Linear and nonlinear properties of ULF waves driven by ring-beam distribution functions

Journal Article · Sat Apr 01 04:00:00 EST 1995 · Journal of Geophysical Research

DOI:https://doi.org/10.1029/94JA02899· OSTI ID:166219

Killen, K; Omidi, N; Karimabadi, H ^[1]

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1. Univ. of California, La Jolla, CA (United States); and others

The problem of the excitation of obliquely propagating magnetosonic waves which can steepen up (also known a shocklets) is considered. Shocklets have been observed upstream of the Earth`s bow shock and at comets Giacobini-Zinner and Grigg-Skjellerup. Linear theory as well as two-dimensional (2-D) hybrid (fluid electrons, particle ions) simulations are used to determine the properties of waves generated by ring-beam velocity distributions in great detail. The effects of both proton and oxygen ring-beams are considered. The study of instabilities excited by a proton ring-beam is relevant to the region upstream of the Earth`s bow shock, whereas the oxygen ring-beam corresponds to cometary ions picked up by the solar wind. Linear theory has shown that for a ring-beam, four instabilities are found, one on the nonresonant mode, one on the Alfven mode, and two along the magnetosonic/whistler branch. The relative growth rate of these instabilities is a sensitive function of parameters. Although one of the magnetosonic instabilities has maximum growth along the magnetic field, the other has maximum growth in oblique directions. The authors have studied the competition of these instabilities in the nonlinear regime using 2-D simulations. As in the linear limit, the nonlinear results are a function of beam density and distribution function. By performing the simulations as both initial value and driven systems, the authors have found that the outcome of the simulations can vary, suggesting that the latter type of simulation is needed to address the observations. A general conclusion of the simulation results is that field-aligned beams do not result in the formation of shocklets, whereas ring-beam distributions can. 47 refs., 19 figs.

• OSTI ID: 166219
• Journal Information: Journal of Geophysical Research, Journal Name: Journal of Geophysical Research Journal Issue: A4 Vol. 100; ISSN JGREA2; ISSN 0148-0227
• Country of Publication: United States
• Language: English

Related Subjects

66 PHYSICS
CYCLOTRON FREQUENCY
EARTH MAGNETOSPHERE
EXCITATION
INSTABILITY
ION BEAMS
MAGNETOACOUSTIC WAVES
MAGNETOPAUSE
OXYGEN IONS
PROTONS
SHOCK WAVES
SOLAR WIND