Small Magnetic Field Strengths Research Articles

Hybrid code simulations of the solar wind interaction with Comet 19P/Borrelly

Detailed observations of the plasma environment of Comet 19P/Borrelly were made by the Plasma Experiment for Planetary Exploration (PEPE) during the Deep Space 1 (DS1) flyby on 22 September 2001. DS1 flew from north to south relative to the plane of the ecliptic on the sunward side of the comet and all plasma boundaries (i.e., first and last pickup ions, bow shock, decelerated solar wind) were observed to be shifted northward. This surprising result was initially attributed to large well‐collimated dayside jets directed ∼10° northward from the nucleus as observed by the DS1 MICAS camera. However, ground‐based observations of Schleicher et al. (2003) indicate a symmetric gas cloud with respect to the comet‐Sun line. In an attempt to resolve this conundrum, we have conducted a series of hybrid plasma simulations to investigate the effects of the large pickup ion gyroradius (i.e., comparable to the length scale of the cometary obstacle) at 1.36 AU. We find that that a maximum asymmetry, consistent with the observations, is generated when the pickup ion gyroradius is comparable to or less than the standoff distance of the bow shock (BIMF ∼ 4 nT). The degree of asymmetry is reduced for larger and smaller magnetic field strengths.

Polarization dynamics of two-photon and cascade lasers in the presence of an arbitrarily directed magnetic field

We study theoretically the polarization dynamics of a new type of quantum oscillator that is based on the two-photon stimulated emission process in the presence of a magnetic field of arbitrary orientation. Both cases of cascade (small intermediate-state atomic detuning) and two-photon (large atomic detuning) lasers are considered. The primary goal of this work is to investigate the origin of recently observed polarization instabilities in a two-photon laser (Pfister et al 2001 Phys. Rev. Lett. 86 4512) using a highly simplified model. It is found that the two-photon laser can emit linearly polarized radiation with its plane of polarization orthogonal to the direction of the magnetic field at small magnetic field strengths. It can also emit elliptically polarized radiation over a large range of magnetic field strengths and orientations. When the magnetic field deviates from a direction perpendicular to the laser cavity axis periodic instabilities can appear through a Hopf bifurcation. This dynamic regime could have contributed to the polarization instabilities observed in the experiment.

Penumbral finestructure: need for larger telescopes

We obtained at the same time G-band images at the Dutch Open Telescope (DOT) on La Palma and spectropolarimetric data in the near infrared at the German Vacuum Tower Telescope (VTT) on Tenerife. The spectropolarimetric data show interesting correlations. Bright filaments have a smaller magnetic field strength, and higher Evershed velocities occur in dark structures. This result is in agreement with some previous observations, but also in contradiction to others. However, we suffer from the fact that the resolution limit of the VTT at a wavelength of 1.565 μm corresponds to 400 km. Spatial power spectra derived from the DOT data indicate a typical width of 250 km for penumbral filaments. Obviously a solar telescope with an aperture of at least 1.5 m is needed to obtain sophisticated results for penumbral structures.

Penumbral finestructure: need for larger telescopes

We obtained at the same time G-band images at the Dutch Open Telescope (DOT) on La Palma and spectropolarimetric data in the near infrared at the German Vacuum Tower Telescope (VTT) on Tenerife. The spectropolarimetric data show interesting correlations. Bright filaments have a smaller magnetic field strength, and higher Evershed velocities occur in dark structures. This result is in agreement with some previous observations, but also in contradiction to others. However, we suffer from the fact that the resolution limit of the VTT at a wavelength of 1.565 μm corresponds to 400 km. Spatial power spectra derived from the DOT data indicate a typical width of 250 km for penumbral filaments. Obviously a solar telescope with an aperture of at least 1.5 m is needed to obtain sophisticated results for penumbral structures.

An experimental study of the influence of a rotating magnetic field on Rayleigh–Bénard convection

A destabilizing vertical temperature gradient and a rotating magnetic field have been applied to a cylindrical column of liquid gallium. The convective flows which arise as a function of these parameters are identified. For small magnetic field strengths, a regime of stationary flow is observed. This regime is bounded by critical values of the Rayleigh and magnetic Taylor numbers. As the rotating magnetic field is increased, the critical Rayleigh number can increase by more than a factor of 10. The rotating magnetic field itself induces an instability at a critical value of the magnetic Taylor number independent of the Rayleigh number. The nature of the bifurcations (whether subcritical or supercritical) and the convective flows occurring at the critical Rayleigh numbers are dependent upon the magnetic Taylor number. For small magnetic Taylor numbers, the experimental observations are consistent with the occurrence of a single asymmetric meridional roll which is driven around the cylinder by the rotating magnetic field.

