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The collisional-radiative ionization and recombination coefficients are shown to be expressible in terms of the 'total probability', from initial level i to final level j, which excludes intermediate returns to the initial level. Applications are made to three-level and four-level systems to illustrate the method of calculation and compare with an approximation currently used.
There exists presently no general agreement concerning the collisional excitation rates of atomic hydrogen. The present investigation is concerned with the results reported by various investigators, taking into account the semiempirical rates of Johnson (1972), a study conducted by Aggarwal (1983), empirically based rates of Crandall et al. (1974), and the rates tabulated in Osterbrock (1974), based on the close-coupling calculations of Burke et al. (1967). It is found that there is a major conflict between the 'best' theoretical and Johnson values on the one hand, and the empirical data on the other. A study of the data shows that the Johnson value for the 1 to 2 rate agrees well with other theoretical calculations, and within 25 percent of empirical measurements, but that the Johnson 1 to 3 rate is 40-75 percent of the 'best' theoretical calculations.
It is pointed out that electron collisional rate coefficients for iron ions are important atomic data for studies of both laboratory and astrophysical plasmas. Datla et al. (1975) have measured the ionization and relative excitation rate coefficients for Fe VIII, Fe IX, and Fe X. The present investigation is concerned with the relative excitation rate coefficients for Fe XI. Absolute intensities of VUV transitions 3s2 3p3 3d-3s2 3p4, 3s 3p5-3s2 3p4, and 3s2 3p3 4s-3s2 3p4 in Fe XI are measured and interpreted in terms of excitation rate coefficients. Data regarding the Fe XI line intensities and plasma parameters, and the relative excitation rate coefficients for Fe XI are provided in tables.
A general theory of atomic dipole coherence under the influence of collisional phase changes, inelastic effects and optically active atom velocity changes, including those due to anisotropic interactions is presented. Velocity change effects are obtained in closed form. Line shapes appear as convolutions of standard pressure broadening contours with velocity-change contours. Width and shift parameters for the He-broadened Na D lines at 2 m bar pressure, 380 K are calculated, as are He-induced photon echo decay rates for these lines. Overall agreement with xperiment is reasonably good.
There are many systems of physical interest for which a set of rate equations for level populations can provide insight. If the system has two (or more) different mechanisms for effecting transition between levels, total rates of transfer are usually taken as the sum of rates that the individual mechanisms would cause acting alone. Using the example of a hydrogen atom subjected to (ionic and electronic) collisions and external radiation, it is shown that when these individual mechanisms overlap, the total transfer rates must be modified to account for correlations between collisional and radiative processes. For a broad-band radiation field the modified rates have a simple physical interpretation. In the case of a narrow-band field the overlapping events may cause new coherences to appear and interpretation of the modified 'rates' is more complicated.
It is shown that the use of the Krook equation greatly simplifies the discussion of the collisional dynamics of particulate disks. Collisions are assumed to be inelastic from the outset. A simple heuristic argument is used to compute the effective collision rate in a disk of spherical particles with a power-law distribution of sizes. For Saturn's rings, the effective collision rate for momentum transport is substantially lower than that conventionally estimated on the basis of an observed optical depth at visual wavelengths. The closure of the vertically integrated set of moment equations without discarding the third-order moments at the outset is discussed, allowing for the possibility of a bent disk. In the limit that the collision frequency is much larger than the orbit frequency, the usual Navier-Stokes equations of viscous hydrodynamics for a thin disk are recovered with an explicit expression for the shear viscosity. The method is generalized to include the effects of gravitational scattering.
A tunable diode laser has been used to observe rotational collisional narrowing or line mixing in the IR absorption spectrum of the Q branch of the (1 1 exp-1 0,0 3 exp-1 0) sub II - 0 0 exp-0 0 band of CO2 at 1932.47/cm over the 100-745 Torr pressure range. The observed absorption coefficients below the band origin are up to 80 percent lower than those calculated on the basis of an isolated Lorentz line approxmation. A simple fitting law is derived for the state-to-state, rotationally inelastic rate constants, on the basis of the known line broadening coefficients; a calculation of the mixing using these rates agrees with observed absorption coefficients to within several percent.
An MHD model is used to calculate the growth rate of the collisional tearing mode instability in a current layer with a Lundquist number in the range 3-100,000. The maximum growth rate is found to be a function of the Alfven transit time in the current sheet and the Lundquist number. When the Lundquist number is below 50, the growth rate asymptotically approaches a value of about -0.075/transit time. The discovery of the dependence of the asymptotic value is considered to be of use in studies of the current sheet in the diffusion region of a magnetic reconnection configuration, where the Lundquist number will be in the range 2-100.
