Transverse Magnetic Perturbations Research Articles

Auroral arcs as sites of magnetic stress release

An analytical model is presented for auroral arcs as the result of a fast release of magnetic shear stresses. The shear stresses are set up by a longitudinal convection that is driven by pressure forces in the outer magnetosphere against the frictional forces exerted in the lower ionosphere. A distorted‐dipole geometry is employed allowing for high plasma beta near the equator. Steep ledges in the radial pressure distribution, extending along the direction of convection, are invoked as the sources of the auroral current sheets. The differential magnetic energy content of these narrow current sheets is released within a few Alfvén transit times by the decoupling of the magnetospheric plasma and field from the ionosphere, owing to the existence of field‐aligned potential drops in the auroral acceleration region, and converted into kinetic energy of the primary auroral particles. A well‐known current‐voltage relation is employed for the formulation of the energy conversion process. This scenario has two important consequences. (1) The loss of magnetic energy creates a concomitant decrease of internal energy of the generator plasma and results in a progression of pressure ledge and auroral current sheet into the more highly stressed magnetic field region. This is the reason for the observed proper motion of auroral arcs with respect to the plasma frame. (2) Plasma and field undergo a rapid stress relief motion along the arc with large but mostly reversible displacements. The net displacement, equivalent to a small S‐shaped contribution to the essentially U‐shaped potential distribution above the auroral arc, is consistent with the transit of the field lines through the progressing current sheet. This scenario is cast into a set of simple relations expressing the key parameters of auroral arcs, such as width, energy flux, potential drop, and proper motion. The main ingredient herein is an auxiliary magnetic perturbation field into which the main properties of the large‐scale current system are condensed. It corresponds to about twice the transverse magnetic perturbation field near the arc and thus to the total shear stresses. Two free parameters are the relative magnitude of the pressure jump at the ledge in the source plasma and the plasma beta. Matching the quantitative results of the relations for the arc properties with observed values suggests pressure jumps of order 10% and beta values between 1 and 5.

New observations, new theoretical results and controversies regarding Pc 3–5 waves

Observations and theories of medium- to long-period (Pc 3–5) magnetic pulsations excited by magnetospheric particles are described. Satellite observations indicate that most pulsations can be classified into two groups according to their magnetic field polarization. One group has a transverse magnetic perturbation and the other strongly compressional perturbation. Despite this difference in polarization they share common characteristics, including large azimuthal wave number, westward propagation, and antisymmetric field-aligned structure. Recent theories describe these observations in a unified framework. It has been pointed out that trapped energetic ions play an important role in determining the instability threshold and the mode structure of the pulsations. Observations and theories of energetic particle response to the excited pulsations are also described.

Effective plasma heat conductivity in 'braided' magnetic field-I. Quasi-linear limit

Anomalous cross-field electron transport in a specified magnetic field with weakly-destroyed flux surfaces is discussed. Following the approach developed previously in the present paper the regimes of effective transverse plasma transport are studied systematically, both in the collisional and collisionless limits. The present analysis incorporates the nonstationary of magnetic perturbations, which was not included in the previous works. Part I of this study deals with the quasi-linear approximation, which may be written as b0L0/ delta <<1, where b0= delta Bperpendicular to /B0 represents the relative magnitude of the transverse magnetic perturbation, while L0 and delta represent the longitudinal and transverse correlation lengths, respectively. It is found that some of the previously-described transport regimes cannot be considered as anomalous (in the sense that effective heat conduction chi eff>> chi perpendicular to ) for time-independent magnetic perturbations. However, these regimes can exist in the presence a finite-frequency magnetic flutter. A unified classification of quasi-linear regimes of anomalous transport is introduced in order to further extend the analysis to the strong magnetic turbulence limit b0L0/ delta >>1 (see Part II), which is considered as a companion paper. (Isichenko, ibid., vol.33, no.7, p.809-26 (1991)).

