Other
Scientific paper
Apr 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995jgr...100.5835k&link_type=abstract
Journal of Geophysical Research (ISSN 0148-0227), vol. 100, no. A4, p. 5835-5852
Other
11
Distribution Functions, Dynamic Stability, Extremely Low Frequencies, Linearity, Magnetic Variations, Nonlinearity, Oxygen, Power Spectra, Protons, Ring Currents, Wave Excitation, Computerized Simulation, Earth (Planet), Giacobini-Zinner Comet, Grigg-Skjellerup Comet, Interplanetary Magnetic Fields, Shock Waves, Two Dimensional Models
Scientific paper
The problem of the exitation of obliquely propagating magnetosonic waves which can steepen up (also known as 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. We 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, we have found that the outcome of the simulations can vary, suggesting that the latter type simulations 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.
Karimabadi Homa
Killen K.
Krauss-Varban Dietmar
Omidi Nojan
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