Pair-tunneling states in semiconductor quantum dots: Ground-state behavior in a magnetic field

Using classical mechanical and quantum Monte Carlo methods we trace the ground-state behavior with an applied magnetic field of localized electron pair states in a quantum dot. By developing a method to treat nonconserved paramagnetic interactions using variational and diffusion quantum Monte Carlo techniques we find (i) a single-triplet transition at very small magnetic field strengths, (ii) enhanced localization of the two electrons with increasing magnetic field, and (iii) a mechanism for pair breakup that is different from that proposed recently by Wan et al. [Phys. Rev. Lett. 75, 2879 (1995)].

Spectroscopy of interface states in HgTe/Hg 1− xCd xTe superlattices

Information on interface states in HgTe/Hg 1− x Cd x Te superlattices has been obtained via studies of interband transitions in an external magnetic field. Interband transitions can exhibit novel characteristic features if interface states are involved. For example, the magnetic field dispersion can be negative at sufficiently small magnetic field strengths if the initial state is an interface state. We discuss in detail the influences of the magnetic field strength, the well and barrier thicknesses, and the temperature on the properties of interband transitions involving interface states.

Shock propagation and the generation of magnetohydrodynamic wave fields in inhomogeneous molecular clouds

We develop a simple one-dimensional model for the interaction of a steady, thin, planar shock wave with a nonrigid cloud which may be in motion relative to the surrounding medium, and we apply the model to shocks impinging on, and propagating through, molecular clouds. Both 'adiabatic' (gamma = 5/3) and radiative (gamma = 1) shocks are considered and we allow for the presence of a uniform magnetic field directed either parallel or perpendicular to the shock normal. The former field orientation is equivalent to the hydrodynamic case, and the latter involves only fast MHD shocks. We focus on the manner in which such shocks can generate internal kinetic motions in the cloud on a range of size and density scales through the direct acceleration of cores and clumps by shocks transmitted into them and through the generation of an MHD wavefield via the reflection of the incident shock at clump boundaries. We find that stronger incident Mach numbers and smaller density contrasts lead to more efficient cloud acceleration, as do isothermal intercloud shocks and small intercloud magnetic field strengths. The acceleration efficiency is insensitive to the adiabatic index and the magnetic field strength in the cloud itself. For typical parameter choices, the direct acceleration of clouds and clumps by strong shocks is found to be substantial and could at least in part account for their observed velocity dispersions. If the shocks are moderately weak, the final velocity of the cloud is linearly related to its initial velocity, with higher acceleration giving shallower slopes (i.e., final velocity distributions which are less sensitive to the initial distribution). Compared to the kinetic energy of the postshock cloud, the energy given to the wavefield at each encounter is small, and the heating of the interclump medium by the dissipation of this wavefield is found to be insufficient to balance the cooling rate in the cloud as a whole (although it may be important in particular regions), even if this medium is warm, unless it is also extremely tenuous (n approximately less than 0.1/cu cm). Nevertheless, the correction for the velocity imparted to the cloud leads to a substantial increase in the critical incident Mach number for wave emission over that reported by Spitzer for the rigid case. The implications of our model for shock-induced star formation are discussed briefly.

A fast pulsar in radio nebula CTB80

A fast pulsar with period of 39.5 milliseconds has been discovered in the peculiar combination nebula CTB80. This nebula has been proposed as the remnant of a possible historical supernova in ad 1408 (refs 1–3), but we argue that neither the nebula nor the new pulsar is linked to this supernova, which is in any case of doubtful veracity4. We suggest instead that the pulsar is at least 10,000 years old and that the observed features in the nebula are the result of the pulsar ploughing through the medium at ˜ 300 km s−1. In contrast to the other young pulsars, this pulsar has a smaller magnetic field strength and is consequently underluminous. We argue that this pulsar is more typical of young pulsars in general, a hypothesis that raises fundamental questions about the birth and evolution of pulsars.