Coupling between alfvén and slow magnetosonic waves in an inhomogeneous finite- β plasma—II. Eigenmode analysis of localized ballooning-interchange instability

Ballooning-interchange instability of guided hydromagnetic waves is examined by performing a numerical eigenmode analysis along a model field-line. This instability is driven by a combined effect of the plasma pressure gradient and the field-line curvature and can be considered as a candidate of the excitation mechanism of storm-time Pc5 pulsations in the outer magnetosphere. The system of eigenmode equations derived in an accompanying paper describes the coupling between Alfvén and slow magnetosonic modes due to inhomogeneity. Such a coupling results in oscillations with hybrid properties between the two, for example, the transverse magnetic perturbation in the radial direction and the out-of-phase relation in perturbed plasma and magnetic pressures ; they are important characteristics of the storm-time Pc5 pulsation. Eigenvalues of the fundamental mode oscillation have a positive imaginary part, that is, the fundamental eigenmode is unstable. The oscillation frequency of this unstable mode is proportional to the azimutha wave number, and is comparable with the ion drift frequency at the equator. These results suggest that the ion drift mode plays an important role in the unstable mode. Although the broad distribution of eigenfunctions along the field-line and a somewhat small value of the oscillation frequency are not consistent with observations, such inconsistencies are due to the oversimplification adopted in the model of plasma and magnetic fields. In practice, high energy particles can be expected to play a decisive role in determining the properties of the pulsations in the magnetosphere. Hence, it is concluded that the ballooning-interchange instability is a strong candidate for the excitation mechanism of the storm-time Pc5 pulsation.

Intense current structures during low geomagnetic activity and their relation to small-scale magnetic perturbations seen by the intercosmos Bulgaria-1300

This paper deals with the initial observational data on the field-aligned currents (FAC) and the small-scale transverse magnetic perturbations (SSTMD) in the cusp region obtained from the magnetometer on board the Intercosmos Bulgaria-1300 satellite. The magnetic field has been investigated by a high accuracy flux-gate magnetometer, IMAP, designed in the Central Laboratory for Space Research (CLSR), Sofia. For low geomagnetic activity 10 meridional passes (September–December 1981) have been examined. Intense FAC are observed in the noon sector of the summer auroral region. SSTMD are superimposed on a weak cusp current as a perturbation in the prenoon sector of the winter auroral region.

Stability properties of a field-reversed ion layer in a background plasma

Stability properties of an intense proton layer (P layer) immersed in a background plasma are investigated within the framework of a hybrid model in which the layer ions are described by the Vlasov equation, and the background plasma electrons and ions are described as macroscopic, cold fluids. Moreover, the stability analysis is carried out for frequencies near multiples of the mean rotational frequency of the layer. It is assumed that the layer is thin, with radial thickness (2a) much smaller than the mean radius (R0). Electromagnetic stability properties are calculated for flute perturbations (∂/∂z=0) about a P layer with rectangular density profile, described by the rigid-rotor equilibrium distribution function f0b= (minb/2π) δ (U−T̂) G (vz), where nb and T̂ are constants, mi is the mass of the layer ions, G (vz) is the parallel velocity distribution, and U is an effective perpendicular energy variable. Stability properties are investigated including the effects of (a) the equilibrium magnetic field depression produced by the P layer, (b) transverse magnetic perturbations (δB≠0), (c) small (but finite) transverse temperature of the layer ions, and (d) the dielectric properties of the background plasma. All of these effects are shown to have an important influence on stability behavior. For example, for a dense background plasma, the system can be easily stabilized by a sufficiently large transverse temperature of the layer ions.

Self-consistent theory of cyclotron maser instability for intense hollow electron beams

A self-consistent theory of the cyclotron maser instability, assuming azimuthally symmetric perturbations about a slowly rotating hollow electron beam propagating parallel to a uniform axial magnetic field B0êz is developed. The stability analysis is carried out within the framework of the linearize Vlasov–Maxwell equations. It is assumed that the beam is thin with radial thickness (2a) much smaller than the beam radius (R0), and that ωp2/ω2c≪1, where ωp and ωc are the electron plasma frequency and electron cyclotron frequency, respectively, in a frame of reference moving with the beam axial velocity cβb. The analysis is carried out for the specific choice of equilibrium electron distribution function in which all electrons have the same value of canonical angular momentum and the same value of energy in a frame of reference moving with axial velocity cβb. Stability properties are investigated including the important influence of finite radial geometry, finite beam temperature, and transverse magnetic perturbations (δB≠0). It is shown that instability exists for a very narrow range of axial wavenumbers satisfying ‖k−βbω/c‖≪1/R0. Detailed stability properties are calculated for a variety of system parameters.

