Charged particle thermonuclear reactions in nucleosynthesis

Thermonuclear reaction rates are calculated at temperatures consistent with nucleosynthesis conditions in stars and supernovae (10⁹T10¹⁰ K). The cross sections as a function of energy are determined by averaging over a density of compound nuclear states, an individual resonance contribution being represented by the Breit-Wigner formula. The proton and -particle channel radii are estimated by fitting experimental (p, n) and (, n) cross sections for nuclei in the mass range 40A200. The influence of the diffuseness of the nuclear surface on barrier penetration is taken into account by multiplying the square well penetration factors by a reflection factor in the manner described by Vogtet al. (1965). Thermonuclear rates calculated by these procedures are compared with sums over experimentally determined resonance strengths for a number of charged-particle reactions involving medium mass nuclei (24A40).     

Radio source orientation and the cosmological interpretation of the angular size-redshift relation for double-lobed quasars

The angular size-redshift test for quasars was compared with various cosmological models including non standard models. The possible effects of radio source orientation and relativistic beaming were taken into account in the analysis.It was found that orientation effects alone were not sufficient to explain the observed-z relation in terms of Friedmann models. In addition, linear size evolution of the formD ~ (1 +z)^–n , with 0.75 n 1.2 would be required for 0 1.0, or possibly an inverse correlation between luminosity and linear size. The non-standard cosmological models all gave better fits to the deprojected data than the Friedmann models in the absence of evolutionary effects, with the tired light effect providing the best fit.     

A coronal loop model for the production of X-ray and radio flares in the RS CVn binary HR 1099

The RS CVn binary stellar system HR 1099 is a source of both X-ray and radio flares. We present here a model of the system in which the two types of flare are produced by the same population of mildly-relativistic ( 10) electrons, injected into a coronal loop. After reviewing possible radiation mechanisms we conclude that, given the probable conditions in the flaring region, the radio emission is gyrosynchrotron radiation and the X-ray emission is thermal bremsstrahlung. The thermal X-ray source must lie in the stellar chromosphere, but the apparent absence of plasma absorption at radio frequencies indicates that the radio source is located high in the coronal loop. Using the relationships given by Dulk and Marsh (1982) for the radio emission from a power-law electron energy spectrum,N() ( - 1)^–, we conclude that 3 7, with 30% of the electron population trapped in the radio source. Some implications of these results for one particular version of the model are discussed.     

Electron–Whistler Interaction in Coronal Loops and Radiation Signatures

Interaction of the 30–300 keV electrons with whistlers in solar coronal loops is studied using a quasi-linear approach. We show that the electron–whistler interaction may play a dominant role in the formation of fast electron spectra within the solar flare loops with the plasma temperature 10⁷ K and plasma density 10¹¹ cm^–3. It is found that Landau damping of whistlers provides weak and intermediate pitch-angle diffusion regimes of fast electrons in coronal loops. The level of whistler turbulence in the weak diffusion regime under flare conditions is estimated as 10^–7 of the energy density in the thermal particles. The `top – footpoint' relations between the hard X-ray flux densities and spectra are derived. The reason for a `broken' spectrum of the flare microwave emission is discussed.     

Composition and adiabatic index of matter at high temperatures

The equation of state and the adiabatic index of thermally dissociated matter composed of nucleons, electrons, positrons, neutrinos, antineutrinos and photons are calculated in the density and temperature ranges, 10⁹q(g cm^–3)10¹³, 2×10¹⁰T K5×10¹¹, respectively. The interaction between nucleons is explicitly included. This leads to a softening of the equation of state. The implications of the results for the problem of supernova collapse are discussed.     