Cyclotron Resonance Spectrum of 2D Polarons

Based on a memory function formalism the cyclotron resonance spectrum of quasi 2D polarons is calculated. From this spectrum we obtain the cyclotron effective polaron mass, which will be compared with the results of other calculations which are based on the calculation of the energy of the Landau levels. We found that the present approach is valid for larger values of α than, e.g., IWBPT.A comparison is made with polaron cyclotron resonance data on InSb-inversion layers and GaAs-heterostructures. Band non-parabolicity, and finite width of the 2D-electron layer are incorporated in our calculation.A survey of the importance of dynamical screening on static and dynamical polaron properties is given in the limit of small magnetic field strength. The importance of screening for finite magnetic fields will be discussed briefly.

Thermally driven hydromagnetic convection in a rapidly rotating sphere

Buoyancy-induced convection is investigated in a rapidly rotating, self-gravitating fluid sphere internally heated by a uniform distribution of heat sources and under the influence of an azimuthal magnetic field whose strength is proportional to the distance ϖ * from the rotation axis. Atten­tion is restricted to relatively small magnetic field strengths (as measured by the parameter ∧ ) such that the dominant force balance remains geo-strophic. Convection is then confined to a thin cylindrical annulus, radius ϖ 0 *, about the rotation axis. A linear analysis is used to find the state of marginal stability together with the corresponding minimum critical value of the modified Rayleigh number, R c (a measure of the buoyancy force required to maintain convective motions). Two distinct modes of instability are found to operate: ‘Rossby’ and ‘magnetic’. When no magnetic field is applied ( ∧ = 0), and when the Prandtl number σ ≪ 1, the instability takes the form of a thermally driven Rossby wave propagating eastward but with group velocity westward. Modifications to this mode due to a non-zero magnetic field result in R c being a complicated function R c ( ∧, q, σ ), where q = k/η and σ = v/k with v , k and η denoting the viscous, thermal and magnetic diffusivities. When it has a significant effect [ σ ≤ O (1)], the magnetic field inhibits the Rossby mode and convection moves towards the axis where the field is weaker ( ϖ 0 *→ 0). The presence of the magnetic field also permits new modes of instability which are facilitated by increasing field strength, with R c ∝ ∧ ─1 . These magnetic modes propagate eastwards (westwards) when q < q m ( q > q m ) with q m ≈ 2.8. Of particular interest to the Earth is the limit q ≪ 1 where the marginally stable magnetic mode takes the form of an undamped buoyancy wave in an unstably stratified fluid. No diffusive processes are operating at leading order and the only effect opposing convection is the geostrophic constraint.

Acoustoelectric Gain in Magnetic Fields

The acoustoelectric attenuation in nondegenerate semiconductors in a magnetic field is usually expressed as an infinite sum of modified Bessel functions. We transform this sum into an exact integral expression. From this expression we derive a simple approximation that is valid at small magnetic field strengths B and coincides with the zero magnetic field attenuation, calculated independently, if we allow B to approach zero. This integral expression has an essential singularity at B = 0; therefore the small magnetic field behavior cannot be described by a power series expansion in B.

Acoustoelectric Effects in Indium Antimonide

The acoustoelectric interaction in InSb is developed and investigated on the basis of a hydrodynamical theory. Comparison of numerical results obtained from this theory with the corresponding numerical results from more detailed microscopic theories reveals that the essential features of the microscopic theories are also present in the simpler hydrodynamical theory. In particular, the hydrodynamical theory accounts for the existence of two distinct modes of acoustic domain formation and for the reduction of peak acoustic gain at low values of magnetic field. Valuable insight into the physical basis of the acoustoelectric effect is obtained and it is shown that in the limit of large electron drift velocities and small transverse magnetic field strengths, the Mode I-type acoustoelectric interaction arises from a drift-enhanced quenching of electron diffusion effects. Moreover it is found that the frequency of maximum acoustoelectric gain, as given by the hydrodynamical theory of the present paper, does not occur at the fixed frequency f0 = (ωRωD)1/2/2π, but rather, it is dependent upon the exact values of drift velocity and magnetic field strength. Several computer-generated plots are presented and discussed in order to fully illustrate the results of this investigation.

