Inverse Compton X-rays from relativistic flare leptons
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| - | == | + | == Radiation from solar atmosphere accelerated electrons == |
| - | + | Flare accelerated electrons are subject to various influences as they move around in the solar atmosphere. They may have close encounters with other electrons and ions, changing energy and direction as they interact via the electrostatic force. They feel a force due to the background magnetic field that keeps them bound in tight spirals around the [http://www.phy6.org/Education/Imagnet.html field lines]. They may exchange energy with optical photons from the photosphere, numerous so close to the solar surface. | |
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| + | Each of these interactions can give to rise to X-rays, at least in principle. From the first of these we get bremsstrahlung radiation, the likely mechanism by which flare hard X-rays are actually produced. All analysis of RHESSI data - including the work described in most other nuggets - supposes this to be the case. In the presence of a magnetic field, very high energy electrons emit [http://en.wikipedia.org/wiki/Synchrotron_radiation synchrotron radiation]. This accounts for radio radiation from flares but getting X-rays this way needs electrons of unfeasibly high energies, for which no other evidence exists. | ||
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| + | [The_Rise_and_Fall_of_The_Low_Energy_Cut_Off The previous nugget] invoked Compton scattering of X-rays by 'cold' (or at least, un-accelerated) electrons in the photosphere. In this process the photons lose energy to the electrons. If photons encounter high energy electrons, however, they may gain energy in [http://venables.asu.edu/quant/proj/compton.html inverse Compton scattering], which we abbreviate here to IC. If sufficiently energetic electrons are present, some of the optical photons streaming away from the photosphere in the course of the Sun's normal luminosity may acquire enough energy to be counted in the X-ray part of the spectrum. | ||
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| + | IC scattering is certainly important in other branches of astrophysics. In the Sunyaev-Zeldovich effect, which has been observed, hot gas in clusters of galaxies inverse Compton scatters photons from the cosmic microwave background to shorter wavelengths. Nice pictures illustrate both Sunyaev-Zeldovich and the basics of IC scattering in [http://science.nasa.gov/newhome/headlines/compton.htm this NASA news article]. | ||
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| + | Alexander Korchak evidently deserves some notoriety as one of the founders of the [http://www.mhg.ru/english Moscow Helsinki Group]. In the 1960s and 1970s, however, in his professional capacity he consideed the likely importance of these three possible methods of generating flare X-rays, concluding that synchrotron radiation would never be important and that bremsstrahlung was almost certainly the dominant mechanism (as we still believe), but leaving open the possibility that inverse Compton radiation could be more important if electrons moved only in low density regions (<em>very</em> roughly speaking, so that they collide more often with photons than with other gas particles). | ||
Revision as of 15:49, 21 November 2008
Radiation from solar atmosphere accelerated electrons
Flare accelerated electrons are subject to various influences as they move around in the solar atmosphere. They may have close encounters with other electrons and ions, changing energy and direction as they interact via the electrostatic force. They feel a force due to the background magnetic field that keeps them bound in tight spirals around the field lines. They may exchange energy with optical photons from the photosphere, numerous so close to the solar surface.
Each of these interactions can give to rise to X-rays, at least in principle. From the first of these we get bremsstrahlung radiation, the likely mechanism by which flare hard X-rays are actually produced. All analysis of RHESSI data - including the work described in most other nuggets - supposes this to be the case. In the presence of a magnetic field, very high energy electrons emit synchrotron radiation. This accounts for radio radiation from flares but getting X-rays this way needs electrons of unfeasibly high energies, for which no other evidence exists.
[The_Rise_and_Fall_of_The_Low_Energy_Cut_Off The previous nugget] invoked Compton scattering of X-rays by 'cold' (or at least, un-accelerated) electrons in the photosphere. In this process the photons lose energy to the electrons. If photons encounter high energy electrons, however, they may gain energy in inverse Compton scattering, which we abbreviate here to IC. If sufficiently energetic electrons are present, some of the optical photons streaming away from the photosphere in the course of the Sun's normal luminosity may acquire enough energy to be counted in the X-ray part of the spectrum.
IC scattering is certainly important in other branches of astrophysics. In the Sunyaev-Zeldovich effect, which has been observed, hot gas in clusters of galaxies inverse Compton scatters photons from the cosmic microwave background to shorter wavelengths. Nice pictures illustrate both Sunyaev-Zeldovich and the basics of IC scattering in this NASA news article.
Alexander Korchak evidently deserves some notoriety as one of the founders of the Moscow Helsinki Group. In the 1960s and 1970s, however, in his professional capacity he consideed the likely importance of these three possible methods of generating flare X-rays, concluding that synchrotron radiation would never be important and that bremsstrahlung was almost certainly the dominant mechanism (as we still believe), but leaving open the possibility that inverse Compton radiation could be more important if electrons moved only in low density regions (very roughly speaking, so that they collide more often with photons than with other gas particles).
