RHESSI Observation of Atmospheric Gamma Rays from Impact of Solar Energetic Particles on 21 April 2002 |
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| Authors: | Share GH Murphy RJ Dennis BR Schwartz RA Tolbert AK Lin RP Smith DM |
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| Institution: | (1) Naval Research, E.O. Hulburt Center for Space Research, USA;(2) NASA Goddard Space Flight Center, Greenbelt, MD, 20771, U.S.A;(3) Space Sciences Laboratory and Dep't. of Physics, University of California, USA |
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| Abstract: | The RHESSI high-resolution spectrometer detected γ-ray lines and continuum emitted by the Earth's atmosphere during impact
of solar energetic particles in the south polar region from 16:00–17:00 UT on 21 April 2002. The particle intensity at the
time of the observation was a factor of 10–100 weaker than previous events when gamma-rays were detected by other instruments.
This is the first high-resolution observation of atmospheric gamma-ray lines produced by solar energetic particles. De-excitation
lines were resolved that, in part, come from 14N at 728, 1635, 2313, 3890, and 5106 keV, and the 12C spallation product at ∼ 4439 keV. Other unresolved lines were also detected. We provide best-fit line energies and widths
and compare these with moderate resolution measurements by SMM of lines from an SEP event and with high-resolution measurements
made by HEAO 3 of lines excited by cosmic rays. We use line ratios to estimate the spectrum of solar energetic particles that
impacted the atmosphere. The 21 April spectrum was significantly harder than that measured by SMM during the 20 October 1989
shock event; it is comparable to that measured by Yohkoh on 15 July 2000. This is consistent with measurements of 10–50 MeV protons made in space at the time of the γ-ray observations. |
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