Uniform Ambient Magnetic Field Research Articles

Electrostatic ion-cyclotron instability caused by a nonuniform electric field perpendicular to the external magnetic field

A new mechanism that can destabilize kinetic ion-cyclotron waves in the presence of a nonuniform electric field perpendicular to the uniform ambient magnetic field is given. In the absence of the electric field, the mode energy is positive, while in the presence of a uniform electric field the mode energy could be negative. However, when the electric field is nonuniform, it is possible for a finite region to be a negative wave energy surrounded by regions of positive wave energy. A nonlocal wave packet couples the two regions so that a flow of energy from the region of negative wave energy to the region of positive wave energy will cause the mode to brow. This gives rise to the instability.

Cosmic-Ray Evolution due to Interactions with Self-Excited Plasma Waves

view Abstract Citations (12) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Cosmic-Ray Evolution due to Interactions with Self-Excited Plasma Waves Lee, Martin A. Abstract We use kinetic equations describing a relativistic plasma embedded in a uniform ambient magnetic field to investigate the time evolution of the cosmic-ray momentum distribution due to interactions between the cosmic-ray particles and their self-excited waves. Within the cosmic-ray lifetime there is negligible change in the cosmic-ray energy spectrum, but for particles with energy less than about 20 0eV there occurs a marked reduction of the cosmic-ray streaming velocity to a value of the order of, but greater than, the AlIven speed, the exact value depending sensitively on the timescale for ion-neutral collisions. Publication: The Astrophysical Journal Pub Date: December 1972 DOI: 10.1086/151838 Bibcode: 1972ApJ...178..837L full text sources ADS |

Self-consistent kinetic equations and the evolution of a relativistic plasma in an ambient magnetic field

A kinetic theory is developed for a relativistic, spatially- homogeneous multispecies plasma embedded in a uniform ambient magnetic field. The theory treats individual particle effects and collective effects on equal footing, and thereby describes both stable and weakly unstable plasma. The three-particle correlation, and nonlinear mode-mode coupling terms are neglected and a 'quasi- longitudinal, weak-field' approximation is employed in calculating the electric and magnetic field fluctuations. The final equations for the long time evolution of the particle distribution functions and the plasma wave intensities emphasize that the evolution of particles and their 'self-excited' waves is coupled in a non-linear manner. Specialized equations for a plasma consisting of cold electrons and protons and a relativistic proton 'tail' are given. These are particularly suited to a discussion of cosmic ray propagation through the interstellar medium no matter whether the cosmic ray distribution is stable or unstable.

Calculation of the Signal Voltage Induced in a Toroidal Proton-Precession Magnetometer Sensor

The signal EMF induced as a function of time in a proton-precession magnetometer sensor of toroidal shape with an elliptical cross-sectional area is calculated in terms of the magnetic properties of the sensor fluid, polarizing current, orientation of the sensor with respect to the direction of the ambient magnetic field, and geometric proportions of the sensor. It is assumed that the sensor is in a uniform ambient magnetic field; the polarizing current is reduced to zero in a time that is short compared to the period of the Larmor precession of the nuclear magnetic moments in the sensor fluid; the nuclear magnetic moments do not interact; and no significant current flows in the sensor winding during the relaxation process in which the EMF is measured. In the specific case that the sensor winding fills the toroid window and that the polarizing supply is a constant voltage source, the calculations permit a simple comparison of variously proportioned sensors in terms of the signal EMF amplitude.

Plasma Instabilities of Streaming Cosmic Rays

view Abstract Citations (58) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Plasma Instabilities of Streaming Cosmic Rays Tademaru, E. Abstract A general expression is derived for the growth rate, due to particle-wave resonance, of plasma waves propagating in a spatially homogeneous, but anisotropic, relativistic plasma embedded in a uniform ambient magnetic field. This expression is good for plasma waves of arbitrary polarizations, wave vectors, and frequencies, provided that the growth rate is much less than the real part of the wave frequency. Applying this expression to an idealized model of cosmic rays streamIng parallel to the magnetic field in a cold background plasma, we find the system to be unstable against waves propagating at an arbitrary angle U to the field direction, provided that the phase velocity of the waves is less than the streaming velocity times the cosine of 0. If the streaming velocity is greater than the Alfvén velocity, then the maxi- mum growth rate occurs at frequencies much less than the cyclotron frequency of the cold ions. These low-frequency instabilities persist over a wide range of propagation angles since the growth rates vary only slowly with angle. These instabilities have e-folding times very much shorter than the lifetime of the cosmic rays in the galactic disk. An estimate of the time for the waves to gain a significant part of the bulk-streaming energy of the cosmic rays is obtained from the second-order equations. It is argued that the main effect of the waves is to reduce the mean streaming velocity by redistribution of momentum pitch angles with little transfer of energy Publication: The Astrophysical Journal Pub Date: December 1969 DOI: 10.1086/150255 Bibcode: 1969ApJ...158..959T full text sources ADS |

Expansion of a Plasma Shell into a Vacuum Magnetic Field

The problem of an initially spherical, superconducting plasma shell expanding into an ambient uniform magnetic field is solved by approximate analytic and numerical methods. The solution is found to depend on only one parameter, the ratio of the initial kinetic energy of the plasma to the magnetic energy initially displaced by the plasma. The dependence of the solutions on this parameter is studied. A criterion for the stability of the solutions is given.

Magnetodynamic turbulence at low magnetic Reynolds number

Golitsyn's equations for the study of magnetodynamic turbulence, and in particular ionospheric turbulence, at low magnetic Reynolds number are discussed. It is shown that the hypothesis of small magnetic Reynolds number is not sufficient to justify these equations and appropriate additional conditions are indicated. The simple case of weak turbulence in the presence of a uniform ambient magnetic field is examined in illustration. The law of energy decay in the final period is obtained by the method of steepest descent—first, from the solutions of Golitsyn's equations, and secondly from the more general equations established by Lehnert. With Lehnert's equations, one finds that the magnetic and the kinetic energies are composed of two contributions, one which decays at t 5/2 as in non-magnetic turbulence and corresponds to equipartition of energy between velocity and magnetic fields, and another one which decays as t ?3 and leads to the partition of energy between the two fields in a proportion which depends on the ratio n of the kinetic and magnetic viscosities. With Golitsyn's equations, one obtains only a t ?3 component. When all the conditions of validity of these equations are satisfied—and not only the condition on Rm—one deduces that the t ?3 component dominates—although it decays faster—until an inaccessibly large time and the t ?3 contributions derived from Lehnert's and Golitsyn's equations are found identical. On the contrary, when certain of these conditions are not verified—even if Rm<1—the t ?3 component is initially comparable to the t 5/2 component and—as it decays faster—becomes rapidly negligible while energy tends to equipartition. Golitsyn's equations lead then to incorrect results. DOI: 10.1111/j.2153-3490.1967.tb01513.x

Magnetohydrodynamic flow of a viscous fluid past a sphere

The flow of a viscous incompressible electrically conducting fluid past a sphere is studied; the uniform ambient flow field is colinear with the ambient uniform magnetic field. The force exerted on the sphere is computed for various conductivities and Reynolds numbers; of particular interest is the distinction in behaviour between the flow with ambient particle speed greater than ambient Alfvén speed and that with particle speed less than Alfvén speed.