Polarization evidence for the isotropy of electrons responsible for the production of 5-20 keV X-rays in solar flares

Details Polarization evidence for the isotropy of electrons responsible for the production of 5-20 keV X-rays in solar flares Polarization evidence for the isotropy of electrons responsible for the production of 5-20 keV X-rays in solar flares

May 1984

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984apj...280..440t&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 280, May 1, 1984, p. 440-447.

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28

Polarization Characteristics, Solar Electrons, Solar Flares, Solar X-Rays, Electron Beams, Electron Distribution, Instrument Compensation, Isotropy, Oss-1 Payload, Polarimetry

Scientific paper

We have flown a solar flare X-ray polarimeter on the third flight (STS 3) of the Space Shuttle Columbia as part of the OSS-1 pallet of instruments. We observed eight solar flares in the 5-20 keV band on 1982 March 28. The signal-to-background ratio in all cases exceeded 25. A preflight contamination problem invalidated the earlier laboratory calibration, and the instrument had to be calibrated in-flight against two flares near the center of the solar disk, which are expected to be unpolarized on geometric grounds in a variety of models. No statistically significant polarization was then detected in any of the other six flares. Upper limits (99% confidence level) range from 2.5% to 12.7%. For two of the observed flares these results disagree with the predictions of a simple radially beamed, linear bremsstrahlung model at greater than 99% confidence. One of these flares had a hard impulsive burst; the measured upper limit on this burst (10%) also disagrees with the predictions of the beamed hypothesis. If the calibration flares were polarized, then the above upper limits can be interpreted as limits on the changes in polarization from flare to flare. Because the observed flares spanned a large longitude range and because the predictions of the beamed models depend fairly sensitively on viewing angle, the small relative polarizations are still difficult to reconcile with simple beamed models. The results are also compared with recent, more sophisticated models of Leach and Petrosian, which generally predict lower polarizations. We find that the observations are marginally inconsistent with a model in which the electrons are initially strongly beamed, but subsequently become largely isotropic as a result of the effects of a converging magnetic field; they are consistent with a model in which the electrons are injected isotropically, but in which the preference for motion along the magnetic field lines is explicitly taken into account. The results are also consistent with a model in which the primary photons are taken to be isotropic and unpolarized and are subsequently slightly polarized by backscattering.

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