Ronald J. Murphy

Energy partition in two solar flare/CME events

Using coordinated observations from instruments on the Advanced Composition Explorer (ACE), the Solar and Heliospheric Observatory (SOHO), and the Ramaty High Energy Solar Spectroscopic Imager (RHESSI), we have evaluated the energetics of two well-observed flare/CME events on 21 April 2002 and 23 July 2002. For each event, we have estimated the energy contents (and the likely uncertainties) of (1) the coronal mass ejection, (2) the thermal plasma at the Sun, (3) the hard X-ray producing accelerated electrons, (4) the gamma-ray producing ions, and (5) the solar energetic particles. The results are assimilated and discussed relative to the probable amount of nonpotential magnetic energy available in a large active region.

High-energy processes in solar flares

A detailed study of high-energy processes in solar flares is reported, including the production of neutrons and pions, and incorporating isobaric and scaling models and a recent compilation of pion production data (Dermer, 1986). The broad-band gamma-ray spectrum resulting from the decay of neutral pions, the bremsstrahlung of positrons and electrons from the decay of charged pions, and the annihilation in flight of positrons is evaluated. Also evaluated is the 0.511 MeV gamma-ray line resulting from the annihilation of the positrons which survive annihilation in flight. Calculations were based on an isotropic, thick-target model using the best available nuclear data and models. Results are compared with extensive observation of the June 3, 1982 flare (10-120 MeV gamma rays), 0.511 and 2.2 MeV line emission, nuclear line emission, high-energy neutrons, and interplanetary charged particles. 75 references.

Observations of GRB 990123 by the Compton gamma ray observatory

GRB 990123 was the first burst from which simultaneous optical, X-ray, and gamma-ray emission was detected; its afterglow has been followed by an extensive set of radio, optical, and X-ray observations. We have studied the gamma-ray burst itself as observed by the Compton Gamma Ray Observatory detectors. We find that gamma-ray fluxes are not correlated with the simultaneous optical observations and that the gamma-ray spectra cannot be extrapolated simply to the optical fluxes. The burst is well fitted by the standard four-parameter GRB function, with the exception that excess emission compared with this function is observed below ~15 keV during some time intervals. The burst is characterized by the typical hard-to-soft and hardness-intensity correlation spectral evolution patterns. The energy of the peak of the νfν spectrum, Ep, reaches an unusually high value during the first intensity spike, 1470 ± 110 keV, and then falls to ~300 keV during the tail of the burst. The high-energy spectrum above ~1 MeV is consistent with a power law with a photon index of about -3. By fluence, GRB 990123 is brighter than all but 0.4% of the GRBs observed with BATSE, clearly placing it on the - power-law portion of the intensity distribution. However, the redshift measured for the afterglow is inconsistent with the Euclidean interpretation of the - power law. Using the redshift value of ≥1.61 and assuming isotropic emission, the gamma-ray energy exceeds 1054 ergs.

Gamma-Ray Measurements of Flare-to-Flare Variations in Ambient Solar Abundances

We have measured uxes of ten narrow-ray lines in 19 X-class solar ares observed by the Solar Maximum Mission spectrometer from 1980 to 1989. These lines originate from interaction of energetic protons and-particles with ambient solar material. Flare-to-are variations in line uxes reveal that the abundances of elements in the are plasma are grouped with respect to their rst ionization potentials (FIP). Line uxes from elements with similar FIPs correlate well with one another; in contrast, the low-FIP (< 10 eV; Mg, Si, Fe) to high-FIP (>11 eV; C, N, O) line ratios vary by as much as about a factor of four from are to are. This factor of four is consistent with the enhancement of low-FIP elements found in the solar corona relative to photospheric abundances. We also nd that the Ne/O line ratio increases as the accelerated particle spectrum becomes softer. This can be explained by excitation cross sections <10 MeV, but the magnitude of the eeect is dependent on the accelerated-particle/proton ratio. After correcting for this spectral dependence, we nd that the Ne/(C + N + O) line ratio is constant from are-to-are, indicating that Ne behaves like these other high-FIP elements in the ambient are plasma. Based on these observations, we discuss a possible explanation for the factor of 3 enhancement in the abundance of Ne, relative to other high-FIP elements, in the 1981 April 27 are found by Murphy et al. (1991).

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.

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.

Accelerated Particle Composition and Energetics and Ambient Abundances from Gamma-Ray Spectroscopy of the 1991 June 4 Solar Flare

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 γ-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 × 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.

The Solar Maximum Mission Atlas of Gamma-Ray Flares

We present a compilation of data for all 258 gamma-ray —ares detected above 300 keV by the Gamma Ray Spectrometer (GRS) aboard the Solar Maximum Mission satellite. This gamma-ray —are sample was collected during the period from 1980 February to 1989 November; covering the latter half of the 21st solar sunspot cycle and the onset of the 22d solar sunspot cycle. We describe the SMM/GRS instrument, its in-orbit operation, and the principal data reduction methods used to derive the gamma-ray —are properties. Utilizing measurements for 185 —ares that were sufficiently intense to allow the derivation of gamma-ray spectra, we present an atlas of time pro—les and gamma-ray spectra. The —are parameters derived from the gamma-ray spectra include bremsstrahlung —uence and best-—t power-law parameters, narrow nuclear line —uence, positron annihilation line —uence, neutron capture line —uence, and an indi- cation of whether or not emissions greater than 10 MeV were present. Since a uniform methodology was adopted for deriving the parameters, this atlas should be very useful for future statistical and correlative studies of solar —ares. Subject headings: catalogsgamma rays: burstsSun: —ares

High-Resolution Spectroscopy of Gamma-Ray Lines from the X-Class Solar Flare of 2002 July 23

The Reuven Ramaty High Energy Solar Spectroscopy Imager (RHESSI) has obtained the first high-resolution measurements of nuclear de-excitation lines produced by energetic ions accelerated in a solar flare, a GOES X4.8 event occurring on 2002 July 23 at a heliocentric angle of ~73°. Lines of neon, magnesium, silicon, iron, carbon, and oxygen were resolved for the first time. They exhibit Doppler redshifts of 0.1%-0.8% and broadening of 0.1%-2.1% (FWHM), generally decreasing with mass. The measured redshifts are larger than expected for a model of an interacting ion distribution isotropic in the downward hemisphere in a radial magnetic field. Possible interpretations of the large redshifts include (1) an inclination of the loop magnetic field to the solar surface so that the ion distribution is oriented more directly away from the observer and (2) extreme beaming of the ions downward along a magnetic field normal to the solar surface. Bulk downward motion of the plasma in which the accelerated ions interact can be ruled out.