Martian Ionosphere: A Component Due to Solar Protons

The small magnetic field strength observed near Mars by Mariner IV suggests that protons from the solar wind may enter the Martian atmosphere and produce ionization in addition to that produced by ultraviolet light and x-rays. It is found that solar protons produce a thin ionized layer at a rate of the order of 3 x 10(3) per cubic centimeter per second at a depth corresponding to the F(1) region in the terrestrial atmosphere. Unless the effective recombinative coefficient is very large (greater than 10(-5) centimeter cubed per second) or unless unusual diffusion effects are present, this layer should have been detected by Mariner IV, and therefore must be present in one of the observed ionized regions. Because of its very compact shape, the subsidiary maximum near 95 kilometers discovered in the Mariner-IV occultation experiment may be the proton ionization peak. If so, the major 120-kilometer maximum is an F(2) layer. Distinction between photon and proton ionization regions can be made by microwave occultation experiments aboard planetary orbiters.

Pressure Dependent Characteristics of an Oak Ridge Type Duoplasmatron Ion Source II. Composition of Hydrogen Ions.

A measurement of the abundance ratio of mass species of hydrogen ion beam extracted from an Oak Ridge type duoplasmatron ion source was made with a mass-spectrometer and processes leading to the formation of each ion were discussed. Each ion current of H1+, H2+, and H3+ can reach a value of 70% or more of extracted beam current depending on the operation conditions. The atomic ion current increases as the arc current increases and it becomes dominant when the spacing between intermediate electrode and anode is small. While, the triatomic ion current is dominant at small arc currents and small magnetic field strengths particularly when the gas pressure is high and the spacing between the intermediate electrode and anode is large. The molecular ion current is dominant only at low gas pressures. In contrast with the case of RF ion source the atomic ions are formed by two-stage processes, namely the reactions, H2+e→ H2++2e and subsequent H2++e → H1++H1+e, have an important role. While, the triatomic ions are produced through the ion-molecule reaction, H2++H2 → H3++H1.

Cerenkov Radiation in a Plasma

This paper treats Cerenkov radiation produced by a charged particle or beam moving in a plasma along an arbitrary direction with respect to the magnetic field. The expressions for the Cerenkov condition and the radiation output are obtained for the cases of a charged particle or beam moving both along and perpendicular to the magnetic field. The frequency ranges of the radiation for two type of waves, i.e. the ordinary and extraordinary waves, are determined by using the Cerenkov condition.For the case of a charged particle of beam moving in a plasma along the magnetic field, the extraordinary waves can be emitted, for a certain range of β=v/c, in the range of frequencies lower as wellas higher than the plasma frequency, and then the regions for the ordinary, and extraordinary waves can overlap. Under the non-relativistic condition (β<<1), the total power radiated by a charged particle or a bunched beam is, respectively, as follows : [dW/dt] _??_β_??_0μ0q2v/16π (2ω2p+ω2c) or[W] _??_β_??_0μ0I2/8·ω3/ω2C, ωp≤ω<√ω2p+ω2c, where μ0 is the permeability in vacuum, ωp and ωc are the plasma and cyclotron angular frequencies, q and v are the charge and velocity of a particle, and I is the beam current. The power output for the extraordinary waves is evaluated numerically. The result shows that Cerenkov radiation occurs with considerably high power at comparatively small magnetic field strengths and also under non-relativistic conditions.For the case of a charged particle or beam moving perpendicular to the magnetic field, the ordinary waves are emitted in the range of frequencies higher as well as lower than the plasma frequency, while the extra-ordinary waves are emitted in the range of frequencies higher than the plasma frequency. The maximum frequency emitted for both types of waves never exceed the frequency defined by ω=√ω2p+ω2c·In the non-relativistic case (β<<1) only the ordinary waves are emitted under the following coherence condition : [cos2θ0 (φ)] β_??_0=1-ω2 (ω2p+ω2c-ω2) /ω2pω2csin2φ, φ≠0, and the frequency ranges are separated into two regions. The radiation pattern is more complicated as compared with the case of a particle or beam moving along the magnetic field; instead of circular cones, we have two non-circular Cerenkov cones for the ordinary and extraordinary waves, with the radiation intensity varying on different generatrices of these conical surfaces.