Evidence for electron excitation of type III radio burst emission

Type III radio bursts observed at kilometric wavelengths ( 0.35 MHz) by the OGO-5 spacecraft are compared with > 45 keV solar electron events observed near 1 AU by the IMP-5 and Explorer 35 spacecraft for the period March 1968–November 1969.Fifty-six distinct type III bursts extending to 0.35 MHz ( 50 R equivalent height above the photosphere) were observed above the threshold of the OGO-5 detector; all but two were associated with solar flares. Twenty-six of the bursts were followed 40 min later by > 45 keV solar electron events observed at 1 AU. All of these 26 bursts were identified with flares located west of W 09 solar longitude. Of the bursts not associated with electron events only three were identified with flares west of W 09, 18 were located east of W 09 and 7 occurred during times when electron events would be obscured by high background particle fluxes.Thus almost all type III bursts from the western half of the solar disk observed by OGO-5 above a detection flux density threshold of the order of 10^–13 Wm^–2 Hz^–1 at 0.35 MHz are followed by > 45 keV electrons at 1 AU with a maximum flux of 10 cm^–2 s^–1 ster^–1. If particle propagation effects are taken into account it is possible to account for lack of electron events with the type III bursts from flares east of the central meridian. We conclude that streams of 10–100 keV electrons are the exciting agent for type III bursts and that these same electrons escape into the interplanetary medium where they are observed at 1 AU. The total number of > 45 keV electrons emitted in association with a strong kilometer wavelength type III burst is estimated to be 5 × 10³².     

Hollow beam distribution of energetic electrons and higher harmonics of electron cyclotron maser

The variations of the growth rates of ECM at first four harmonics in X-, Z-, and O-modes excited by a hollow beam distribution of weakly relativistic electrons with a parameter _p / _e are presented in this paper. The results show that the second harmonic of ECM in X-mode dominates the instability if < 1, and if 1.2 , 2 or 2.2 3 the third or fourth harmonic will dominate. The second and third harmonics of Z-mode waves grow faster only if 2 2.2 and 3 3.2, respectively, so it would not be a competition in most cases. It is suggested that the ECM emission at these harmonics in X-mode is a possible mechanism to produce solar spike emissions with high brightness temperature at shorter and longer decimetric wavelengths.Proceedings of the Workshop on Radio Continua during Solar Flares, held at Duino (Trieste), Italy, 27–31 May, 1985.On leave from the Department of Astronomy, Nanjing University, Nanjing, The People's Republic of China.     

Diagnostics of solar flare hard X-ray sources

The dynamics of hard X-ray producing electron beams in solar flares can be strongly affected by the occurrence of a reverse current. The parameter diagram for a beam can be divided into three regimes, one of which is the usual thick target case, the two others being due to two different possible consequences of the reverse current. The use of this parameter diagram as a possible diagnostic tool for solar flare hard X-ray sources is discussed, together with the necessary observations and their interpretation.The forthcoming Solar Maximum Mission, complemented with concurrent ground-based efforts provide the next possibility to obtain these observations, given a good coordination of observing programs. We stress the importance of microwave (GHz) ratio observations with good temporal (few sec) and spatial resolution (1) in one dimension, and of reliable spectroscopic methods to determine the density in solar flare hard X-ray sources.     

