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What Gamma-Ray Deexcitation Lines Reveal about Solar Flares  

Ronald Murphy   Submitted: 2005-03-25 13:36

Gamma-ray emission from solar flares reveals information about the nature of the accelerated particles and about the physical conditions of the medium through which the accelerated particles are transported. In this paper, we present the gamma-ray line production and loop transport models used in our calculations of high-energy emission. We discuss the calculated interaction time history, the depth distribution, the interacting-particle angular distribution, and fluence ratios of the narrow gamma-ray lines. We show the relationship between the g-ray observables and the parameters of the transport and line-production models. For illustration, we use calculations of 4.44 MeV 12C nuclear deexcitation line production. Applications of the calculations to flare observations by both SMM and RHESSI are also presented.

Authors: R.J. Murphy & G.H. Share
Projects: RHESSI

Publication Status: aceepted to Adv. Sp. Res.
Last Modified: 2005-03-25 13:36
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The Physics of Positron Annihilation in the Solar Atmosphere  

Ronald Murphy   Submitted: 2005-03-25 08:34

We compile the most recently measured and calculated cross sections for processes relevant to annihilation of positrons in solar flares and present the final evaluated cross sections. We calculate the 511 keV annihilation line spectrum and relative strength of the 3-gamma continuum for each of the annihilation processes, using a Monte Carlo code that simulates the thermalization of and positronium production by ~MeV positrons. We calculate the thermally-averaged annihilation and positronium production rates and the positronium quenching rates. We apply the results to four specific environments (fully ionized, neutral, partially ionized and a non-LTE model of the quiet solar atmosphere), calculating the relative strengths of each process and the combined total annihilation line spectrum. The results are compared with data obtained recently from the high spectral-resolution detectors of RHESSI. We find that positron annihilation in solar flares can occur in a wide variety of environments. The cross sections presented here are also useful for evaluation of positron annihilation in other environments such as the interstellar medium.

Authors: R. J. Murphy, G. H. Share, J. G. Skibo & B. Kozlovsky
Projects: RHESSI

Publication Status: submitted to ApJ
Last Modified: 2006-03-15 10:53
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Physical Implications of RHESSI Neutron Capture-Line Measurements  

Ronald Murphy   Submitted: 2003-06-03 13:52

We report high spectral-resolution measurements of the 2.223 MeV neutron capture-line obtained with the Reuven Ramaty High Energy Spectroscopic Imager from the 2002 July 23 solar flare. The time history of this line is affected by both the photospheric 3He abundance and the angular distribution of the interacting flare-accelerated particles producing the neutrons. The measured time history is compared with predicted time histories calculated using a magnetic loop model with a magnetic field perpendicular to the solar surface at the footpoints. The model 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. This is the first analysis of the neutron-capture line using such a physically-based model. The derived constraints on the interacting angular distribution require that the accelerated particles had to suffer pitch-angle scattering during their transport through the coronal portion of the loop. The derived photospheric He/H ratio was not well-constrained, primarily due to uncertainties associated with the measured nuclear deexcitation-line flux used to represent the neutron-production time profile.

Authors: R.J. Murphy, G.H. Share, X.-M. Hua, R.P. Lin, D.M. Smith, and R.A. Schwartz
Projects: RHESSI

Publication Status: In press ApJ Letters
Last Modified: 2003-06-03 13:52
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Nuclear Deexcitation Gamma Ray Lines from Accelerated Particle Interactions  

Ronald Murphy   Submitted: 2002-03-13 13:01

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 Earth's atmosphere, planetary atmospheres and surfaces, the interstellar medium and galactic nebulae.

Authors: Kozlovsky, B., Murphy, R. J., & Ramaty, R.
Projects:

Publication Status: Astrophysical Journal Supplement (accepted)
Last Modified: 2002-10-28 14:21
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Simultaneous Observations of Solar Flares with CGRO/OSSE and Yohkoh/WBS  

Ronald Murphy   Submitted: 2000-02-04 17:04

Yohkoh and the Compton Gamma Ray Observatory are the only satellites currently measuring of gamma radiation from solar flares. In this paper we present preliminary results of a cross-calibration of the Yohkoh/Wide Band Spectrometer (WBS) and the Oriented Scintillation Spectrometer Experiment (OSSE) on CGRO. Except for one flare whose spectrum was very hard, total flux measurements by the two instruments agreed. Differences of narrow-line fluxes are probably attributable to the poorer energy resolution of the Yohkoh/WBS.

