the flare lies around this wavelength? And why chromosphere ?

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    $\begingroup$ This isn't my area, so I won't risk a full answer, but the intensity of the $H_\alpha$ radiation is correlated to the temperature so a brightening means the area you're looking at is hotter. This correlates with flares simply because the flares are hotter - much, much hotter :-). $\endgroup$ – John Rennie Dec 7 '14 at 8:37
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The basic model for a solar flare starts with the magnetic field in the corona. You can think of the topology of the magnetic field to consist of loops that poke up out of the photosphere and extend into the corona. However, the photosphere of the Sun is turbulent and constantly in motion due to convection and differential rotation. Whilst a loop may be formed in a minimum energy state, it can get twisted and stressed by these motions.

At some point an instability is reached and the magnetic field can undergo a "reconnection" event, to flip back to a lower energy configuration. During this event, charged particles are accelerated and travel down the magnetic field lines towards the photosphere.

Before they get there, they encounter the chromosphere, which is where the bulk of the particle kinetic energy is deposited. This results in excess H alpha emission from material at 10-20 thousand kelvin, but some chromospheric material is also heated and evaporated such that it fills the magnetic loops with X-ray emitting plasma at temperatures of more than a million kelvin. Some of the flare energy may also be used to accelerate material away from the Sun in a "coronal mass ejection".

Flares can bee seen at a wide variety of wavelengths. There are signatures to be seen in white light, ultra-violet, hard and soft X-rays and radio waves. You can generally divide this between thermal (white light, UV, soft -ray) and non-thermal (hard X-rays and radio waves) processes. The total energies in solar flares follow a roughly power law distribution such that $dN/dE \propto E^{-\alpha}$, with $\alpha \simeq 2.5$. The largest flares are therefore very infrequent (on the Sun, but not necessarily for other stars).

The largest "recent" flare was the Carrington event of 1859. This was a once every 100-500 year flare with a total energy of roughly a few $10^{26}$ Joules. It is speculated that larger (and more rare) events are possible and indeed are seen on other stars, which may be ordersof magnitude more active. Other large solar events associated with coronal mass ejections (as pointed out by honeste_vivere below) have been seen recently and may indicate that this is a lower limit, and that these highly energetic events may be somewhat more common than once thought.

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  • $\begingroup$ There have been several flares and CMEs since the Carrington event that were of similar magnitude. In fact, there was a 2012 CME that propagated as fast, if not faster, than the one associated with Carrington event. The flare and CMEs associated with events in 1972 and 1989 (both associated with power grid issues or failures) were also on par with the Carrington event. So they are not as rare as once every 100-500 years. $\endgroup$ – honeste_vivere Jan 18 '15 at 13:52
  • $\begingroup$ @honeste_vivere Interesting - references? Of course the Carrington event energy estimate must be quite rough. $\endgroup$ – Rob Jeffries Jan 18 '15 at 14:32
  • $\begingroup$ Baker et al., "A major solar eruptive event in July 2012: Defining extreme space weather scenarios," Space Weather 11, pp. 585–591, doi:10.1002/swe.20097, 2013. $\endgroup$ – honeste_vivere Jan 19 '15 at 14:20
  • $\begingroup$ Anderson et al., "Outage of the L4 system and the geomagnetic disturbances of 4 August 1972," Bell Syst. Tech. J. 53 (9), pp. 1817–1837, 1974. $\endgroup$ – honeste_vivere Jan 19 '15 at 14:23
  • $\begingroup$ Vaisberg, O.L., and G.N. Zastenker "Solar wind and magnetosheath observations at Earth during August 1972," Space Sci. Rev. 19, pp. 687, 1976. $\endgroup$ – honeste_vivere Jan 19 '15 at 14:24
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Solar flares, now distinguished from coronal mass ejections, are defined as a temporally abrupt, spatially localized enhancement in electromagnetic radiation in the upper atmosphere of a star. They typically release upwards of 10$^{27}$-10$^{32}$ ergs (or 10$^{20}$-10$^{25}$ joules) of energy by our sun (can much higher at extrasolar stars). In other words, these electromagnetic outbursts release the energy equivalent of exploding >10$^{15}$ tons of TNT or ~100,000 times the worlds yearly energy consumption in 2010.

For practical purposes, solar flares are generally identified using X-ray observations by the GOES spacecraft. We can and do observe them with H$_{\alpha}$ emissions. This emission tends to occur in the chromosphere because the density of neutral hydrogen is much higher (which is necessary for H$_{\alpha}$ emission to occur). In the corona, where temperatures are much higher and densities much lower, it is very difficult for hydrogen to remain neutral. By comparison, the majority of X-ray and UV emission occurs in the corona. This is generally due to thin and thick target Bremsstrahlung emission. The thin target emission often occurs high in the coronal and is observed as soft X-rays while thick target occurs at the base of the corona as hard X-rays. The RHESSI mission was designed to investigate the source of these emissions.

The processes that lead to the electromagnetic emissions are accelerating charged particles, as inferred from the thin and thick target Bremsstrahlung and observed by in situ spacecraft as impulsive solar energetic particles or SEPs. So we can and do observe solar flares in many different ways, not just H$_{\alpha}$ emission.

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