10–100 keV electron acceleration and emission from solar flares

We present an analysis of spacecraft observations of non-thermal X-rays and escaping electrons for 5 selected small solar flares in 1967. OSO-3 multi-channel energetic X-ray measurements during the non-thermal component of the solar flare X-ray bursts are used to derive the parent electron spectrum and emission measure. IMP-4 and Explorer-35 observations of > 22 keV and > 45 keV electrons in the interplanetary medium after the flares provide a measure of the total number and spectrum of the escaping particles. The ratio of electron energy loss due to collisions with the ambient solar flare gas to the energy loss due to bremsstrahlung is derived. The total energy loss due to collisions is then computed from the integrated bremsstrahlung energy loss during the non-thermal X-ray burst. For > 22 keV flare electrons the total energy loss due to collisions is found to be 10⁴ times greater than the bremsstrahlung energy loss and 10² times greater than the energy loss due to escaping electrons. Therefore the escape of electrons into the interplanetary medium is a negligible energetic electron loss mechanism and cannot be a substantial factor in the observed decay of the non-thermal X-ray burst for these solar flares.We present a picture of electron acceleration, energy loss and escape consistent with previous observations of an inverse relationship between rise and decay times of the non-thermal X-ray burst and X-ray energy. In this picture the acceleration of electrons occurs throughout the 10–100 sec duration of the non-thermal X-ray burst and determines the time profile of the burst. The average energy of the accelerated electrons first rises and then falls through the burst. Collisions with the ambient gas provide the dominant energetic electron loss mechanism with a loss time of 1 sec. This picture is consistent with the ratio of the total number of energetic electrons accelerated in the flare to the maximum instantaneous number of electrons in the flare region. Typical values for the parameters derived from the X-ray and electron observations are: total energy in > 22 keV electrons total energy lost by collisions = 10^28–29 erg, total number of electrons accelerated above 22 keV = 10³⁶, total energy lost by non-thermal bremsstrahlung = 10²⁴erg, total energy lost in escaping > 22 keV electrons = 10²⁶erg, total number of > 22 keV electrons escaping = 10^33–34.The total energy in electrons accelerated above 22 keV is comparable to the energy in the optical or quasi-thermal flare, implying a flare mechanism with particle acceleration as one of the dominant modes of energy dissipation.The overall efficiency for electron escape into the interplanetary medium is 0.1–1% for these flares, and the spectrum of escaping electrons is found to be substantially harder than the X-ray producing electrons.Currently at Tokyo Astronomical Observatory, Mitaka, Tokyo, Japan.     

On the relation between solar-flare gamma-ray emission and proton escape into interplanetary space

It is shown that escaping of solar flare energetic protons into interplanetary space as well as their relation to the flare gamma-ray emission depend on the parameter = 8p/B ₀ ² , where p is the pressure of hot plasma and energetic particles and B ₀ is the magnetic field in a flaring loop. If 1, the bulk of the energetic protons escape to the loss cone because of diffusion due to small-scale Alfvén-wave turbulence, and precipitate into the footpoints of the flaring loop. The flare then produces intense gamma-ray line emission and a weak flux of high energy protons in interplanetary space. If >_*0.3-1.0, then fast eruption of hot plasma and energetic particles out of the flaring loop occurs, this being due to the flute instability or magnetic-field-plasma nonequilibrium. The flare then produces a comparatively weak gamma-radiation and rather intense proton fluxes in interplanetary space. We predict a modulation of the solar flare gamma-ray line emission with a period 1 s during the impulsive phase that is due to the MHD-oscillations of the energy release volume. The time lag of the gamma-ray peaks with respect to the hard X-ray peaks during a simultaneous acceleration of electrons and protons can be understood in terms of strong diffusion.     

Diffuse galactic FUV radiation and interstellar dust grains

A re-analysis of the diffuse far UV radiation ( 1350–1480 Å) observed in the sky region ofl ^II180° and 0°b ^II40° is presented, as a revised version of a paper by Hayakawaet al. (1969). In comparison with the previous one, the value of the half optical depth of the Galazxy in our wavelength region is reduced, and the values of the albedo coefficient and the forward phase functiong are not well determined. If, however, we combine our results with the theoretical model of interstellar grains by Gilra, the value of is given by 0.13(5)0.18(5).     

The interrelationship between cosmic dust and the microwave background

Attention is given to the radiation of microwaves by charged dust in space. Presently-used particle distributions do not restrict the presence in space of large numbers of small (r<10^–6 cm) silicate grains, but it is shown that such densities (10^–25–10^–26 g cm^–3) of small grains would produce a microwave background with an energy density of the same order of magnitude as the energy density of the (presumed) cosmological 3 K background. Limits set by the isotropy of the latter are: (HI clouds)10^–26, (Galactic plane)10^–30, (Halo)10^–32, (Local Group)10^–34 g cm^–3. These limits imply that either there is a cutoff in particle distributions atr10^–6 cm, or that the density of silicate grains in space has been generally overestimated, or that cosmic rays have broken up a lot of grains so that they now form a population of grains of very small size (10^–7 cm) which are difficult to detect by conventional methods. One way to look for the latter population is by studying expected distortions of the 3 K spectrum to the short wavelength side of the portion hitherto observed (grains may have a size distribution able to give an approximate black-body curve for radiation from larger grains of 10^–6 cm size), and by testing the effective energy density of the 3 K field in other galaxies.     

