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  • ASTROPHYSICS  (9)
  • 1990-1994  (9)
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  • 1
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    Nonlinear stability of Halley comethosheath with transverse plasma motion (1994)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: Weakly nonlinear Magneto Hydrodynamic (MHD) stability of the Halley cometosheath determined by the balance between the outward ion-neutral drag force and the inward Lorentz force is investigated including the transverse plasma motion as observed in the flanks with the help of the method of multiple scales. The eigenvalues and the eigenfunctions are obtained for the linear problem and the time evolution of the amplitude is obtained using the solvability condition for the solution of the second order problem. The diamagnetic cavity boundary and the adjacent layer of about 100 km thickness is found unstable for the travelling waves of certain wave numbers. Halley ionopause has been observed to have strong ripples with a wavelength of several hundred kilometers. It is found that nonlinear effects have stabilizing effect.
    Keywords: ASTROPHYSICS
    Type: Astrophysics and Space Science (ISSN 0004-640X); 222; 1-2; p. 113-125
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  • 2
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    Numerical solution of wave equations for the stability of the inner cometo-sheath (1993)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Numerical solution of the MHD wave equations for stability of the cometary sheath determined by the balance between the inward Lorentz body force and the outward ion-neutral drag force is obtained by using a two-point boundary value method. The eigenvalues and the eigenfunctions are obtained numerically by treating the cometary inner sheath as a layer of finite thickness, bounded by the contact surface, i.e., the diamagnetic cavity boundary. The magnetic field structure discovered in the ionosphere of Comets Halley and Giacobini-Zinner is found to be unstable. The effects of finite plasma pressure, dissociative recombination, and mass loading due to photoionization are found to be stabilizing but are unable to quench the instability completely. It is also found that the higher the neutral production rate the lesser is the growth rate for the instability.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 409; 2; p. 782-789.
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  • 3
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    Acceleration of cometary H2O group pickup ions by obliquely propagating nonlinear magnetosonic waves (1993)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The observations made during the encounter with comet Giacobini Zinner show that the character of MHD turbulence is governed by the magnetosonic (MS) waves generated by the pickup ions via a resonant cyclotron instability. The interaction of cometary ions in the mass loaded solar wind with MS waves propagating away from the comet and oblique to the interplanetary magnetic field (IMF) is investigated using the test particle approach. Ion trajectories, distribution functions, widths of pitch angle scattering and energy diffusion are obtained. We have also investigated the relationship between pitch angle scattering and the three parameters, namely, alpha, the angle between the solar wind flow direction and the ambient magnetic field, theta(sub BK), the angle between the ambient magnetic field and the wave propagation vector, and the ion injection velocity.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; A12; p. 21,023-21,037
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  • 4
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    Cometary plasma waves and instabilities (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Various plasma waves and instabilities that have been observed near comets are discussed. Observational results are ordered by plasma parameters and compared with specific instabilities. The variation in the cometary ion and electron beam densities and velocities are studied by examining regions far from and near the comet nucleus. Spacecraft observations relevant to nonlinear wave evolution, wave cascading, and the development of turbulence are reviewed. The implications of the results for wave-particle interactions - linear, nonlinear, resonant, and nonresonant - and their effects on stochastic particle acceleration are addressed. Higher-frequency ELF/VLF wave observations are also reviewed, and comparisons between the various measurements are made.
    Keywords: ASTROPHYSICS
    Type: In: Comets in the post-Halley era. Vol. 2 (A93-13551 02-90); p. 1171-1210.
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  • 5
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    Discrete phase changes within nonlinear steepened magnetosonic waves - Comet Giacobini-Zinner (1990)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Some features of steepened magnetosonic waves are discussed with reference to the Giacobini-Zinner data set. In particular, attention is given to the discovery of discrete intervals of both phase rotation and lack of phase rotation within a single wavelength and also to the presence of intervals of 'backward' rotations (right-hand polarized in the spacecraft frame) within the magnetosonic wave. Possible explanations of these features are reviewed, and it is suggested that these features are nonlinear manifestations of the wave steepening process.
    Keywords: ASTROPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 17; 1817-182
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  • 6
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    Stability of the Halley cometosheath with resistivity and plasma motion (1993)
    In:  Other Sources
    Details
    Publication Date: 2011-08-24
    Description: The MHD stability of the cometary inner sheath determined by the balance between the inward Lorentz body force and the outward ion-neutral drag force is investigated by numerically solving the wave equations which include resistivity, plasma motion and plasma pressure with the help of two-point boundary value method. The eigenvalues and the eigenfunctions are obtained numerically by treating the cometary inner sheath as a layer of finite thickness, bounded by the contact surface, that is, the diamagnetic cavity boundary. To gain insight into the problem, certain limiting cases of the wave equations are also discussed. The diamagnetic cavity boundary and the adjacent layer of about 100-km thickness of Comet Halley is found to be unstable. The effects of finite plasma pressure, dissociative recombination, mass loading due to photoionization, resistivity, and plasma motion are found to be stabilizing but are unable to quench the instability completely. Motion of the Halley ionopause has been confirmed by observations.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; A9; p. 15,263-15,273.
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  • 7
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    Comets - A laboratory for plasma waves and instabilities (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The review discusses the various plasma waves and instabilities detected in the vicinity of comets. Particular emphasis is placed on nonlinear wave evolution, as well as the role of nonlinear waves in wave-particle interactions. Areas that need further research are identified to help persons who wish to contribute to the field.
    Keywords: ASTROPHYSICS
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  • 8
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    Highly nonlinear magnetic pulses at Comet Giacobini-Zinner (1990)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: Nonlinear solitary magnetic pulses have been detected near the outbound bow shock/wave of Comet Giacobini-Zinner. The pulses have peak field-to-background compression ratios ranging from 2.3 to 7.0 and full-width half-maximum durations from 12 to 72s, comparable to the H2O group ion gyroperiod. Because all of the solitary waves have the same spacecraft-frame polarizations, it is suggested that the waves may have phase speeds greater than the solar wind speed. The role that the waves may play with the bow shock is discussed briefly.
    Keywords: ASTROPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 17; 757-760
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  • 9
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    Comment on 'Comparison of observed and calculated implanted ion distributions outside Comet Halley's bow shock' by T. I. Gombosi, M. Neugebauer, A. D. Johnstone, A. J. Coates, and D. E. Huddleston (1993)
    In:  Other Sources
    Details
    Publication Date: 2019-08-28
    Description: It is argued that the formulation used by Gombosi (1991) does not distinguish between two fundamental types of resonant interactions (those between cometary pickup ions and LF electromagnetic waves). Two problems arise from using their overly simplified expressions. The wave polarization of the solar wind Alfven wave is not purely right-handed, and values from alpha sub com and alpha sub sw are not well known, nor are the 'polarizations' of the 'backward'-going waves in either case. Gombosi et al. discuss the antisunward vs the sunward propagating solar wind Alfven wave, and assume a reasonable value of 0.2 for alpha sub sw. However, alpha sub com is totally unknown. It is concluded that a more correct analysis of wave polarization can lead to significant changes in the formulation and results from Gombosi et al. In his reply Gombosi argues that his simplified wave-particle interaction model is consistent with Giotto observations and plasma kinetic simulations and it leads to a reasonable approximation of the implanted ion transport coefficients. The more complicated approach suggested by Tsurutani & Thorne (1993) requires the introduction of several ad hoc parameters and it does not lead to any change of the energy diffusion coefficient.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; A3; p. 3623-3625; Author
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