B. Kozlovsky

Nuclear gamma-rays from energetic particle interactions

Gamma ray line emission from nuclear deexcitation following energetic particle reactions is evaluated. The compiled nuclear data and the calculated gamma ray spectra and intensities can be used for the study of astrophysical sites which contain large fluxes of energetic protons and nuclei. A detailed evaluation of gamma ray line production in the interstellar medium is made.

Light Elements and Cosmic Rays in the Early Galaxy

Observations of Be and B in low-metallicity halo stars formed during the first 109 yr of Galactic evolution show that cosmic-ray acceleration must have taken place in the early Galaxy. The observed abundances of these elements relative to Fe, which, in the early Galaxy, is almost exclusively produced in Type II supernovae, strongly suggest that the cosmic-ray acceleration is also related to such supernovae with the particles being accelerated out of freshly nucleosynthesized matter before it mixes into the ambient, essentially nonmetallic interstellar medium. The observed abundances require that about 3 × 1049 to 2 × 1050 ergs per Type II supernova be imparted to these metallic cosmic rays, depending on whether or not H and He are accelerated along with the metals. The current data, however, are not sufficient to decide whether these cosmic rays are predominantly low energy or high energy. But, in any case, arguments of energetics imply a hard-energy spectrum extending up in energy to at least 50 MeV nucleon-1. This rules out Be and B production by supernova ejecta without further acceleration. In addition to production by cosmic rays, there must also be significant 11B production by neutrinos. This argument is driven by the observed 11B/10B ratio in meteorites that is very difficult to reproduce by cosmic-ray interactions. Observations of 6Li and Li in the early Galaxy provide information on the acceleration of nonmetallic cosmic rays out of the interstellar medium.

Solar Atmospheric Abundances and Energy Content in Flare-accelerated Ions from Gamma-Ray Spectroscopy

We used SMM gamma-ray data from 19 solar flares to study ambient elemental abundances in the solar atmosphere. We found that the abundance ratios of low FIP (first ionization potential) to high FIP elements are enhanced relative to photospheric abundances, but that the variability of these ratios from flare to flare is limited to a narrower range than that inferred from EUV and X-ray observations. The mean of the gamma-ray derived Mg/O (a low FIP to high FIP element abundance ratio) is coronal and the individual values are always higher than the photospheric Mg/O. The value of Ne/O (~0.25) is higher than the coronal value of 0.15 obtained from solar energetic particle data, but not inconsistent with some EUV and X-ray determinations. To avoid Ne/O higher than 0.3 a steep accelerated particle energy spectrum extending down to about 1 MeV per nucleon is needed. This implies that a large fraction of the available flare energy is contained in accelerated ions.

Nuclear Deexcitation Gamma-Ray Lines from Accelerated Particle Interactions

Total cross sections for the production of gamma-ray lines from nuclear deexcitation as a function of the projectile energy are evaluated and presented. Included are proton and α reactions with He, C, N, O, Ne, Mg, Al, Si, S, Ca, and Fe. Such functions are essential for interpretation of gamma-ray line observations of astrophysical sites which contain large fluxes of energetic particles such as solar flares, the Earths atmosphere, planetary atmospheres and surfaces, the interstellar medium, and galactic nebulae.

Light-Element Evolution and Cosmic-Ray Energetics

Using cosmic-ray energetics as a discriminator, we investigate the viability of evolutionary models for the light elements Li, Be, and B (LiBeB). We find that models in which the cosmic rays are accelerated mainly out of the average interstellar medium which is increasingly metal-poor at early times significantly underpredict the measured Be abundance of the early Galaxy, the possible increase in [O/Fe] with decreasing [Fe/H] indicated by some recent data notwithstanding. On the other hand, if the cosmic-ray metals are accelerated primarily out of supernova ejecta-enriched superbubbles, such that the cosmic-ray source composition as a function of [Fe/H] remains similar to that of the current epoch, the measured Be abundances are consistent with a cosmic-ray acceleration efficiency that is in very good agreement with the current epoch data. This model requires the incorporation of neutrino-produced 11B. We show that, even though the production histories of the cosmic-ray-produced B and Be and the neutrino-produced 11B are different, B/Be can remain essentially constant as a function of [Fe/H]. We also find that neither the above cosmic-ray origin models nor a model employing low-energy cosmic rays originating from the supernovae of only very massive progenitors can account for the 6Li data at values of [Fe/H] below -2.

