A Solar Hard X-Ray Halo: Exploring the Quiet Sun 2

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Number: 21
1st Author: Alec MacKinnon
2nd Author: Hugh Hudson
Published: 27 January 2006
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This nugget is a continuation in our quiet Sun series (see our previous quiet Sun nugget). During solar minimum, RHESSI has a wonderful opportunity to search for X-rays not related to solar flares. This nugget describes a new component of the hard X-ray emission from the quiet Sun.

The usual process through which X-rays are produced on the Sun is bremsstrahlung. In the context of the Sun, this term usually refers to radiation produced by free energetic electrons deflected by other charged particles. Another variety of this bremsstrahlung process exists, "inner" bremsstrahlung. When a neutron beta-decays, which it does with a half-life of about ten minutes, the beta particle (another name for an electron) - can emit an X-ray as it "exits" the decaying neutron. Such neutrons can be produced on the Sun through galactic cosmic rays interacting with the solar material. These interactions produce secondary neutrons which can then scatter back out of the Sun into the corona and beta-decay.

Unfortunately, several factors conspire to make this process inefficient. Low-energy cosmic rays are excluded by the Sun's magnetic field and the primary cosmic rays are protons. Such protons require still higher energies in order to interact with the solar atmosphere (consisting of protons itself) to produce neutrons. Finally, most of the neutrons will decay beneath the photosphere, from which their bremsstrahlung cannot escape.

Nevertheless this process exists and has been observed at Earth (which has a smaller magnetic field), for which the CRAND mechanism ("Cosmic Ray Albedo Neutron Decay") actually populates the inner Van Allen Belts with protons (another product of the beta decay).


Inner bremsstrahlung occurs with a reasonably high probability, about 1%, (compared with an efficiency of about 10-5 for more ordinary direct bremsstrahlung by a 20 keV free electron). This is because the emitting electron doesn't have to rely on a chance close encounter with a nucleus - it comes into existence close to one. The resulting spectrum is unusual: a flat distribution of photon energies up to 780 keV, as described in the only paper in the astrophysical literature on this subject - by Vahé Petrosian and RHESSI's own Reuven Ramaty. By comparison the hard X-rays from solar flares and from the diffuse cosmic X-ray background radiation, have steep falling spectra. Thus, this component should be easier to detect at high energies.

To calculate the size and shape of this X-ray halo, we must know the spectrum of neutrons escaping from the Sun. We estimate this with an energy spectrum f(E) = (E + E0)-δ, where we take δ to be the power-law index of the primary cosmic rays, namely 3.3. At low energies the neutron energy spectrum is determined not by the energies of the galactic cosmic rays but by what happens to the neutrons in the solar atmosphere, and the parameter E0 = 10 MeV reflects this crossover.

Our calculations suggest a large hard X-ray halo - some 20 degrees across - that is fainter than the cosmic X-ray background radiation (note that the explanation of these cosmic hard X-rays recently led to a Nobel Prize for Riccardo Giacconi). Therefore this X-ray halo may be hard to detect. Yet, several parameters in this calculation are speculative and we are not sure. For example, the detection of this halo could teach us about the penetration of galactic cosmic rays to the solar surface. In addition the solar hard X-ray halo may contribute a background source for other astrophysical or physical measurements.

A halo source like this (or any symmetrical coronal source, for that matter) would appear in the X-ray sky as an object with a dimple, as illustrated here:

The brightness of the solar disk is reduced by about a factor of two because the body of the Sun obscures that part of the halo source behind it. Accordingly the solar disk would appear dim in the image, rather than bright. The actual details near the Sun are complex and not well determined by these calculations, but that is all the more reason to try to make direct observations.


We have described the calculation of a heretofore unrecognized component of the quiet-Sun hard X-ray spectrum. This component may be hard to detect, but in principle it's there. Free neutrons are produced during flares, in much greater numbers than those produced more steadily by galactic cosmic rays, and one can observe neutron-decay protons directly, via detectors on space probes out in the heliosphere. Neutrons are among the most difficult particles to observe, because of their charge neutrality and ephemeral existence outside nuclei. The inner bremsstrahlung process may give us the means to detect the presence of neutrons on the Sun indirectly.

An unfortunate consequence of this physics is that this halo may obscure a comparable halo resulting from the decay of a theoretical particle, the axions. We may do a future RHESSI science nugget on this topic (any volunteers?) - suffice it to say here that the detection of axions - a possible form of dark matter would certainly contribute to our understanding of cosmology and particle physics.

Biographical note: Alec MacKinnon is an astrophysicist at Glasgow University (UK) where solar physics research is supported by the UK's PPARC, and Hugh Hudson is a RHESSI researcher at U.C. Berkeley.

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