Energetic ions

3.5 Energetic ions

Gamma-ray lines between 0.8 and 20 MeV are emitted atomic nuclei excited by impinging ions. The neutron-capture line at 2.223 MeV (forming deuterium) is usually the strongest of the many lines (Figure 20

). Protons and other nuclei accelerated from about 10 MeV to $∼>$ 100 MeV per nucleon collide with the nuclei in the dense chromosphere and produce neutrons. Neutrons thermalize and, after traveling less than 1000 km, are captured by an ambient proton to form deuterium (Ramaty and Kozlovsky, 1974 ; Hua and Lingenfelter, 1987 ). Thus images in the 2.223 MeV line indicate the site of high-energy ion precipitation. Comparison of imaged and spatially integrated fluences observed by the RHESSI satellite show that in all cases most of the emission was confined to compact sources within the active region (Hurford et al., 2006

). In some events double footpoints were observed. Thus the flare associated gamma-rays are produced by ions accelerated in the flare process and not by a large-scale shock driven by a coronal mass ejection.

Surprisingly, RHESSI found that the footpoints of the 2.223 MeV line – indicating ion precipitation – and the footpoints of the non-thermal continuum emission – produced by precipitating electrons – do not coincide (Figure 21 ). The discrepancy demonstrates that protons and electrons are accelerated differently, or originate, as proposed by Emslie et al. (2004b ), in large and small loops, respectively.

Figure 20:

RHESSI gamma-ray spectrum from 0.3 to 10 MeV integrated over the duration of the flare of July 23, 2002. The lines show the different components of the model used to fit the spectrum (Lin et al., 2003 ).

Figure 21:

Location of the gamma-ray sources observed by the RHESSI satellite on October 28, 2003. The contours at 50%, 70%, and 90% of the peak value show in blue the deuterium recombination line at 2.223 MeV and red the electron bremsstrahlung at 200 – 300 keV. The centroid positions of the bremsstrahlung emission at different times are indicated by plus signs. The FWHM angular resolution is 35”, given at bottom left. The RHESSI data are overlaid on the negative of a TRACE 195 Å image dominated by the emission of Fe xii (Hurford et al., 2006 ).