Extreme ultraviolet (EUV) consists of electromagnetic radiation with wavelengths between 0.12 pm and 0.01 pm. EUV makes up only a tiny part of the spectrum of stars with photospheric temperatures under 10000 K - main sequence stars of spectral class B,A,F,G,K and M. (Recall that our Sun is a class G star). It nonetheless fuels the chemistry and physics of the outer atmosphere. EUV photons have sufficient energy to break up otherwise stable atmospheric compounds, allowing their components to combine into less stable forms that would not otherwise exist in appreciable quantities in the atmosphere. Further, because EUV photons are energetic enough to penetrate and interact with the electron clouds of atoms and molecules, the absorption cross section is so high that significant heating rates can be sustained despite the low EUV flux. This is not the case for the more abundant visible or near-ultraviolet photons, to which the tenuous outer atmosphere is largely transparent. For this reason, EUV absorption is an important source of energy available to sustain atmospheric escape. Because of the nearly ubiquitous role of EUV in what is to follow, it is useful to pause at this point and provide some general background on EUV radiation.
EUV is not produced by blackbody radiation in a star's photosphere. Instead, it is produced high in the star's thin outer atmosphere - it's corona - where temperatures are brought to extremely high values by a variety of arcane and poorly understood heating mechanisms. The production of EUV in the hot corona has three important consequences. First, it allows the EUV energy flux to be far in excess of the blackbody value corresponding to the photospheric temperature. For example, at the Earth's orbit the photospheric blackbody flux from the Sun in the wavelength range from 0.055 pm to 0.060 pm would be a mere 7 • 10-11W/m2, whereas the observed solar EUV flux in this range is 9 • 10_5W/m2. The second important consequence is that the EUV fluctuates considerably in response to the solar activity cycle - loosely speaking, the sunspot cycle - because activity is intimately connected with the cycle of a star's magnetic field. The magnetic environment, in turn has a great effect on the corona. Thus, while the total solar luminosity varies very little over the course of the 11-year solar cycle, the net EUV flux varies by a factor of two or more - typically between .003W/m2 and .007W/m2 at Earth's orbit. In some sub-bands of EUV the variation is even more pronounced. The third consequence is that cool stars such as M-dwarfs can nonetheless have high sporadic EUV output, because the processes determining the coronal temperature are quite distinct from those controlling the photospheric temperature. M-dwarfs in particular have strong activity cycles, which give rise to the kind of events that effectively produce EUV. The strong and sporadic EUV output of such stars should give rise to novel aspects regarding the escape and chemistry of the atmospheres of planets orbiting these stars. One should also bear in mind that the Faint Young Sun could have had EUV output out of proportion to the dimness of the rest of the spectrum. In fact, while total stellar energy output increases with time, it is generally agreed that EUV output is higher for young Main Sequence stars. Solar EUV output is often assumed to be 3-6 times greater than at present in the first two billion years of the Sun's life as a star, though neither stellar models nor observations of other stars offer the possibility of precise estimates at present.
EUV is so strongly absorbed by the outer atmosphere that it can only be observed from outer space. The EUV output of stars other than our Sun is of extreme importance, but it was long thought that EUV astronomy was a dead-end, on account of absorption by interstellar hydrogen. It turns out, however, that there are sufficient fluctuations in the interstellar hydrogen density to permit a great deal of useful information to be obtained about stellar EUV output. Results from this emerging science will have a great bearing on the evolution of atmospheres of extrasolar planets.
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