Showing papers by "Jonathan Tennyson published in 1998"

JIM: A time‐dependent, three‐dimensional model of Jupiter's thermosphere and ionosphere

Abstract: We present the Jovian Ionospheric Model (JIM), a time-dependent, three-dimensional model for the thermosphere and ionosphere of Jupiter. We describe the physical inputs for the hydrodynamic, thermodynamic and chemical components of the model, which is based on the UCL Thermosphere Model of Fuller-Rowell and Rees [1980]. We then present the results of an illustrative simulation in which an initially neutral homogeneous planet evolves for approximately 4 Jovian rotations, under the influence of solar illumination and auroral (electron) precipitation at high latitudes. The model shows that solar zenith angle, auroral activity, ion recombination chemistry and, to a lesser degree, magnetic field orientation, all play a role in forming the dayside and nightside global ionization patterns. We compare auroral and nonauroral/equatorial ionospheric compositions and find the signature of ion transport by fast winds. We also include a localized spot' of precipitation in our model and comment on the associated ionization signatures which develop in response to this To-like aurora. The simulation also develops strong outflows with velocities up to ∼600 m s -1 from the auroral regions, driven mainly by pressure gradients. These pressure gradients, in turn, arise from the differences in chemical composition between the auroral and nonauroral upper thermospheres, as evolution proceeds. This preliminary study indicates a strong potential for JIM in analysis of two-dimensional image data and simulation of time-dependent global events.

Calculation of the rotation–vibration states of water up to dissociation

Abstract: We present rotation–vibrational levels of water up to the dissociation limit using two recent, global potential energy surfaces. These calculations are performed using our recently developed discrete variable representation (DVR) based parallel code (PDVR3D), which runs on computers with massively parallel processors. Variational tests on the convergence of these results show convergence within 0.5 cm−1. Analysis of the highest wave functions for the vibrational energy levels are also shown. Tests on previous calculations performed using conventional computers suggest that convergence for high-lying rotationally excited states is not as good as claimed.

Electron- scattering resonances as a function of bond length

Abstract: Resonances in low-energy electron- collisions are studied using the R-matrix method for eight total symmetries: , over a range of bond lengths from 0.8 to . A rich and complicated structure of resonances is apparent. Where possible, the resonances are fitted using the more appropriate of the eigenphase sum and time-delay methods. Positions and widths are tabulated. Potential curves of individual resonant states are derived and used for nuclear motion calculations. Excellent agreement with experiment has allowed the explanation and assignment of all the resonances below 13 eV where previously there had been confusion and contradiction among different experiments and theory.

Calculated high-temperature partition function and related thermodynamic data for H216O

Abstract: The partition function, Q, of water is calculated by explicit summation of ∼200 000 vibration–rotation levels computed using variational nuclear motion calculations. Temperatures up to 6000 K are studied. Estimates are obtained for the heat capacity (Cp), the Gibbs enthalpy factor (gef), the Helmholtz function (hcf), and the entropy (S) of gas-phase water as a function of temperature. To get converged results at higher temperatures it is necessary to augment the accurate list of energy levels. This is done using estimates for all the vibrational band origins to dissociation and rotational levels calculated using Pade approximants. The widely used method of computing the internal partition function as the product of vibrational and rotational partition functions is tested and found to overestimate the partition function by up to 10%. The present estimates of Q(T), Cp(T), gef(T), hcf(T), and S(T) are probably the most accurate available for water at temperatures, T, above 2000 K. Errors, as a function of te...

Calculated rotational and vibrational excitation rates for electron-HeH+ collisions

Abstract: Molecular R-matrix calculations are performed at a range of energies to give rotational and vibrational excitation and de-excitation cross-sections and, hence, rates for electron collisions with HeH+ up to electron temperatures of 20 000 K. Critical electron densities are also given. The rotational calculations include the Coulomb-Born completion of the cross-sections for high I values. Rates for the transition j = 0 --> 2, which have previously been assumed to be negligible, are found to be up to half those for j = 0 --> 1, raising the prospect of observing the HeH+ j = 2 --> 1 emission line at 74.8 mu m.

On the calculation of electron-impact rotational excitation cross sections for molecular ions

Abstract: Methods of computing rotational excitation cross sections for electron collisions with diatomic molecular ions are examined for impact energies up to 5 eV. The HeH and NO ions are used as test cases and calculations are performed at various levels of approximation. Previous studies have all used the Coulomb-Born approximation assuming only dipolar potentials. This approximation is found to be unreliable in a number of aspects: short-range and threshold effects are important, and the widely made assumption that only processes need to be considered is particularly questionable. Conversely, full inclusion of vibrational motion is found to be less important.

Water on the Sun: the Sun yields more secrets to spectroscopy

Abstract: Analysis of sunlight, which started the discipline of spectroscopy, has been the key to a number of major scienti® c discoveries. Sunspots, which are much cooler than most of the Sun’s surface, have particularly rich and complicated spectra which has long been thought to be due to very hot water. The challenge of analysing this spectrum has stimulated the development of new theoretical procedures based on full quantum mechanical treatments of the vibrational and rotational motion of the water molecule. The result has been the identi® cation of novel spectral features and a deeper understanding of how excited molecules such as superheated water behave. This work has applications ranging from the models of cool star atmospheres and rocket exhausts to the possible automated detection of forest ® res. Perhaps the most interesting result is the insight given to understanding how our own atmosphere absorbs sunlight, and the possible consequences that this may have for modelling the greenhouse eŒect. 1. Spectroscopy and the Sun In 1814 Fraunhofer allowed a beam of sunlight from a narrow opening in his shutters to pass through a prism and to be projected onto a white wall. What Fraunhofer saw was not only the colours of the rainbow, which had been

Continuum states of

Abstract: R-matrix calculations are performed on symmetry resonance states of the for internuclear separations in the range 0.8 - . Position, widths and quantum defects are given for resonances converging on the and states of . Calculations focus on which is produced in decay of and is thus important for neutrino mass measurements. The resonances may also be important in collisions.