Irregular Magnetic Field Research Articles

The Three-Phase Interstellar Medium Revisited

▪ Abstract The interstellar medium in the vicinity of the Sun is arranged in large-scale structures of bubble walls, sheets, and filaments of warm gas, within which close to the midplane there are subsheets and filaments of cold dense material; the whole occupies roughly half the available volume and extends with decreasing mean density to at least a kiloparsec off the plane. The remainder of the volume is in bubble interiors, cavities, and tunnels of much lower density, with some but not all of those lower density regions hot enough to be observable via their X-ray emission. This entire system is pervaded by a rather strong and irregular magnetic field and cosmic rays, the pressures of which are confined by the weight of the interstellar gas, particularly that far from the plane where gravity is strong. Observations suggest that the cosmic rays and magnetic field have an even more extended vertical distribution than the warm gas, requiring either the weight of additional coronal material or magnetic tension to confine it to the disk. Adjusting one's perception of this medium to embrace the known aspects is difficult. After this adjustment, there are many problems to solve and prejudices to overcome—the weak role of thermal instability, the suppression of certain gravitational instabilities, the problem of determining the state in the low-density regions, the twin difficulties of not having too much OVI (O+5) and getting enough diffuse 3/4 keV X-ray emission, the possible importance of large old-barrel–shaped supernova remnants in clarifying matters, the possible role of dust evolution in adjusting the heating to make clouds stable, the factors influencing the magnitudes of the interstellar pressure and scale height—things that global models of the medium might examine to clarify some of these matters; attention to these details and more constitute the challenge of this subject.

Particle Acceleration at Shocks Moving through an Irregular Magnetic Field

We use nondiffusive, nonrelativistic, test-particle numerical simulations to address the physics of particle acceleration by collisionless shocks. We focus on the importance of the shock normal angle, θBn, in determining the energy spectrum of the accelerated particles. For reasonable parameters, we find that the injection velocity is weakly dependent on the mean shock normal angle and that low-energy particles are readily accelerated to high energies irrespective of θBn. Our results are applicable for shocks that are nearly planar on scales larger than the coherence scale of the upstream magnetic turbulence and for particles whose gyroradii are smaller than this scale. We confirm previous results showing that the acceleration rate is larger for nearly perpendicular shocks compared to parallel shocks. However, we also find that the acceleration rate at parallel shocks moving through large-scale magnetic fluctuations is larger than that predicted by simple first-order Fermi acceleration. Our results can be understood in terms of the nature of the large-scale fluctuations and their effect on particle transport.

Modeling the effects of pitch-angle scattering processes on the transport of solar energetic particles along the interplanetary magnetic field

We present a Monte-Carlo technique to study the time-dependent transport of energetic particles in the interplanetary medium. We use the guiding center approximation between discrete finite pitch-angle scatterings to quantify the competing effects of focusing and pitch-angle scattering on energetic particles propagating along a Parker spiral magnetic field. We consider that the pitch-angle scattering process is produced by small-scale magnetic field irregularities frozen in the expanding solar wind. We also include the effects of both solar wind convection and adiabatic deceleration. We use a joint probability distribution P( h, μ′) = p( h; μ′) q( μ′; μ) to describe both the distance traveled by the particle between two scattering processes and the change in the particle pitch-angle after a scattering process. Here, p( h; μ′) is the conditional probability that the particle travels a distance h along the field line before the next scattering if it had a pitch-angle cosine μ′ after the previous scattering, and q( μ′; μ) is the conditional probability for the pitch-angle cosine μ′ if the pitch-angle cosine was μ before the scattering. We consider several functional forms to describe the processes of pitch-angle scattering, such as an isotropic scattering without any memory of the initial particle’s pitch-angle or processes in which the scattering result depends upon the initial particle’s pitch-angle. The results of our simulations are pitch-angle distributions and time-intensity profiles that can be directly compared to spacecraft observations. Comparison of our simulations with near-relativistic (45–290 keV) electron events observed by the Electron, Proton and Alpha Monitor on board the Advanced Composition Explorer allows us to estimate both the time dependence of the injection of near-relativistic electrons into the interplanetary medium and the conditions for electron propagation along the interplanetary magnetic field.

