Gamma-rays (and white light)

3.7 Gamma-rays (and white light)

Protons are also accelerated during the flare process. They can penetrate deeply into the solar atmosphere, into layers which are usually not involved with the flare. These protons can excite both: white light emission and Gamma-rays. Proton energies exceeding some 20 keV are required. This explains why only the really big flares are visible in white light (remember Carrington’s observation in 1859) and Gamma-rays.

There are at least six different physical processes that contribute to Gamma-ray emission: 1) electron bremsstrahlung continuum emission, 2) nuclear de-excitation line emission, 3) neutron capture line emission, 4) positron-electron annihilation line emission at 0.511 MeV, 5) pion-decay radiation at > 50 MeV (pions may be generated whenever protons are accelerated into the 1 GeV energy range), and 6) neutron production. The strongest lines in the Gamma-ray spectrum of flares are the positron-electron annihilation line at 511 keV and the 2.223 MeV line produced when neutrons are captured by protons (see, e.g. Ramaty et al. (2002 ); Lin et al. (2003 ) and Figure 14 ). Note that both the positrons and the neutrons must first be produced in the flare process itself. The hardest Gamma line detected so far is the 6.129 MeV line due to ¹⁶O nuclei de-excitation. The electron bremsstrahlung continuum was found with energies up to some GeV (Kanbach et al., 1993 ). It consists of two types: from electrons directly accelerated in the flare and from secondary electrons and positrons released in high-energy reactions (involving pion decay and muon production). For further details see, e.g. Rieger and Rank (2001 ).

Figure 14:

RHESSI Gamma-ray count spectrum from 0.3 to 10 MeV, integrated over the interval 0027:20–0043:20 UT. The lines show the different components of the model used to fit the spectrum. From Lin et al. (2003 ).