Measurements of Coronal Proton Velocity Distributions

Details Measurements of Coronal Proton Velocity Distributions Measurements of Coronal Proton Velocity Distributions

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmsh21a0298k&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #SH21A-0298

Astronomy and Astrophysics

Astrophysics

7514 Energetic Particles (2114), 7549 Ultraviolet Emissions

Scientific paper

The Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory is being used to measure precise coronal H I Ly-alpha spectral line profiles out to several Doppler half widths. Such observations can be used to reveal the proton velocity distribution along the line-of-sight. Departures from a Maxwellian distribution are believed to be needed for the acceleration of solar energetic particles (SEPs) by coronal mass ejection (CME) shocks. Our initial attempt to measure suprathermal proton velocity distributions has been described by Kohl et al. (2006). We have made considerable additional progress on such measurements since then. Improvements include the following: a much more accurate instrument spectral line profile, an increase in the wavelength range used for the observations, an increase in the statistical accuracy of the observations by increasing the observation time, and inclusion of a background measurement as part of every set of observations. We have also investigated the sensitivity to the detector high voltage, investigated the effects of diffraction in the instrument, determined the stray light effects and the Thompson scattering effects, which both turn out to be small except for scattering of Si III 120.6 nm. That scattered light is out of the primary wavelength range of interest. We believe that we have now demonstrated that UVCS has the sensitivity to distinguish between a Gaussian coronal velocity distribution and a kappa = 4 or smaller distribution. It is generally believed that the required seed particle population needed to produce SEPs of interest with a CME shock would have a velocity distribution with 0.001 to 0.01 of the particles with speeds that exceed 1000 km/s. Assuming a kappa distribution that is symmetric in the tangential plane and Maxwellian in the radial direction, this would correspond to a distribution with kappa = 3.5 or smaller. This paper will report the results of examining a fairly large body of new observations obtained with the new procedure and report the departures from a Maxwellian distribution. It will also report the CME predecessor history of each observation. This work is supported by the National Aeronautics and Space Administration (NASA) under Grant NNX07AL72G to the Smithsonian Astrophysical Observatory. Kohl J. L., Cranmer, S. R., Fineschi, S., Gardner, L. D., Phillips, D. H., Raymond, J. C., and Uzzo, M., Proc. SOHO 17 - 10 Years of SOHO and Beyond (ESA SP-617, July 2006).

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