Weekly Notes from the Yohkoh Soft X-Ray Telescope

(Week 39, 2002)

Science Nugget: Sept. 27, 2002

Coronal Compton scattering?


The Yohkoh science nuggets have dealt with many aspects of solar observations in X-rays, but have not yet discussed scattering in the corona. The solar corona (the "K-corona") is strongly polarized as a result of the what the astronomers call "electron scattering" and the physicist knows as Thomson scattering in its non-relativistic treatment. The basic idea is that a photon will strike a coronal electron, altering (in general) its direction of motion, energy, and state of polarization as a result of this interaction. The result is the glorious solar corona, visible to the naked eye during a total eclipse - or rather, the K-corona is one component, one should not forget the F-corona or the E-corona.
corona A view of the solar corona in white light.

Hard X-rays

Thomson scattering does not have any wavelength dependence, although its relativistic extension Compton scattering does. The full physics is embodied in the Klein - Nishina cross-section which is too complicated to do more than link here. So why should the very same glorious solar corona not scatter hard X-rays, to be observed by instruments such as HXT or RHESSI? It does, obviously enough, but hard X-rays are so "hard" to detect that the observation of Compton-scattered coronal hard X-rays has not yet happened (or rather, been reported).

The special circumstances that would be needed are shown in the beautiful sketch below:

Geometry of Compton scattering for hard X-rays in the solar corona. We envision a flare (blue loop, red footpoints) viewed over the limb. The lines-of-sight represent observers 10 degrees and 20 degrees around the limb. The black squiggles represent an unrelated prominence visible at the 20-degree occultation, which conceals the flare loops. geometry

The significance of this geometry is the following: our hard X-ray imaging instruments suffer from poor image dynamic range (contrast), in the sense that the brighter the object is, the higher the background level and its fluctuations. The result is fog-like. But, for a flare hidden behind the limb, this fog would not be present and we could see pure scattering sources, in principle. Of course there would need to be over-dense features in the solar corona, at sufficient altitudes (as shown), to make this happen.

Why is this interesting?

Hard X-ray polarization represents kind of a last solar observational frontier, because it is so hard to do. Over the years a small community has persisted in trying to get at this diagnostic tool, and now there is a chance with RHESSI in orbit. The occulted sources could approach 100% polarization degree and thus would make an ideal test for this capability. More than this, even without the polarization signature, the scattered X-rays will compete (albeit weakly, diluted by about four orders of magnitude!) with direct hard X-ray emissions.

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September 27, 2002

Hugh Hudson hhudson@ssl.berkeley.edu