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

G.H. Share, R.J. Murphy, A.J. Tylka (NRL) B.R. Dennis, R.A. Schwartz (GSFC) R.P. Lin, D.M. Smith (UCB)

The RHESSI high-resolution spectrometer detected gamma-ray lines and continuum emitted by the earth's atmosphere during impact of solar energetic particles from hours UT on 21 April The particle intensity at the time of the observation was a factor of 10 to 100 times 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 SEP 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 on 15 July 2000.

Earth’s Atmosphere Glows in  -Rays Atmospheric  rays are produced by interactions of cosmic rays and SEPs in the Earth’s atmosphere. Protons from the 1989 October 20 solar energetic particle event increased the gamma-ray line flux observed by SMM by over a factor of 100.

SEP-Produced Atmospheric Lines Observed by SMM

Comparison of >10 MeV Proton Fluxes in 3 SEP Events GOES integral proton fluxes >10 MeV for 3 SEP events detected in atmospheric  rays. The flux at the time of the 21 April 2002 RHESSI observations was below the fluxes of the SMM (1989) and Yohkoh (2000) observations.

RHESSI 4-7 MeV Rates 21 April April 2002 Comparison of rates in the 4-7 MeV band observed by RHESSI at the same high magnetic latitudes during the SEP event on 21 April and 2 days earlier. The spectrum accumulated between the lines on 19 April was used to remove background from the 21 April SEP event.

RHESSI Background and SEP Spectra a) Overall RHESSI background spectrum b) Typical spectrum observed at high latitude when no SEP present. c)RHESSI spectrum of 21 April 2002 SEP event showing nuclear line features in the earth’s atmosphere.

Fits to the RHESSI 511 and ~720 keV Lines The atmospheric annihilation line is the strongest feature in the spectrum and originates from positrons produced in  + decays and showers. The feature near ~720 keV is fit with lines at 718 keV from spallation product 10 B and the 728 keV 14 N de-excitation line.

The RHESSI 1.6 and 2.3 MeV 14 N Lines The feature near 1585 keV is instrumental and partly masks the atmospheric line at 1635 keV from the transition from the 2 nd excited state to the ground state of 14 N. The strong 2313 keV line comes from the first excited state of 14 N.

Fits to the RHESSI High-Energy Lines There may be as many as 10 lines in the 3.5 to 8 MeV region of the spectrum. Three lines that are well defined in the RHESSI spectrum are at 3890 and 5106 keV from 14 N and at 4439 keV, primarily from spallation product 12 C.

Comparison of Line Energies and Widths RHESSI width and energy measurements are a significant improvement over the SMM measurements in spite of weakness of the SEP event. They are in good agreement with the HEAO-3 measurement of lines produced by cosmic-ray interactions.

SEP Spectra Derived from Line Flux Ratios Comparison of 2.31 MeV de-excitation and 4.44 MeV spallation line fluxes provides information on the spectrum of SEP protons that impact the atmosphere. The measurements are consistent with a softer proton spectrum during the 20 October 1989 shock event. We have used the solar  ray production code (Kozlovsky, Murphy and Ramaty, 2002) to show that the line ratios for the three events are consistent the measured proton spectra in space (paper in progress).