Statistical properties of the radial transport in the magnetic field with radially bounded stochastic region

Statistical properties of the radial heat transport of electrons are clarified in the various types of the radially bounded irregular magnetic field inside an axisymmetric torus plasma, where the collisional stochasticity due to the Coulomb collision and the magnetic stochasticity due to a radially bounded perturbed field coexist. Extensive Monte Carlo numerical analysis are performed in the two dimensional parameter space ( s b / s bc , ν/ ν t ) with a delta function initial distribution in the radial direction. The parameters s b and ν are the strength of a magnetic field perturbation and the collision frequency, respectively. The normalization parameter s bc corresponds to the islands overlapping criterion, and ν t is the characteristic frequency of the passing particle orbits in the corresponding regular magnetic field. It is shown that the type of the transport process is closely connected with the locality of the particle radial displacements. As s b / s bc (⩾1) increases and ν/ ν t decreases, the particle distribution spreads out in a whole stochastic region after the short time ballistic (dynamical) phase reflecting the non-locality of radial displacements. As a consequence the subdiffusive process ( s b / s bc ⩾1, ν/ ν t ⩾0) or the uniform mixing process ( s b / s bc ⪢1, ν/ ν t =0) are observed in the long time limit. The standard diffusion approach is applicable only in the limited parameter region when the fairly frequent collisions inside the radially bounded stochastic region with the mixture of regular and irregular domains can recover the locality of particle displacements.

Increase in high energy particle flux affected by an interplanetary shock wave

An energetic proton event was observed at a geosynchronous orbit at nearly the same time as an interplanetary shock wave reached the Earth's magnetosphere, indicating that these two events interacted with each other during their passage to Earth. The variation of the energetic proton flux is not explained by the shock acceleration mechanism because there was no diffusive behavior upwind of the shock wave. These protons could have been ejected by a solar flare, because an energetic flare occurred at a well-connected position before the proton event, however, the delay time for the propagation of the energetic particles was too long if this flare was the origin of those particles. In the previous paper, we showed that the peculiar behavior of high-energy particles could be explained by the following scenario: the protons produced during a proton flare, which occurred after the CME, catch up with it and enter the turbulent region behind the shock wave, they are then scattered by the irregular magnetic field, that is, the Fermi acceleration, and are captured in the turbulent region behind the shock wave. In this paper, we obtain a scale of the irregularity of the turbulent magnetic field using Fourier transformation, compare it with a mean free path consistent with the one required from the observational data and the Larmor radius, and show that the proposed acceleration mechanism explains these events.

MOTION CONTROL OF LINEAR SYNCHRONOUS MOTORS WITH FORCE RIPPLE COMPENSATION USING CURRENT SHAPING

The main problem in improving the tracking performance of linear synchronous motors is the presence of force ripple caused by the irregular magnetic field of the permanent magnets and inaccuracy in electronic commutation by the servo amplifier. In this paper a method to compensate the force ripple based on shaping of the phase current waveforms is presented. The method generates optimized current waveforms which produces minimal copper losses and maximize motor efficiency. The current waveforms are based on a Fourier series approximation. A comparison of the tracking performance with and without force ripple compensation is given.

Application of a generalized total variation diminishing algorithm to cosmic ray transport and acceleration

We have applied a modified total variation diminishing (TVD) method within the framework of a finite difference method for solving a transport equation for cosmic rays in an irregular magnetic field. We employ Roe's superbee scheme to control the numerical diffusion at a sharp profile in the cosmic ray particle density. Furthermore, we have modified the TVD concept for the case of a general Courant number. Comparison with previous results shows that this technique can give an accurate solution with an increased grid size and much shorter running time. This has also enabled the first simulations of the pitch angle transport and acceleration of cosmic rays crossing an oblique shock discontinuity at moderate energies.

Decay of Magnetic Field Irregularities Observed by Ulysses

We study the solar cycle, radial, and latitudinal dependence of the characteristics of magnetic field irregularities in the Heliosphere. The frequency of magnetic field discontinuities is determined, using high time resolution magnetic field observations by Ulysses, covering the time interval from 1992 to 2000. The quasi-linear scattering mean free path of particles is also calculated. These investigations aim at understanding/exploring transport properties of energetic charged particles in the Heliosphere. We find that the travel time of solar wind plasma from the Sun to the observer is the key parameter of the process, by controling the decay of the irregularities.

Properties of Alfvén waves in the outer solar corona from two-station faraday rotation observations

Faraday rotation fluctuations (FRF) of S-band polarized radio signals observed during the 1981 solar conjunction of the Helios spacecraft are used to investigate Alfvén waves in the solar corona. The occultation geometry enabled coronal radio sounding of the heliographic equatorial region at solar offset distances from 3–12 R⊙. Spectral analysis was performed with the FRF data to determine the turbulence regime of the magnetic field irregularities. Cross-correlation functions of signals received simultaneously at two widelyseparated ground stations were used to obtain the propagation speed of Alfvén waves traversing the ray paths.

The spectrum of magnetic field irregularities in the solar corona and in interplanetary space

Results are presented of a spectral analysis of Faraday rotation fluctuations observed during the solar occultations of the Helios spacecraft. The temporal spectra of Faraday rotation fluctuations in the frequency range between 10 −3 Hz and 10 −2 Hz usually assume the form of a power law. The power-law exponent is ⋍1.6 at heliocentric distances 3 R ⊙ < r < 6 R ⊙ (R ⊙ = solar radius) and ⋍1.1 at distances 6 R ⊙ < r < 12 R ⊙. The Faraday rotation data are compared with in situ measurements of magnetic field fluctuations in the range of solar distances 0.3 < r < 1 AU.

