The Role of Nuclei-Nuclei Interactions in the Production of Gamma-Ray Lines in Some Solar Flares

1. The Role of Nuclei-Nuclei Interactions in the Production of Gamma-Ray Lines in Some Solar Flares Boris M. Kuzhevskij (1), Wei-Qun Gan (2), and Leonty I. Miroshnichenko (3, 4) (1) SINP, Moscow State University, Moscow, RUSSIA (2) Purple Mountain Observatory (PMO), Nanjing, CHINA (3) Instituto de Geofísica, UNAM, MEXICO, leonty@geofisica.unam.mx(4) N.V. Pushkov Instiute IZMIRAN, Troitsk, Moscow, RUSSIA Chinese Journal of Astronomy and Astrophysics, 2005, v.5, No.3, p.295-301

2. Abstract • Dramatic extensions of experimental possibilities (spacecraft RHESSI, CORONAS-F, INTEGRAL and others) in solar gamma-ray astronomy call for urgent and detailed consideration of a set of physical problems of solar activity and solar-terrestrial relationships that earlier may have only been outlined. • We undertake a theoretical analysis of issues related to the production of gamma-radiation in the processes of interactions of SEPs - energetic (accelerated) heavy and middle nuclei with the nuclei of the solar atmosphere (the so-called heavy-heavy, or ij-interactions). We also make an estimate of the contribution of these interactions to the formation of nuclear and isotopic abundances of the solar atmosphere in the range of light and rare elements. • The analysis is curried out for SEP spectra in the wide range of their spectral indices. We compare our theoretical estimates with RHESSI observations for the flare of 23 July 2002. It was shown that the 24Mg gamma-ray emission in this event was produced by the newly generated 24Mg nuclei. With a high probability, the gamma-ray line emission of 28Si from this flare was generated by the same processes.

3. Relative abundances of some elements in the solar atmosphere and cross sections of i-j and p-k interactions Variant (a), Aller (1963): N(O)/N(Mg) = 36.4; N(C)/N(Mg) = 20.8; Variant (b), Cameron (1973): N(O)/N(Mg) = 22.2; N(C)/N(Mg) = 13.3 ________________________________________________ Cross sections of i-j interactions (0.73-7.2 MeV/n): 250-300 mb (Dyer et al., 1981; Kuzhevskij, 1985). Cross sections of p-k interactions (5.0-30 MeV): 300-550 mb (Dyer et al., 1981; Kuzhevskij, 1985). Ratios of gamma-ray fluxes approach to ~1 for spectral index S ≥3.

4. Table 1. Contribution of 12C and 16O Nuclei into Gamma-Ray Flux from Excited 24Mg Nuclei _______________________________________ S 2 3 4 5 6 7 _______________________________________ G(C) 0.016 0.20 1.70 15.4 25.90 154 G(C+O) 0.035 0.40 3.40 30.0 50.00 300 _______________________________________ G is a ratio of gamma-ray fluxes produced by p-k and i-j interactions; S – power-law index.

5. Table 2. Ratios of Gamma-Ray Flux from 12C nuclei to that from Other Nuclei Ratios Experiment Calculations for p-k and • (23 July 2002) alpha-k interactions ____________________________________________________ 12C/24Mg 1.01, max 1.90 10.40 (a); 4.40 (b) 12C/20Ne 1.34, max 2.47 2.65 (a); 1.53 (b) 12C/28Si 1.67, max 3.31 31.0 (a); 12.0 (b) 12C/56Fe 3.81, max 8.02 30.00 (a); 2.60 (b) ____________________________________________________ Observational RHESSI data are taken from Smith et al. (2003) ___________________________________

6. Conclusions • Our analysis of the gamma-ray line fluences from the RHESSI flare of 23 July 2002 confirms that i-j interactions are very important for nuclei whose initial abundances in the solar atmosphere are small. Amongst them are light elements Li, Be, and B, as well as some rare elements, e.g., Na, and even Mg, Si. • The role of i-j interactions grows, especially, in cases where the SEP has a soft spectrum (S≥3). Note that such a situation is very often encountered in processes of particle acceleration at/near the Sun.

7. Implications and Prospects • The nuclei of 24Mg and 28Si may be effectively produced in the solar active region during the flare due to i-j interactions, e.g., between C-O and O-O. • The contribution of C-O interactions to the generation of 20Ne is small, and the 56Fe nucleus cannot be created due to C-O and/or O-O interactions. • Standard solar composition (Grevesse & Sauval, 1998) does not change our preliminary conclusion about the role of i-j interactions in the production of gamma-ray lines from 24Mg and 28Si nuclei.

8. ACKNOWLEDGEMENTS This work was supported partly by the Russian Foundation for Basic Research (RFBR, projects 02-02-39032, 03-02-96026), Federal Purpose Scientific and Technical Program Section I, Project 4), and President’s Grant of Russian Federation (project 1445.2003.2). The work by W. Gan was supported by the National Natural Science Foundation (NNSFC) of China via grants 10173027, 10221001, 10333040 and by grant G2000078402 from the Ministry of Science and Technology of China.

9. Acknowledgements • This work was greatly inspired by and carried out due to enormous efforts of Prof. Boris M. Kuzhevskij (SINP MSU, Moscow) who drastically passed away on 28 February 2005. His contribution to the investigation of different aspects of solar gamma rays remains very significant.

10. Important references • B.M. Kuzhevskij. Nuclear Processes in Solar Atmosphere and Solar Cosmic Radiation. Moscow, Energoatomizdat, 1985. • N. Grevesse & A.J. Sauval. Space Sci. Rev., 85, 161 (1998). • B. Kozlovsky, R.J. Murphy, and R. Ramaty. Ap. J. Suppl., 141, 523 (2002). • D.M. Smith, G.H. Share, R.J. Murphy et al. (in all 6 authors). Ap. J. Lett., 595, L81(2003).