Solar gamma rays

R. Ramaty*, B. Kozlovsky, R. E. Lingenfelter

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


The theory of gamma-ray production in solar flares is treated in detail. Both lines and continuum are produced. The strongest line predicted at 2.225 MeV with a width of less than 100 eV and detected at 2.24±0.02 MeV, is due to neutron capture by protons in the photosphere. Its intensity is dependent on the photospheric 3He abundance. The neutrons are produced in nuclear reactions of flare accelerated particles which also produce positrons and prompt nuclear deexcitation lines. The strongest prompt lines are at 4.43 MeV from 12C and at ∼6.2 from 16O and 15N. These lines result from both direct excitation and spallation. The widths of individual prompt lines are determined by nuclear kinematics. The width of the 4.43 MeV line is ∼100 keV and that of the 6.2 MeV feature is ∼300 keV. Both these lines have been observed from a solar flare. Other potentially observable lines are predicted at 0.845 and 1.24 MeV from 56Fe, at 1.63 MeV principally from 14N and 20Ne, at 1.78 MeV from 28Si, at ∼5.3 MeV from 15O and 15N, and at 7.12 MeV from 16O. The widths of the iron lines are only a few keV, while those of the other lines are about 100 keV. The only other observed line is at 0.511 MeV from positron annihilation. The width of this line is determined by the temperature, and its temporal variation depends on the density of the ambient medium in the annihilation region. Positrons can also annihilate from the 3S state of positronium to produce a 3-photon continuum below 0.511 MeV. In addition, the lines of 7Li and 7Be at 0.478 keV and 0.431 keV, which have kinematical widths of ∼30 keV, blend into a strong feature just below the 0.511 MeV line. From the comparison of the observed and calculated intensities of the line at 4.4 MeV to that of the 2.2 MeV line it is possible to obtain information on the spectrum of accelerated nuclei in flares. Moreover, from the absolute intensities of these lines the total number of accelerated nuclei at the Sun and their heating of the flare region can be estimated. We find that about 1033 protons of energies greater than 30 MeV were produced in the 1972, August 4 flare. The gamma-ray continuum, produced by electron bremsstrahlung, allows the determination of the spectrum and number of accelerated electrons in the MeV region. From the comparison of the line and continuum intensities we find a proton-to-electron ratio of about 10 to 102 at the same energy for the 1972, August 4 flare. For the same flare the protons above 2.5 MeV which are responsible for the gamma-ray emission produce a few percent of the heat generated by the electrons which make the hard X-rays above 20 keV.

Original languageEnglish
Pages (from-to)341-388
Number of pages48
JournalSpace Science Reviews
Issue number3
StatePublished - Dec 1975
Externally publishedYes


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