Particle Trapping in Axisymmetric Electron Holes

Electron orbits are calculated in solitary two‐dimensional axisymmetric electrostatic potential structures, typical of plasma electron holes, in order to establish the conditions for the particles to remain trapped. Analytic calculations of the evolution of the parallel energy caused by the perturbi...

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Bibliographic Details
Published in:Journal of geophysical research. Space physics 2020-08, Vol.125 (8), p.n/a
Main Author: Hutchinson, I. H.
Format: Article
Language:English
Subjects:
Algorithms
detrapping
Distribution functions
Electric fields
electron holes
Electrons
Field strength
Gyrofrequency
Harmonics
Holes (electron deficiencies)
islands
Magnetic fields
Magnetic moments
Mathematical analysis
Orbital resonances (celestial mechanics)
Orbits
Perturbation
plasma
stochasticity
trapped orbits
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Summary:Electron orbits are calculated in solitary two‐dimensional axisymmetric electrostatic potential structures, typical of plasma electron holes, in order to establish the conditions for the particles to remain trapped. Analytic calculations of the evolution of the parallel energy caused by the perturbing radial electric field (breaking magnetic moment invariance) are shown to agree well with full numerical orbit integration Poincaré plots. The predominant mechanism of detrapping is resonance between the gyrofrequency in the parallel magnetic field and harmonics of the parallel bounce frequency. A region of phase space adjacent to the trapped‐passing boundary in parallel energy is generally stochastic because of island overlap of different harmonics, but except for very strong radial electric field perturbation, more deeply trapped orbits have well‐defined islands and are permanently confined. A simple universal quantitative algorithm is given, and its results plotted as a function of magnetic field strength and hole radial scale length, determining the phase space volume available to sustain the electron hole by depression of the permanently trapped distribution function. Key Points Electron holes of finite transverse extent are subject to trapped orbit losses arising from the violation of magnetic moment conservation Detrapping is calculated analytically and verified by numerical orbit integration, finding the phase space that can sustain the hole The results appear to offer a qualitative explanation for some trends concerning electron hole aspect ratio observed in space plasmas
ISSN:2169-9380
2169-9402
DOI:10.1029/2020JA028093