Editing Stellar corona (section)

===Flares===
{{main|Solar flares}}
[[File:Magnificent CME Erupts on the Sun - August 31.jpg|thumb|On August 31, 2012, a long filament of solar material that had been hovering in the Sun's outer atmosphere, the corona, erupted at 4:36 p.m. EDT]]

Flares take place in active regions and are characterized by a sudden increase of the radiative flux emitted from small regions of the corona. They are very complex phenomena, visible at different wavelengths; they involve several zones of the solar atmosphere and many physical effects, thermal and not thermal, and sometimes wide reconnections of the magnetic field lines with material expulsion.

Flares are impulsive phenomena, of average duration of 15 minutes, and the most energetic events can last several hours. Flares produce a high and rapid increase of the density and temperature.

An emission in white light is only seldom observed: usually, flares are only seen at extreme UV wavelengths and into the X-rays, typical of the chromospheric and coronal emission.

In the corona, the morphology of flares is described by observations in the UV, soft and hard X-rays, and in Hα wavelengths, and is very complex. However, two kinds of basic structures can be distinguished:
<ref>{{cite journal|doi = 10.1086/155452|last1 = Pallavicini |first1 = R.|last2 = Serio|first2 = S.|last3 = Vaiana|first3 = G. S.|title = A survey of soft X-ray limb flare images – The relation between their structure in the corona and other physical parameters
  | journal = The Astrophysical Journal  | volume = 216| page = 108| year = 1977 | bibcode=1977ApJ...216..108P}}</ref>
* ''Compact flares'', when each of the two arches where the event is happening maintains its morphology: only an increase of the emission is observed without significant structural variations. The emitted energy is of the order of 10<sup>22</sup>&nbsp;– 10<sup>23</sup> J.
* ''Flares of long duration'', associated with eruptions of prominences, transients in white light and ''two-ribbon flares'':<ref>{{cite journal|doi = 10.1038/344842a0|last1 = Golub |first1 = L.|last2 = Herant|first2 = M.|last3 = Kalata|first3 = K.|last4 = Lovas|first4 = I.|last5 = Nystrom|first5 = G.|last6 = Pardo|first6 = F.|last7 = Spiller|first7 = E.|last8 = Wilczynski|first8 = J. | title = Sub-arcsecond observations of the solar X-ray corona | journal = Nature | volume = 344 | pages = 842–844| year = 1990|bibcode = 1990Natur.344..842G | issue=6269|s2cid = 4346856 }}</ref> in this case the magnetic loops change their configuration during the event. The energies emitted during these flares are of such great proportion they can reach 10<sup>25</sup> J.

[[File:Solar-filament.gif|thumb|Filament erupting during a solar flare, seen at EUV wavelengths ([[TRACE]])]]

As for temporal dynamics, three different phases are generally distinguished, whose duration are not comparable. The durations of those periods depend on the range of wavelengths used to observe the event:
* ''An initial impulsive phase'', whose duration is on the order of minutes, strong emissions of energy are often observed even in the microwaves, EUV wavelengths and in the hard X-ray frequencies.
* ''A maximum phase''
* ''A decay phase'', which can last several hours.
Sometimes also a phase preceding the flare can be observed, usually called as "pre-flare" phase.