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  • GEOPHYSICS  (5)
  • SOLAR PHYSICS  (1)
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  • 1
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    Theory of 2 omega(sub pe) radiation induced by the bow shock (1994)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: A new radiation emission mechanism is proposed to explain electomagnetic radiation observed at twice the electron plasma frequency, 2 omega(sub pe), in the upstream region of the Earth's bow shock. This radiation had its origin at the electron foreshock boundary where energetic electron beams and intense narrow-band Langmiur waves are observed. The proposed emission mechanism results from the interaction of the electron beam and Langmuir waves that are backscattered off thermal ions. This interaction is described by a nonlinear dispersion equation which incorporates an effect owing to electron trajectory modulation by the backscattered Langmuir waves. Subsequent analysis of the dispersion equation reveals two important consequences. First, a long-wavelength electrostatic quasi-mode with frequency at 2 omega(sub pe) is excited, and second, the quasi-mode and the electomagnetic mode are nonlinearly coupled. The implication is that, when the excited 2 omega(sub pe) quasi-mode propagates in an inhomgeneous medium with slightly decreasing density, the quasi-mode can be converted directly into an electromagnetic mode. Hense the electomagnetic radiation at twice the plasma frequency is generated. Numerical solutions of the dispersion equation with the choice of parameters that describe physical characteristics of the electron foreshock are presented, which illustrates the viability of the new mechanism.
    Keywords: SOLAR PHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 99; A12; p. 23,481-23,488
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  • 2
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    Kinetic friction attributed to enhanced radiation by cyclotron maser instability (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Along the auroral field lines, a fraction of the energetic electrons injected from the magnetotail is reflected by the earth's convergent geomagnetic field. The reflected loss-cone electrons are unstable with respect to the cyclotron maser instability, resulting in the auroral kilometric radiation. This paper investigates the kinetic friction force exerted on the energetic electrons by the enhanced radiation field. It is found that the enhanced radiation results in a deceleration of reflected electrons, thereby providing an effective resistivity. In addition, the rate of decrease (increase) of effective perpendicular (parallel) kinetic temperatures is also evaluated. The analysis is carried out over various physical parameters such as the degree of loss cone, average particle energy, and the ratio of plasma frequency to cyclotron frequency.
    Keywords: GEOPHYSICS
    Type: Physical Review A (ISSN 1050-2947); 44; 6819-682
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  • 3
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    Kilometric radio waves generated along auroral field lines observed by ground facilities - A theoretical model (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: A theory of generation of radio waves observed by ground-based facilities in the frequency range 150-700 kHz is discussed. This work is a continuation of an earlier discussion (Wu et al., 1989) in which it was proposed that the trapped electrons along the auroral field lines can lead to a cyclotron instability which amplifies the whistler waves observed at ground level. The objective of the present study is to investigate the propagation effect on the wave amplification and to examine whether the proposed mechanism is indeed viable.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 96; 1495-150
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  • 4
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    A theory of electron cyclotron waves generated along auroral field lines observed by ground facilities (1989)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: A generation mechanism for radio waves in the frequency range 150 - 700 kHz observed by ground facilities is suggested in terms of an electromagnetic electron cyclotron instability driven by auroral electrons. The excited waves can propagate downward along the ambient magnetic field lines and are thus observable with ground facilities. The trapped auroral electrons are supposed to play an important role in the generation process, because they give rise to a thermal anisotropy which consequently leads to the instability. The present work is a natural extension of the theory proposed earlier by Wu et al. (1983) which was discussed in a different context but may be used to explain the observed waves originated at low altitudes. This paper presents a possible wave generation mechanism valid in the entire auroral field-line region of interest.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 16; 1461-146
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  • 5
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    Quasi-linear analysis of ion Weibel instability in the earth's neutral sheet (1993)
    In:  Other Sources
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    Publication Date: 2019-08-27
    Description: A quasi-linear analysis of the ion Weibel instability (IWI) for waves with parallel propagation is carried out for parameters appropriate to the earth's neutral sheet during the substorm interval. For ion drift speed reaching sizable fraction of the ion thermal speed, unstable waves grow to a nonlinear regime in a time interval greater than an ion gyroperiod. The saturation level is attained with current density reduced to about 15-28 percent of its preactivity level. The unstable wave amplitude normalized to the initial ambient field is found to be in the range of 0.2-0.8. This is accompanied by ion heating along the magnetic field with the parallel temperature being enhanced by 25-90 percent. Thus, the IWI can provide nonadiabatic heating of ions in current disruptions during substorms. The associated anomalous resistivity is estimated to be about 1 x 10 exp -7 to 1 x 10 exp -6 s, which is about 11 to 12 orders of magnitude above the classical resistivity.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; A1; p. 153-163.
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  • 6
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    Nonlinear analysis of generalized cross-field current instability (1993)
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    Publication Date: 2019-08-27
    Description: Analysis of the generalized cross-field current instability is carried out in which cross-field drift of both the ions and electrons and their temperatures are permitted to vary in time. The unstable mode under consideration is the electromagnetic generalization of the classical modified-two-stream instability. The generalized instability is made of the modified-two-stream and ion-Weibel modes. The relative importance of the features associated with the ion-Weibel mode and those of the modified-two-stream mode is assessed. Specific applications are made to the Earth's neutral sheet prior to substorm onset and to the Earth's bow shock. The numerical solution indicates that the ion-Weibel mode dominates in the Earth's neutral sheet environment. In contrast, the situation for the bow shock is dominated by the modified-two-stream mode. Notable differences are found between the present calculation and previous results on ion-Weibel mode which restrict the analysis to only parallel propagating waves. However, in the case of Earth's bow shock for which the ion-Weibel mode plays no important role, the inclusion of the electromagnetic ion response is found to differ little from the previous results which treats ions responding only to the electrostatic component of the excited waves.
    Keywords: GEOPHYSICS
    Type: Physics of Fluids B (ISSN 0899-8221); 5; 3; p. 836-853.
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