Acceleration of electrons in absence of detectable optical flares deduced from type III radio bursts,Hα activity and soft X-ray emission

The relationship between H absorption features, type III radio bursts and soft X-ray emission has been examined in order to determine the characteristics of the particle acceleration process operating when a H-flare may not be detectable. It is found that transient H activity observed in the absence of reported flares is associated with production of relatively weak type III radio and soft X-ray emission. Since such optical phenomena are much more frequent than flares themselves, it is concluded that instabilities generating fast particles may be produced in the corona in a quasi-continuous way with coincident perturbations in the lower solar atmosphere.The soft X-ray component, which is similar to the precursor in flares, is not necessarily the direct product of fast particles, but is probably associated with some type of heating since both the soft X-ray emission and the H features exhibit a similar evolution, the type III bursts occurring near the maximum of this perturbation. The observations are consistent with a model in which the electron acceleration region is located at an altitude where the ion density is 10⁹ cm^–3 and most of the accelerated electrons( 20 keV) are confined to coronal altitudes where the ion density is 10¹⁰ cm^–3.     

SOLAR ENERGETIC ION AND ELECTRON LIMITS FROM HIREGS OBSERVATIONS

The HIgh-REsolution Gamma-ray and hard X-ray Spectrometer (HIREGS) consists of an actively shielded array of twelve liquid-nitrogen-cooled germanium detectors designed to provide unprecedented spectral resolution and narrow-line sensitivity for solar gamma-ray line observations. Two long-duration, circumpolar balloon flights of HIREGS in Antarctica (10–24 January, 1992 and 31 December, 1992–10 January, 1993) provided 90.9 and 20.4 hours of solar observations, respectively. During the observations, eleven soft X-ray bursts at C levels and above (largest M1.7) occurred, and three small solar hard X-ray bursts were detected by the Compton Gamma-Ray Observatory. HIREGS detected a significant increase above 30 keV in one. No solar gamma-ray line emission was detected. Limits on the 2.223-MeV line and the hard X-ray emission are used to estimate the relative contribution of protons and electrons to the energy in flares, and to coronal heating. For the 2.223-MeV line, the upper limit fluence is 0.8 ph cm^-2 in the flares, and the upper limit flux is 1.8 × 10^-4 ph s^-1 cm^-2 in the absence of flares. These limits imply that 6 × 10³⁰ (2) protons above 30 MeV were accelerated in the flares, assuming standard photospheric abundances and a thick target model. The total energy contained in the accelerated protons >30 MeV is 4 × 10²⁶ ergs, but this limit can be more than 10³⁰ ergs if the spectrum extends down to 1 MeV. The upper limit on the total energy in accelerated electrons during the observed flares can also exceed 10³⁰ ergs if the spectrum goes down to 7 keV. Quiet-Sun observations indicate that 10²⁶erg s^-1 are deposited by energetic protons >1 MeV, well below the10²⁷ –10²⁸ erg s^-1 required for coronal heating, while <3 × 10²⁷ erg s^-1 are deposited by energetic electrons, which does not exclude the possibility of coronal heating by quiet-time accelerated electrons. The quiet-Sun observations also suggest that if protons stored in the corona are to supply the energy for flares, as suggested by Elliot (1964), the proton spectrum must extend down to at least 2 MeV. However, collisional losses at typical coronal-loop densities prevent those low-energy protons from being stored for 10⁴ s. It therefore seems unlikely that the energy for flares could come from energetic protons stored over long periods.     

