Linear and nonlinear properties of ULF waves driven by ring-beam distribution functions
- 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, Vol. 100, Issue A4; Other Information: PBD: 1 Apr 1995
- Country of Publication:
- United States
- Language:
- English
Similar Records
Kinetic study of solar wind mass loading and cometary bow shocks
Subcritical dispersive shock waves upstream of planetary bow shocks and at comet Giacobini-Zinner