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Title: Ring-beam driven maser instability for quasiperpendicular shocks

Abstract

The cyclotron maser instability is a well-known radiation emission mechanism responsible for radio emissions in magnetized planets and for laboratory microwave generation devices. The present paper discusses mechanisms and properties of cyclotron maser instability driven by a ring-beam distribution of energetic electrons with application to the quasiperpendicular collisionless shock. It is shown that the fast extraordinary and ordinary electromagnetic waves as well as slow upper-hybrid Z-mode are excited over a wide range of physical parameters. The implications of the present findings for actual applications including the coronal type II radio source are also discussed.

Authors:
; ;  [1];  [2]
  1. Institute for Physical Science and Technology, University of Maryland, College Park, Maryland 20742 (United States)
  2. (China)
Publication Date:
OSTI Identifier:
20974843
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 2; Other Information: DOI: 10.1063/1.2437118; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; BEAM-PLASMA SYSTEMS; CYCLOTRONS; ELECTRON BEAMS; MASERS; MICROWAVE RADIATION; PLASMA; PLASMA INSTABILITY; SHOCK WAVES; TAIL ELECTRONS

Citation Formats

Yoon, Peter H., Wang, C. B., Wu, C. S., and CAS Key Laboratory of Basic Plasma Physics, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026. Ring-beam driven maser instability for quasiperpendicular shocks. United States: N. p., 2007. Web. doi:10.1063/1.2437118.
Yoon, Peter H., Wang, C. B., Wu, C. S., & CAS Key Laboratory of Basic Plasma Physics, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026. Ring-beam driven maser instability for quasiperpendicular shocks. United States. doi:10.1063/1.2437118.
Yoon, Peter H., Wang, C. B., Wu, C. S., and CAS Key Laboratory of Basic Plasma Physics, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026. Thu . "Ring-beam driven maser instability for quasiperpendicular shocks". United States. doi:10.1063/1.2437118.
@article{osti_20974843,
title = {Ring-beam driven maser instability for quasiperpendicular shocks},
author = {Yoon, Peter H. and Wang, C. B. and Wu, C. S. and CAS Key Laboratory of Basic Plasma Physics, School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230026},
abstractNote = {The cyclotron maser instability is a well-known radiation emission mechanism responsible for radio emissions in magnetized planets and for laboratory microwave generation devices. The present paper discusses mechanisms and properties of cyclotron maser instability driven by a ring-beam distribution of energetic electrons with application to the quasiperpendicular collisionless shock. It is shown that the fast extraordinary and ordinary electromagnetic waves as well as slow upper-hybrid Z-mode are excited over a wide range of physical parameters. The implications of the present findings for actual applications including the coronal type II radio source are also discussed.},
doi = {10.1063/1.2437118},
journal = {Physics of Plasmas},
number = 2,
volume = 14,
place = {United States},
year = {Thu Feb 15 00:00:00 EST 2007},
month = {Thu Feb 15 00:00:00 EST 2007}
}
  • We study the cyclotron maser instability (CMI) driven by an energetic ring-beam distribution by a particle simulation to explain possible generation mechanisms of intense radiation phenomena observed in space. The main objective is to understand the nonlinear processes that control saturation of the emission process. Our study reveals new issues that have been overlooked in past literature. It is found that electrostatic wave modes excited by the electron beam instability compete with the electromagnetic waves excited by the CMI. Nonlinear effects of these electrostatic modes tend to redistribute the energy of the energetic electrons and make the physics more complicated.more » The CMI can be much less effective in a realistic case than it is anticipated theoretically.« less
  • The overtaking of one collisionless shock by another is studied by means of hybrid numerical simulations. The two shocks merge into a stronger shock and trailing nonshock discontinuities. The strong shock continues to propagate in the same direction as the two weaker shocks. The merging is shown to occur by a self-consistent process involving the interaction of ions reflected at the overtaking shock with the plasma upstream of the leading shock. The characteristic time scale for the merging is typically {Omega}{sup {minus}1}{sub {ital i}}, where {Omega}{sub {ital i}} is the ion gyrofrequency. For exactly perpendicular shocks, the trailing discontinuity ismore » a tangential discontinuity. It has a width of 2--3 ion Larmor radii. For oblique shocks, a contact discontinuity is present in the downstream plasma state. These results are of relevance to shock interactions in the very distant solar wind as well as in other energetic astrophysical situations such as solar flares.« less
  • The behavior of minor ions just downstream of a low Mach number quasiperpendicular shock is investigated both theoretically and by computer simulations. Because all ions see the same cross shock electric field their deceleration depends on their charge to mass ratio, yielding different downstream velocities. It is shown that these differences in velocity can lead to coherent wave structures in the downstream region of quasiperpendicular shocks with a narrow transition layer. These waves are shown to be multi ion hybrid waves in contrast to mirror waves and ion cyclotron waves. Under favorable conditions these waves should be observable both atmore » interplanetary shocks and at planetary bowshocks.« less
  • One-dimensional full particle simulations of almost perpendicular supercritical collisionless shocks over a wide Alfven Mach number range are presented. The physical ion to electron mass ratio has been used; however, due to computer time limitations a value of the ratio of the electron plasma frequency to the electron gyrofrequency of 4 has been assumed. The shock structure in the density and magnetic field consists of a foot, formed by reflected ions, and a steeper ramp leading to an overshoot. It is shown that the shock ramp scale in units of the upstream ion inertial length is more or less constantmore » and close to 1 over the Mach number regime investigated, i.e., up to M{sub A}{approx_equal}14. Further, the convective ion gyroradius in units of the upstream ion inertial length is also constant with the Mach number when the gyroradius is evaluated with the magnetic field strength in the overshoot. Thus the shock transition also scales with the convected gyroradius. When a hyperbolic tangent function is fitted to the density profile the neglect of the overshoot essentially results, for high Mach number shocks, in a fit of the foot and not of the ramp, i.e., the shock transition scale is grossly overestimated. The simulations suggest that in a regime above the critical Mach number the nonlinear steepening is balanced by gyroviscosity of the reflected ions as the shock ramp scale is given by the convected gyroradius in the overshoot. At higher Mach numbers the shock becomes unsteady the ramp scale can become as small as several electron inertial length during a part of the reformation cycle. At still higher Mach number microinstabilities in the foot may have growth times much shorter than the inverse ion gyrofrequency so that they can lead to ion heating, and a steady resistive shock will result.« less
  • A purely growing mode propagating exactly parallel to the ambient magnetic field, driven by a cross-field current was first discussed by Chang {ital et} {ital al}. (Phys. Rev. Lett. {bold 65}, 1104 (1990)). This paper examines the nonlinear evolution of this instability and the resulting heating of the plasma with particular emphasis on application to the quasiperpendicular collisionless shock wave. It is found that when the cross-field current is due to electrons, the instability saturates to extremely low level and the resulting heating becomes insignificant, whereas if the ions are allowed to drift the saturation level of the unstable wavemore » is very large with substantial heating of both electrons and ions. On the other hand, if the electron drift is determined by equilibrium conditions such as the magnetic field and density inhomogeneities and local electric field at the shock front, it is found that the plasma heating becomes greatly enhanced.« less