Gamma-ray bursts from high velocity neutron stars

We investigate the viability of the Galactic corona model of -ray bursts by calculating the spatial distribution of neutron stars born with high velocities in the Galactic disk, and comparing the resulting brightness and angular distribution with the BATSE data. We find that the Galactic corona model can reproduce the BATSE peak flux and angular distribution data for neutron star kick velocities 800 km s^–1, source turn-on ages 10 Myrs, and sampling depths 100 kpc d _max 400 kpc.     

Evidence for a two-component injection of cosmic rays from the solar flare of 1969, March 30

The solar flare of 1969 March 30, occurring 20° behind the west limb, produced very extensive 80 MHz radio emission at the Sun, and gave rise to the deployment of cosmic radiation over 360°long, in interplanetary space. The wide spread of this event may reflect a similar spread of coronal magnetic fields from the flare site. We interpret the solar proton data recorded by spacecraft at two separate points both at 1 AU, in terms of a two-component injection of particles at the Sun consisting of: (i) a soft component which arrived promptly; (ii) a harder component which arrived later. The radio spectral and positional data provide evidence of shock waves which propagated far and wide from the flare; we attribute the precursor injection of the soft ( 10 MeV) proton component to one of these shock waves.Radiophysics Publication RPP 1590, May, 1972.Now at University of California, LASL, Los Alamos, N.M., U.S.A.     

Preliminary interpretation of the polarization measurements performed on ‘Intercosmos-4’ during three X-ray solar flares

Analysis of the X-ray polarization data at 0.8 Å for three major chromospheric flares shows that during the hard phase of the flare the X-rays are polarized in the plane, the projection of which on the solar disc is going approximately from the flare region to the center of the disc. Simultaneously performed measurements of the spectral energy distribution have proved that observed X-rays are produced by the bremsstrahlung of the accelerated electrons with the energies in the range 10–100 keV. The experimental data are in good agreement with the flare model, which deals with the radial movement of accelerated electrons towards the photosphere, together with the continuous injection of these electrons into the emitting region.Presented to International Meeting on Solar Activity, IZMIRAN, November 15–22, 1971.     

The physical relationship between flares and surges observed in the extreme ultraviolet

A search was made for EUV surges among the EUV flares recorded by the Harvard spectroheliometer on ATM. Out of a large set of partial observations of such flares, a subset of 24 complete events was chosen. More than 24 associated surges were found, many of them multiple events. The flare-surge correlation is therefore considerably higher in the EUV than in H, presumably because EUV surges generally appear in emission, and in high contrast compared to H. In over 70% of the cases, the surges were found to grow out of the flare structure. Making reasonable assumptions, it was possible to infer the magnitude of the gas pressure gradient from the flare core into the surge by using the EUV intensity gradient. The inferred pressure gradient appears sufficient to drive the surge, although higher resolution observations will be required to corroborate this, and rule out the importance of magnetic Lorentz force.     

Superthermal plasma nodules and their relation to solar flares

We define superthermal plasma nodules as bright points (diameter 20), visible on high resolution X-ray heliograms. Flares appear to show a strong tendency to occur at the places of these nodules. There are indications that (part of) the hot plasma produced by consecutive flares is accumulated and confined in the superthermal plasma nodules, and that with increasing energy content of a nodule the probability for a drastic change of its magnetic structure increases, thus reducing the possibility for more flares to occur.     

Gamma-ray and microwave evidence for two phases of acceleration in solar flares

Relativistic electrons in large solar flares produce gamma-ray continuum by bremsstrahlung and microwave emission by gyrosynchrotron radiation. Using observations of the 1972, August 4 flare, we evaluate in detail the electron spectrum and the physical properties (density, magnetic field, size, and temperature) of the common emitting region of these radiations. We also obtain information on energetic protons in this flare by using gamma-ray lines. From the electron spectrum, the proton-to-electron ratio, and the time dependences of the microwave emission, the 2.2 MeV line and the gamma-ray continuum, we conclude that in large solar flares relativistic electrons and energetic nuclei are accelerated by a mechanism which is different from the mechanism which accelerates 100 keV electrons in flares.Research supported by NASA Grant 21-002-316 at the University of Maryland, College Park.