Nuclear processes and accelerated particles in solar flares

A detailed review of nuclear processes and particle acceleration in solar flares has been completed recently (Ramaty and Murphy, 1987). Included in this review were a comprehensive discussion of the theory of gamma-ray and neutron production, as well as the results of comparisons of calculations with gamma-ray, neutron and charged-particle observations of solar flares. The implications of these comparisons on particle energy spectra, total numbers, anisotropies and electron-to-proton ratios, as well as on acceleration mechanisms and the interaction site were also discussed. In addition, elemental and isotopic abundances of the ambient gas, derived from gamma-ray observations, were compared to abundances obtained from observations of escaping accelerated particles and other sources. The present paper is a synopsis of this review     

Nuclear processes and accelerated particles in solar flares

A detailed review of nuclear processes and particle acceleration in solar flares has been completed recently (Ramaty and Murphy, 1987). Included in this review were a comprehensive discussion of the theory of gamma-ray and neutron production, as well as the results of comparisons of calculations with gamma-ray, neutron and charged-particle observations of solar flares. The implications of these comparisons on particle energy spectra, total numbers, anisotropies and electron-to-proton ratios, as well as on acceleration mechanisms and the interaction site were also discussed. In addition, elemental and isotopic abundances of the ambient gas, derived from gamma-ray observations, were compared to abundances obtained from observations of escaping accelerated particles and other sources. The present paper is a synopsis of this review

     

Nuclear and non-nuclear processes in the production of peculiar A stars

Recent theories of the origin of the abundance anomalies on Ap stars are reviewed. The abundance anomalies are mainly the results of supernova explosions of companion stars and magnetic accretion from the supernova remnants and the interstellar medium. Large rare-earth excesses indicate that magnetic accretion has taken place, while large excesses of Pt and Hg are due to the transfer of material from nearby supernovae. Nuclear reactions involving protons may also have taken place around Ap stars. There are several objections to Michaud's diffusion theory as a comprehensive explanation for Ap abundance anomalies.     

On the fissions of a solid body under influence of tidal force; with application to the problem of twin craters on the moon

The internal strain due to the tidal force in the proximity of a tide-generating body (in the present case, the Moon) is calculated according to the Lord Kelvin theory of Earth tides. The conditions for which uniform elastic sphere possessing a definite tensile strength is crushed near the surface of the Moon is investigated. The state of internal stress is almost independent of the value of elastic constants. Many lunar features, such as twin craters, craterous walled plains of irregular forms, compound craters, may be explained by fission of the meteoritic material before impact.     

Nuclear spectra of polar-ring galaxies

We report the results of spectroscopic observations of eight southern polar-ring galaxies (PRGs), in the wavelength range 5900–7300 Å. We find that five out of eight galaxies contain LINERs or Sy nuclei. Taking into consideration all PRGs with available spectral data, we estimate that about half of all PRGs and PRG candidates have either LINER or Seyfert nuclei. The observed widths of the [N  ii ] λ 6583 line in the nuclei of early-type PRGs follow the linewidth–absolute luminosity relation for normal E/S0 galaxies. We found that one of the observed galaxies – ESO 576-G69 – is a new kinematically-confirmed polar-ring galaxy with a spiral host.     

Surface processes and tectonics: Forcing of continental subduction and deep processes

It is now well accepted that surface processes provide a critical feedback on the surface tectonic deformation, whatever it is, orogenic building or basin evolution. However, the idea that the influence of these processes may go below the crustal levels, is less common. In this preliminary study, we use coupled thermo-mechanical numerical models to investigate the possible influence of surface processes on the styles of continental collision, in particular, continental subduction. For that, we further exploit the recent successful model of continental subduction of the early stages of India–Asia collision by Toussaint et al. [Toussaint G., Burov, E., and J.-P. Avouac, Tectonic evolution of a continental collision zone: a thermo-mechanical numerical model, Tectonics, 23, TC6003, doi:10.1029/2003TC001604, 2004b.]. On the example of India–Asia-like settings, we show that not only the surface topography but also the total amount of subduction may largely vary as function of denudation rate (controlled by the coefficient of erosion, k). Erosion provides a dynamic discharge of the hanging wall of the major thrust zone, whereas the sedimentation increases loading on the footwall and this helps down-thrusting of the lower plate. Both processes reduce the resistance of the major thrust and subduction channel to subduction. However, very strong or very slow erosion/sedimentation enhance the possibility of plate coupling and promote whole-scale thickening or buckling. The maximal amount of subduction is thus achieved for some intermediate values of erosion rates when the tectonic uplift rate is fine-balanced by the denudation rate. In our case the optimal balance is reached for the values of k on the order of 3000 m²/yr. We then extended our model beyond the conditions of India–Asia collision, in terms of the tested range of k and convergence rates. The experiments suggest that for provided settings, both extra slow (k < 50–100 m²/yr) and extra rapid erosion (k > 6000–8000 m²/yr) limit, by up to 50%, the total amount of subduction, if not totally prevent it. The model demonstrates the large capability of surface processes to adopt to different deformation styles: the orogenic building and subduction successfully develop (subduction number, S > 0.5) in the range of k between 500 m²/yr and 6000 m²/yr at convergence rates ranging from 1 cm/yr to 6 cm/yr. Within this range, some peculiar features of orogenic style such as the geometry of the accretion prism, amount of upper crustal subduction, horizontal progression of the mountain range/thrust fault and the amount of exhumation of metamorphic facies are sometimes quite different. We conclude that surface processes may control deep, mantle level tectonic evolution.     