Authors: R. J. Murphy, G. H. Share and M. Yoshimori
Projects:

Publication Status: Submitted to the HESSI Workshop Proceedings
Last Modified: 2002-10-28 14:22
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Accelerated Particle Composition and Energetics and Ambient Abundances from Gamma-Ray Spectroscopy of the 1991 June 4 Solar Flare  

Ronald Murphy   Submitted: 1999-11-08 18:02

The Oriented Scintillation Spectrometer Experiment (OSSE) on board the COMPTON Gamma Ray Observatory observed the 1991 June 4 X12+ solar flare, one of the most intense nuclear gamma-ray line flares observed to date. Using these OSSE observations, we have derived time profiles of the various components of gamma-ray emission and obtained information about the accelerated particle spectra and composition and about the ambient plasma at the flare site. The main results are: (1) the nuclear reactions associated with the impulsive phase of the flare continued for at least 2 hours and resulted from ions that were probably continuously accelerated rather than impulsively accelerated and trapped; (2) the total energy in these accelerated ions exceeded the energy in >0.1 MeV electrons; (3) the accelerated α proton ratio was closer to 0.5 than to 0.1; (4) there is evidence for a decrease of the accelerated heavy ion-to-proton ratio as the flare progressed (5) there is evidence for a temporal change in the composition of the flare plasma; (6) the ratio of electron bremsstrahlung to the flux in narrow gamma-ray lines decreased as the flare progressed; (7) the high-energy (>16 MeV) component of the electron spectrum was much more impulsive than the lower-energy ~MeV component; (8) a model-dependent upper limit of 2.3 x 10-5 was obtained for the photospheric 3He/H abundance ratio; and (9) energetic ions may have been present for several hours prior to and following the impulsive phase of the flare.

Authors: R.J. Murphy, G.H. Share, J.E. Grove, W.N. Johnson, R.L. Kinzer, J.D. Kurfess, M.S. Strickman, and G.V. Jung
Projects:

Publication Status: Ap. J., 490, 883 (1997)
Last Modified: 2002-10-29 05:56
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Surprisingly Intense Neutron Emission from a Flare Behind the Limb of the Sun  

Ronald Murphy   Submitted: 1999-11-08 13:03

The Oriented Scintillation Spectrometer Experiment on board the Compton Gamma Ray Observatory observed a strong flux of neutrons from the behind-the-limb flare that occurred on 1991 June 1. This is surprising if the neutrons were produced by thin target interactions in the Sun's corona as suggested by gamma-ray observations made by Granat/PHEBUS of this flare. We compare neutron and gamma-ray observations of the June 1 flare with thick target emissions observed from a flare three days later where the interactions took place in the chromosphere and photosphere. A very hard spectrum for the accelerated particles is required to account for the number of neutrons observed on June 1 if they were produced by thin-target interactions in the corona.

Authors: R.J. Murphy, G.H. Share, K. W. DelSignore, X.-M. Hua
Projects:

Publication Status: Ap. J., 510, 1011 (1999)
Last Modified: 2002-10-29 05:57
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Abstracts by Author
What Gamma-Ray Deexcitation Lines Reveal about Solar Flares
The Physics of Positron Annihilation in the Solar Atmosphere
Physical Implications of RHESSI Neutron Capture-Line Measurements
Nuclear Deexcitation Gamma Ray Lines from Accelerated Particle Interactions
Simultaneous Observations of Solar Flares with CGRO/OSSE and Yohkoh/WBS
Accelerated Particle Composition and Energetics and Ambient Abundances from Gamma-Ray Spectroscopy of the 1991 June 4 Solar Flare
Surprisingly Intense Neutron Emission from a Flare Behind the Limb of the Sun

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