Fluorescence lidar measurements of the hydroxyl radical require detailed information concerning collision induced processes in order to deduce the radical number density from a lidar return. The Goddard SFC OH lidar currently utilizes a broadband detector which precludes the necessity of fully understanding collisional redistribution of rotational energy within the excited state. Numerous advantages result however from the inclusion of a detector with a bandpass only slightly larger that the Doppler width of a rotational line. This however places more stringent requirements on the spectroscopy. Measurements were accordingly made of rotationally resolved quenching rates for collisions with O2, N2, and H2O. Rotational transfer rates were also measured for the same colliders. Quenching rates were measured using a Nd-YAG pumped Rh6G dye laser doubled into the UV. The OH lifetimes were measured as a function of pressure of quenching gas at total pressures of between 50 and 250 microns. Rotational transfer rates were measured by recording the emission spectrum on an intensified diode array and integrating over 10.000 laser shots.
The process of collisional catastrophic disruption has played a significantly role in structuring the solar system. Diverse populations of bodies such as the asteroid belt, small satellites of Jupiter and Saturn and perhaps even the rings of Saturn have been created or substantially changed by catastrophic distruption. Understanding the outcome of large scale impacts is essential to learning about the early history of the solar system in the asteroid zone and the reason why a planet failed to form there.
Theoretical calculations are used to determine the collisional excitation rates of methyl cyanide under interstellar molecular cloud conditions. The required Q(L,M) as a function of kinetic temperature were determined by averaging fixed energy IOS (infinite order sudden) results over appropriate Boltzmann distributions of collision energies. At a kinetic temperature of 40 K, rates within a K ladder were found to be accurate to generally better than about 30 percent.
The behavior of nonthermal electrons ejected into a tapered magnetic loop, under the action of both Coulomb collisional and magnetic field gradient forces is studied. An approximate analytic formula for the heating rate as a function of distance along the loop is developed, and found to be in good agreement with exact numerical solutions of the relevant equations. Such a formula is useful as a source term in many situations, such as hydrodynamic simulations of atmospheric response to flare energy input.
Hameen-Anttila's (1984) theory of self-gravitating collisional particle disks is extended to include the effects of particle spin. Equations are derived for the coupled evolution of random velocities and spins, showing that friction and surface irregularity both reduce the local velocity dispersion and transfer significant amounts of random kinetic energy to rotational energy. Results for the equilibrium ratio of rotational energy to random kinetic energy are exact not only for identical nongravitating mass points, but also if finite size, self-gravitating forces, or size distribution are included. The model is applied to the dynamics of Saturn's rings, showing that the inclusion of rotation reduces the geometrical thickness of the layer of cm-sized particles to, at most, about one-half, with large particles being less affected.
Numerical simulations of the generalized Rayleigh-Taylor instability are presented. The model and simulations are applicable to bottomside and topside spread F, unstable barium cloud dynamics, and collisional interchange instability in general. The principal result is that the evolution of the instability tends to an anisotropic state consisting of nearly sinusoidal (quasi-periodic) variation along the effective electric field, and shocklike structures propagating perpendicular to E(eff) along the extrema of the quasi-periodic structures. The spectral properties of the nonlinear state are analyzed using one-dimensional power spectra calculated along spatial trajectories for selected angles to E(eff). In this way a direct comparison to in situ probe data can be made. The inherent anisotropy of the nonlinear state is reflected in major qualitative differences between the spectra taken parallel to and perpendicular to E(eff). The fundamental finding of the present work is that anisotropy in interchange dynamics is much greater than had been previously reported. This strong anisotropy can explain much of the spectral and spatial structural characteristics of both bottomside and topside spread F. In a companion paper a comparison of the simulation results to various in situ data sets is given.
The implications of gas buildups around space vehicles are investigated in terms of induced photon emissions. Results indicate that significant effects arise from gas collisions between the relatively energetic ambient atoms and molecules and the atoms and molecules in the shuttle environment. It is suggested that such collision may transfer considerable energy to the gases in the shuttle environment, with the most likely emissions being vibrational and rotational radiation losses in the infrared due to direct collisional excitation.
The origin and evolution of the Eos and Koronis families are addressed by an analysis of Binzel's (1987) observational results. The Maxwellian distribution of the Eos family's rotation rates implies a collisionally-evolved population; these rates are also faster than those of the Koronis family and nonfamily asteroids. While the age of the Eos family may be comparable to the solar system's, that of the Koronis family could be considerably younger. Greater shape irregularity may account for the Koronis family's higher mean lightcurve amplitude.
Restricted three-body calculations of high-inclination low-impact-parameter encounters between a disk galaxy and its companion are used to demonstrate that the shell-like ripples noted in a number of disk galaxies are also collisional artifacts. It is suggested that some of the ripples may be the results of internal oscillations following such encounters. It is assumed that the target is an early-type disk with a sufficiently low gas fraction that recent star formation does not dominate the appearance of the disturbed disk.
Collisional excitation rates for C3H2, calculated using the coupled states approximation at temperatures of 10-30 K, are presented. C3H2 produces a number of spectral line pairs whose members are close together in frequency but arise from levels with different excitation energies. The rates are used in statistical equilibrium calculations to illustrate the excitation properties and density-dependent behavior of various C3H2 line ratios.