Influence of magnetic shear on the lower-hybrid-drift instability in toroidal reversed-field pinches

The influence of magnetic shear on the lower-hybrid-drift instability is investigated, including application to microstability properties of post-implosion reversed-field-pinch profiles. Two complementary analyses are performed. One analysis includes the influence of magnetic shear and finite ion beta in a fully electromagnetic model that assumes Te/Ti≪1, but otherwise incorporates the effects of transverse magnetic perturbations (δB≠0). The other analysis includes the influence of magnetic shear and finite Te/Ti in an electrostatic model that neglects finite plasma beta effects. In both cases, it is found that sufficiently strong magnetic shear can completely stabilize the lower-hybrid-drift instability. The theoretical model is used to investigate the microstability properties of post-implosion reversed-field-pinch profiles. It is shown that the lower-hybrid-drift instability persists in regions of moderate magnetic shear only if the local density gradient is sufficiently large. Moreover, for the diffuse profiles anticipated in the Los Alamos ZT-40 experiment, the lower-hybrid-drift instability is completely stabilized.

Kinetic description of negative-mass instability in an intense relativistic nonneutral E layer

The negative-mass stability properties of a relativistic nonneutral E layer are investigated within the framework of the Vlasov–Maxwell equations. It is assumed that the E layer is thin with radial thickness (2a) much smaller than the mean radius (R0), and that ν/γ0≪1, where ν is Budker’s parameter and γ0mc2 is the electron energy. The equilibrium electron charge is partially neutralized by a positive ion background with density n0i(r) =fn0e(r), where f=const=fractional charge neutralization. Moreover, no a priori restriction is made to low beam density (ω2p≪ω2c is not assumed), and the stability analysis is carried out for perturbations about the electron equilibrium f0e= (N0/2Δ) δ (U−γ̂mc2) ⊕[Δ2−(Pϑ−P0)2], where N0, Δ, γ̂, and P0 are constants, U is an effective energy variable, and Pϑ is the canonical angular momentum. The negative-mass growth rate is calculated including the effects of (a) equilibrium self-fields, (b) transverse magnetic perturbations (δB≠0), (c) the presence of inner and outer cylindrical conductors, and (d) a spread in canonical angular momentum (2Δ). All of these effects are shown to have an important influence on stability behavior.

Plasma Resonance Interaction with a Spatially Rotating Static Magnetic Field

The ion cyclotron resonance interaction of plasma with a rotating transverse magnetic perturbation on a longitudinal magnetic field is investigated experimentally and theoretically. The plasma is injected axially from a coaxial hydromagnetic gun along a longitudinal magnetic field on which there is superimposed a transverse spatially rotating perturbation field. A resonance transfer of ion energy from the longitudinal to the transverse direction is observed when the spatial period or wavelength of the perturbation field and the plasma velocity and ion cyclotron frequency satisfy the relation λz = 2πvz/ΩB. Measurements of plasma diamagnetism and transit time show an increase of up to a factor of two in diamagnetism coupled with a decrease in axial velocity corresponding to a reduction to half the initial longitudinal energy. Resonance may be observed over a range of longitudinal energies by varying the parameters of the system. For the plasma gun and perturbation field helix used in the experiment, optimum resonance was obtained with plasmas of longitudinal energy of ∼ 2.5 keV and particle densities of ∼ 1015cm−3(β ≳ 0.8). A theoretical account of the effect is given which considers the excitation by the perturbation field of circularly polarized electromagnetic waves in the plasma at ion cyclotron frequency and the transfer of longitudinal ion energy into transverse motion through the intermediate action of the waves.