Atomic processes in astrophysics

We review the atomic processes responsible for astrophysical emission line spectra, paying particular attention to the reliability of the rates used to determine the physical conditions in the emiting gas. We discuss particular cases where often-neglected processes play important roles.     

Oscillatory processes in prominences

A new type of oscillations, being long-period oscillations of line-of-sight velocities, with periods ranging from 42 to 82 min and amplitudes in excess of 200 m s^-1 has been discovered in prominences. These oscillations may be interpreted as a combination of torsional and longitudinal ones.     

Aeolian processes on Venus

In this pre-Magellan review of aeolian processes on Venus we show that the average rate of resurfacing is less than 2 to 4 km/Ga, based on the impact crater size frequency distribution derived from Venera observations, reasonable values of the impact flux, and the assumption of steady state conditions between crater production and obliteration. Viscous relaxation of crater topography, burial by volcanic deposits, tectonic disruption, chemical and mechanical weathering and erosion, and accumulation of windblown sediments probably all contribute to resurfacing. Based on the rate of disappearance of radar-bright haloes around impact craters, the rate of removal of blocky surfaces has been estimated to be about 10^–2 km/Ga. Pioneer-Venus altimetry data show that the average relative permittivity (at 17 cm radar wavelength) of the surface is too high for exposure of soils 10 cm deep, except for ~5% of the planet located primarily in tessarae terrains. The tectonically disrupted tessarae terrains may be sites of soil generation caused by tectonic disruption of bedrock and the presence of relatively steep slopes, or they may be terrains that serve as traps for windblown material. The overall impression is that Venus is a geologically active planet, but one dominated by volcanism and tectonism. On the other hand, theoretical considerations and experimental data on weathering and transport of surface materials suggest rather different conditions. Thermochemical arguments have been advanced that show: (1) CO₂ and SO₂ incorporate into weathering products at high elevation, (2) transport of weathered material by the wind to lower-elevation plains, and (3) re-equilibration of weathered material, releasing both CO₂ and SO₂. In addition, kinetic data suggest a rate of anhydrite formation of 1 km/Ga, a value comparable to the soil erosion rate on Mars, a planet with an active aeolian environment. Experiments and theoretical studies of aeolian processes show that measured surface winds are capable of moving sand and silt on Venus. Assuming that there is a ready sand supply, the flux could be as high as 2.5 × 10^–5 g/cm/s, a value comparable to desert terrains on Earth. In an active aeolian abrasion environment, sand grains could have lifetimes <10³ years. In addition, comminuted debris may be cold-welded to surfaces at the same time as abrasion is occurring. Magellan altimetry and SAR observations should allow assessment of which model for venusian surface modification (active vs. inactive surficial processes) is correct, given the global coverage, high spatial resolution, the calibrated nature of the data, and the potential during extended missions of acquiring multiple SAR views of the surface.Geology and Tectonics of Venus, special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci. Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT, Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).     

Nuclear line spectroscopy of solar flares

The study of nuclear line spectra from solar flares holds a rich promise for elucidating the properties of both the accelerated particles and the interaction or target region. We review the observations and the analysis of the large nuclear line rich flare which occurred near the west limb starting at 08:03 UT on 27 April, 1981. The observed spectrum from this flare contains three intense and isolated gamma-ray lines which can be analyzed in a model independent way. The measured energies are 1.628 ± 0.008, 4.430 ± 0.011, and 6.147 ± 0.022 MeV, identifying them as the de-excitation lines of ²⁰Ne (1.634 MeV), ¹²C (4.438 MeV), and ¹⁶O (6.129 MeV). Elemental abundances of the ambient gas at the site of gamma-ray line production in the solar atmosphere are deduced using these gamma-ray line observations. The resultant abundances are different from local galactic abundances which are thought to be similar to photospheric abundances.Resident Research Associate at NRL under the NRC Associateship Program.     

