RHESSI Observation of Atmospheric Gamma Rays from Impact of Solar Energetic Particles on 21 April 2002

The RHESSI high-resolution spectrometer detected γ-ray lines and continuum emitted by the earth's atmosphere during impact of solar energetic particles from 1600-1700 hours 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. De-excitation lines were resolved that, in part, come from ¹⁴N at 728, 1635, 2313, and 5105 keV, and from ¹¹B and ¹²C spallation products at ~4440 keV. Several other unresolved lines were also detected. We provide best-fit line energies and widths and compare these measurement with atmospheric line measurements made by HEAO 3 and SMM. We use line ratios to estimate the spectrum of solar energetic particles that impact 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 particles made in space at the time of the γ-ray observations.