POSITRON-EMITTER PRODUCTION IN SOLAR FLARES FROM ^3He REACTIONS

We treat in detail positron production from the decay of radioactive nuclei produced in nuclear reactions of accelerated 3He. Because of their large cross sections and low threshold energies, these reactions can significantly contribute to positron production in solar flares with accelerated-particle compositions enriched in 3He. The addition of these 3He reactions extends earlier calculations of positron production by accelerated protons and α-particles. 3He reactions not only add significantly to the total positron yield in flares, but can also yield a positron depth distribution that peaks higher in the solar atmosphere. We discuss the impact these reactions have on the analysis of the annihilation line observed with RHESSI from the 2002 July 23 flare. A significant contribution from 3He reactions expands the utility of the annihilation line as a sensitive tool for investigating the structure of the flaring solar atmosphere.

Angular and Energy-dependent Neutron Emission from Solar Flare Magnetic Loops

We have developed new neutron production kinematics and thoroughly updated the neutron production cross sections, and we have included ion pitch-angle scattering and magnetic mirroring in our Monte Carlo simulation programs, to make new calculations of anisotropic neutron emission produced in the solar flare magnetic loop models. The anisotropy in these models arises from the combined effects of converging magnetic field lines and a rapidly increasing ambient density in the portion of the loop below the chromosphere-corona transition. We have carried out new calculations of the depth, time, angle, and energy dependences of the neutron production, the angle distributions and energy spectra of the escaping neutrons, and the energy spectrum of the surviving neutrons at the distance 1 AU from the Sun. These new calculations will now allow much more reliable and detailed analyses of the various solar flare neutron spectral observations.

Acceleration in solar flares: Interacting particles versus interplanetary particles

Abstract We study the properties of flare accelerated particles using gamma ray observations. We derive ion and electron energy spectra, electron to proton ratios, and numbers of interacting particles. We investigate the effects of the abundance variations implied by the gamma ray data on these parameters. We compare the results with interplanetary observations of solar flare particles.

Solar atmospheric abundances from gamma ray spectroscopy

We used SMM gamma ray data from 19 solar flares to study ambient elemental abundances in the solar atmosphere. We found that the abundance ratios of low FIP (first ionization potential) to high FIP elements (Mg/O, Si/O, Fe/O) are enhanced relative to photospheric abundances and may vary around their respective coronal values. For the high FIP elements (C, O, Ne) we showed that: (i) The gamma ray data allows a good determination of the C to O abundance ratio; the data are consistent with a C/O which does not vary from flare to flare; and the best fit value is C/O=0.4. (ii) The derived value of Ne/O (∼0.25) is higher than the coronal value of 0.15 obtained from solar energetic particle data and some EUV and X‐ray observations of photospheric material. To avoid Ne/O higher than 0.3 a steep accelerated particle energy spectrum extending down to about 1 MeV/nucl is needed. This implies that a large fraction of the available flare energy is contained in accelerated ions.

Using Gamma-Ray and Neutron Emission to Determine Solar Flare Accelerated Particle Spectra and Composition and the Conditions within the Flare Magnetic Loop

The measurable quantities associated with γ-ray and neutron observations of solar flares are nuclear-deexcitation line shapes, shifts, fluences, and time histories; neutron capture and annihilation line fluences and time histories; and energy-dependent escaping neutron fluence and time history. A comprehensive understanding of these quantities requires a model for ion acceleration, transport, and interaction. In this paper we address transport and interaction using a magnetic loop model that includes energy losses due to Coulomb collisions, removal by nuclear reactions, magnetic mirroring in the convergent flux tube, and MHD pitch-angle scattering in the corona. The accelerated ions are assumed to have a given kinetic energy spectrum and composition. Each measurable quantity depends to varying degree on the parameters of the loop model and of the accelerated ions. We explore these dependences in detail and construct a self-consistent approach to the analysis of high-energy flare data that provides an optimum set of parameters with meaningful uncertainties. To illustrate this approach, the calculations are applied in a comprehensive analysis of the γ-ray and neutron observations of the 1991 June 4 solar flare obtained with OSSE on CGRO. We find that the loop model can account for these observations with physically reasonable values for the parameters. In addition, our analysis of the neutron data shows that the accelerated ion spectrum for this flare was not an unbroken power law but had to steepen sharply above ~125 MeV nucleon-1. The paper also provides yields and yield ratios calculated with assumed abundances and spectral forms currently considered appropriate for solar flares. They can be used by other researchers analyzing high-energy solar flare data.