Performance of the Nuclear Compton Telescope

On 1 June 2005, the prototype Nuclear Compton Telescope (NCT) flew on a high altitude balloon from Fort Sumner, New Mexico. NCT is a balloon-borne soft γ-ray (0.2–10 MeV) telescope for studying astrophysical sources of nuclear line emission and γ-ray polarization. Our program is designed to develop and test technologies and analysis techniques crucial for the Advanced Compton Telescope; however, our detector design and configuration is also well matched to the focal plane requirements for focusing Laue lenses. The NCT prototype utilizes two, 3D imaging germanium detectors (GeDs) in a novel, ultra-compact design optimized for nuclear line emission in the 0.5–2 MeV range. Our prototype flight provides a critical test of the novel detector technologies, analysis techniques, and background rejection procedures developed for high resolution Compton telescopes.     

Direct simulation of collision processes

The mass distribution of protoplanets is studied by a Monte Carlo technique. It is assumed that the bodies coalesce on collision and that their self-gravitation is important. It is found that over most of the mass range a distribution of the formn(m)αm ^?q develops, withq tending to ～1.8. However, there is an overabundance of large masses relative to the power law: in particular the simulations indicate the growth of a large nucleus which progressively dominates the growth process. It is also found that the axial inclinations of the planets are best explained if they grew out of a coagulating medium comprising planetesimals with random velocities ～5 km s^?1 and masses up to 10^?2 of the embryo masses. The inclinations of the giant planets are consistent with a floccule-type origin of these bodies.     

Reaction rates for neutrino processes

Some integrals involved in neutrino processes are evaluated by transformation to a special system of reference—usually to the center of mass system (CM). Rather simple analytic expressions are obtained for reaction rates and, though less simple, for moments. An interesting result thus obtained is for an isotropic interaction (in CM) of a neutrino with a monoenergetic isotropic gas of extreme relativistic electrons: it is found that the probability of the scattered neutrino to have energy in a certain range is independent of this energy.     

Observability of coronal heating processes

The mechanisms that could possibly heat the corona are briefly reviewed with emphasis on their observability. Observing enhanced wave flux at footpoints of active regions would confirm wave heating. Observation of nonthermal electrons in tiny coronal events (nanoflares) would confirm dissipation of current sheets. Presence of large scale flows in coronal arcades would underline the importance of turbulent resistivity for coronal heating. A comparison of HeI absorption in quiet and active regions demonstrates the difficulty of interpreting data that connect chromospheric dynamics with coronal heating. Finally, the implications of the search for observations of coronal heating processes are mentioned.     

Neutrino processes in dense matter

Expressions for the source and collision terms of the neutrino transport equation relevant to the neutrino transport supernova model are derived in the framework of the theory of neutral currents. In particular we study the capture and emission of neutrini (and anti-neutrini) by free nucleos, the inelastic scattering by free nucleons, the coherent scattering by nuclei, as well as the corresponding muonic processes.We also derive an analytical formula for the neutrino scatteringrate on electrons, valid for large electron degeneracy.     

Collisional processes in stellar systems

The theory of collisional relaxation in stellar systems is discussed in terms of an expansion in powers of 1/N, the inverse of the total number of stars. The results are expressed in terms of the concept of gravitational polarization.     

Thick-target processes and white-light flares

Observations indicate that fast electrons in solar flares, which cause the hard X-ray burst and the impulsive microwave burst, lose energy predominantly by collisional processes. This requires a thick-target theory of the emission, for which the electron spectrum inferred from the X-ray spectrum becomes 1.5 powers steeper than in the usual thin-target theory.The low-energy end of this spectrum contains enough energy above about 5 keV to supply the white-light continuum emission occasionally observed in major flares. The penetration of the nonthermal electrons creates long-lived excess ionization which enhances the free-free and free-bound continuum in the heated medium. The emission will occur high above the photosphere at small optical depth in the visible continuum. Thus its spectrum will extend into the infrared and ultraviolet.     

Direct simulation of collision processes

A Monte Carlo technique for examining the mass distribution of mutually colliding bodies is described. The technique is simple to program and can handle a wide variety of physical circumstances. Some illustrative cases are given.     

Varying Constants, Nuclear Physics and Unification

Precision measurements of gauge couplings at particle accelerators strongly suggest that the standard model fields are unified at about 10¹⁶ GeV. This talk will consist of a brief review of this evidence and also how in such a setting variations in α _em lead to much larger variations in Λ _QCD . I will then describe how variations in the strong force will impact upon nuclear astrophysics, considering in particular the stability of di-nucleons and stellar and primordial nucleosynthesis. (Based upon collaborations with T. Dent and with A. Csoto, H. Oberhummer and H. Schlattl.) This revised version was published online in July 2006 with corrections to the Cover Date.