Natalie Mandzhavidze

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.

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.

Implications of solar flare charged particle, gamma ray and neutron observations: Rapporteur Paper II for the High Energy Solar Physics Workshop

Solar flares provide a unique site for the study of energy release and particle acceleration in astrophysics. This paper summarizes the charged particle and gamma ray aspects of the Workshop on High Energy Solar Physics held at the Goddard Space Flight Center in August 1995. The X‐ray aspects of the Workshop are summarized in another paper. Here are our highlights: (i) both the charged particle, and now also gamma ray observations have shown that stochastic acceleration due to gyroresonant interactions with plasma waves is very important in solar flares; (ii) CME driven shocks accelerate the particles observed in space from gradual flares; (iii) accelerated MeV/nucl ions can contain a large fraction of the energy released in flares; (iv) GeV ions are either trapped at the Sun or accelerated for hours; (v) nuclear gamma ray spectroscopy has become a tool for abundance determinations in the solar atmosphere; (vi) behind‐the‐limb flares can provide imaging information for solar gamma ray emission.

Determination of the Abundances of Subcoronal 4He and of Solar Flare-accelerated 3He and 4He from Gamma-Ray Spectroscopy

A series of narrow gamma-ray lines at 0.339, 0.429, 0.478, 0.937, 1.00, 1.04, 1.05, and 1.08 result almost exclusively from the bombardment of ambient He, O, and Fe by accelerated α-particles and 3He nuclei. Study of these lines, combined with the 56Fe line at 0.847 MeV and the 16O line at 6.129 MeV, allows the determination of the abundances of α-particles and 3He accelerated in solar flares and of the ambient He in the gamma-ray production region in the solar atmosphere. Using the Solar Maximum Mission/GRS and Compton Gamma Ray Observatory/OSSE data for 20 flares, we find that with significance not exceeding about 2.5 σ there are flares that exhibit α-particle abundance enhancements (α/p>0.1), show evidence for the presence of accelerated 3He, and indicate ambient He abundance enhancements. In some cases the accelerated 3He/4He~1 , and for essentially all of the flares 3He/4He could be 0.1, consistent with our earlier conclusion that in both impulsive and gradual flares the particles that interact and produce gamma rays are always accelerated by the same mechanism that operates in impulsive flares, namely, stochastic acceleration through gyroresonant wave-particle interactions. The ambient He abundance enhancements suggest that there are chromospheric regions where He/O exceeds its photospheric value, a result that could have important implications for solar atmospheric dynamics.

Gamma rays from pion decay : evidence for long-term trapping of particles in solar flares

The authors analyze the energy spectrum and time dependence of the 50 MeV to 2 GeV gamma rays observed from the 1991 June 11 solar flare. It is shown that the emission detected at the late phase of this flare with EGRET on the COMPTON Observatory can be best explained by a model in which the bulk of the particles were accelerated during the impulsive phase and subsequently trapped in a coronal magnetic loop. The authors fit the observed spectrum with a combination of pion decay radiation and primary electron bremsstrahlung. The trapping of the ions and relativistic electrons in the loop on time scales of hours requires that the level of the plasma turbulence and strength of the ambient coronal magnetic field be sufficiently low. The comparison of the 1991 June 11 data with data available for the 1982 June 3 and 1991 June 15 flares indicates that all three of these flares probably produced pions under similar conditions.

High-energy gamma-ray emission from pion decay in a solar flare magnetic loop

The production of high-energy gamma rays resulting from pion decay in a solar flare magnetic loop is investigated. Magnetic mirroring, MHD pitch-angle scattering, and all of the relevant loss processes and photon production mechanisms are taken into account. The transport of both the primary ions and the secondary positrons resulting from the decay of the positive pions, as well as the transport of the produced gamma-ray emission are considered. The distributions of the gamma rays as a function of atmospheric depth, time, emission angle, and photon energy are calculated and the dependence of these distributions on the model parameters are studied. The obtained angular distributions are not sufficiently anisotropic to account for the observed limb brightening of the greater than 10 MeV flare emission, indicating that the bulk of this emission is bremsstrahlung from primary electrons.

Solar Atmospheric and Solar Flare Accelerated Helium Abundances from Gamma-Ray Spectroscopy

From the measured fluences of αα and narrow gamma-ray lines we derive He abundances (He/O) in the gamma-ray production region of solar flares, most likely subcoronal regions located above the photosphere. The αα line is a spectral feature due to de-excitations in 7Li and 7Be produced by the interactions of α-particles with He. We find that the derived values of He/O are consistent with the photospheric He/O only if the accelerated particle α/p 0.5. More conventional values (α/p 0.1) imply a higher He abundance in the subcoronal regions than in the photosphere. Indications for the existence of such He-enriched regions in the solar atmosphere are obtained from solar wind observations. We show how future high-resolution observations with Ge detectors could distinguish between the two possibilities of high α/p or high He/O (and hence high He/H) values by resolving an already observed spectral feature at ~1.02 MeV. This feature is dominated by lines produced exclusively in α-particle interactions with 56Fe and 3He interactions with 16O. In addition to the determination of α/p and the ambient He abundance, this will also determine the accelerated 3He/4He and its time development as the acceleration progresses. The relative intensities of the αα and narrow de-excitation lines confirm our previous finding that in the gamma-ray production region the abundance of Ne exceeds its measured coronal and generally accepted photospheric value. Thus, the subcoronal abundances of the two elements with the highest first ionization potential, He and Ne, are higher than their corresponding coronal abundances.

Theoretical models for high‐energy solar flare emissions

We discuss gamma ray production mechanisms and ion transport processes in solar flares. We investigate the implications of the extended GeV gamma ray emission observed from the 11 June and 15 June 1991 flares. We find that this extended emission could be produced by ions trapped in loops, provided there is a suitable combination of size, twist, field convergence, and turbulent energy density. We also consider in some detail the possibility of continuous acceleration by Alfven turbulence. We find that this would require the continuous presence of turbulence with energy density of at least 1 erg cm−3. The strong pitch angle scattering caused by this turbulence leads to anisotropic pion decay emission with much steeper spectrum than observed. We discuss various alternatives, including the possibility of episodal acceleration.

The Giant 1991 June 1 Flare: Evidence for Gamma-Ray Production in the Corona and Accelerated Heavy Ion Abundance Enhancements from Gamma-Ray Spectroscopy

We investigated the implications of the gamma-ray line emission observed with Granat/PHEBUS from the behind-the-limb flare on 1991 June 1. We showed that thin target interactions are required to account for the very high observed ratio of the nuclear line emission in the 1.1-1.8 MeV and 4.1-7.6 MeV energy bands, which are populated predominantly by gamma rays from de-excitations of Ne-Fe and C-O, respectively. We found that the composition of the accelerated particles that produce gamma rays in this extremely powerful gradual flare clearly shows the heavy-element abundance enhancements characteristic of acceleration via resonant wave-particle interactions. Furthermore, the heavy-element abundance enhancements increase with time, reaching, toward the end of the flare, the highest values observed in space from impulsive solar energetic particle events. We showed that the energy deposited in the coronal interaction region is comparable to the largest total accelerated particle energy content in previously observed gamma-ray flares. However, this deposited energy is small in comparison with the energy content in greater than 20 keV electrons inferred from hard X-ray observations with Ulysses for which the 1991 June 1 flare was in full view. Approximate energy equipartition between accelerated electrons and ions then implies that the total ion energy content is much larger than the portion deposited in the corona, requiring a short ion residence time τ and a low ambient density nH in the coronal interaction region (nHτ 2.5 × 109 cm-3 s).