Topological Formation of Small Scales in Magnetohydrodynamics: A Fast Dissipation Mechanism

The dissipation of Alfvén wave packets propagating in an inhomogeneous three-dimensional magnetic field is numerically studied in the Wentzel-Kramers-Brillouin (WKB) approximation. The dissipation rate is found to scale proportionally to the logarithm of viscosity and/or resistivity, i.e., much faster than the scaling found for two-dimensional configurations (phase mixing, resonance absorption). This phenomenon is related to the exponential separation of neighboring rays; estimations of the corresponding rate are consistent with the behavior of the Kolmogorov entropy in this kind of structure. The scaling law td ∝ ln S (td is the e-folding dissipation time and S is the Reynolds number) holds not only in an irregular magnetic field (which was studied by Similon & Sudan), but is also verified in quasi-uniform magnetic structures, which contain very small chaotic regions. This study can apply to the heating of low-collision plasmas, e.g., in astrophysical contexts such as the solar corona.

A practically zero horizontal magnetic field in a planar undulator used in the Tristan Super Light Facility.

The Tristan Main Ring (MR) at KEK (High Energy Accelerator Research Organization) has been used as a third-generation synchrotron radiation ring with an undulator installed in the MR as a third-generation light source. The magnetic field irregularities of the undulator were examined precisely. Measurement of the magnetic field after synchrotron radiation experiments showed that the magnetic irregularities in both the horizontal and vertical directions are negligibly small. This result suggests that our previous estimates of emittance, made using the measured undulator spectrum, should correctly represent the properties of the electron beam in the MR.

Electromagnetic radiation induced current steps in biepitaxial Josephson junctions

Ordinary Shapiro steps were observed in the I - V curves of biepitaxial Josephson junctions at when microwave radiation at f = 11 GHz was applied to the junction. However, pronounced current steps were observed at twice the period, when the d.c. Josephson current was suppressed by high power microwave radiation. The extension of the voltage region, within which the double period steps were seen, increased with microwave power. Modified biepitaxial heterostructures of , with as a seed layer and as an intermediate buffer layer were grown by laser ablation on . The I - V characteristics for the biepitaxial Josephson junctions agreed well with the RSJ model for T > 50 K but showed hysteresis at T < 30 K. No large excess current was observed. The critical current density, about at 4 K, was an order of magnitude higher than for bicrystal junctions and prevalent biepitaxial junctions with a misorientation between the crystal planes on either side of the grain boundary. An irregular magnetic field dependence of the critical current, , indicated an uneven current distribution within the junction. The products for the junctions were about 1.5 mV at 4.2 K and 20 - 60 at 77 K. Pronounced current steps in the I - V dependencies due to a.c. Josephson current coupling to self-induced electromagnetic modes at the biepitaxial boundary were used to characterize the junctions.

Production of Antiprotons in Interstellar Space by Propagating Cosmic Rays under Conditions of Diffusive Reacceleration

view Abstract Citations (30) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Production of Antiprotons in Interstellar Space by Propagating Cosmic Rays under Conditions of Diffusive Reacceleration Simon, M. ; Heinbach, U. Abstract The propagation of cosmic rays in the Galaxy is understood as a diffusive process in which the magnetic field irregularities serve as scatter centers. Particles that interact with turbulent magnetic field irregularities can gain energy because of stochastic, second-order, Fermi type of acceleration. We could show that such a propagation scenario, in which the particles are primary preenergized at the outsets and then reaccelerated during propagation, is completely compatible with the observation of cosmic-ray nuclear data, such as cosmic-ray spectra and the secondary/primary ratio of boron/carbon and subiron/iron. In this context, we present here results of the expected interstellar secondary antiproton flux via p + p → bar-p + x interactions in the framework of this diffusive reacceleration model. We can show that the interstellar bar-p spectrum under these conditions looks very different from that calculated in the Standard Leaky Box situation. Publication: The Astrophysical Journal Pub Date: January 1996 DOI: 10.1086/176676 Bibcode: 1996ApJ...456..519S Keywords: ACCELERATION OF PARTICLES; ISM: COSMIC RAYS; ELEMENTARY PARTICLES full text sources ADS |

Propagation of galactic cosmic rays under diffusive reacceleration

view Abstract Citations (62) References (43) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Propagation of Galactic Cosmic Rays under Diffusive Reacceleration Heinbach, U. ; Simon, M. Abstract The propagation of cosmic rays in the galaxy is understood as a diffusive process where the magnetic field irregularities serve as scattering center. In this paper we pursue the idea that particles which interact with turbulent magnetic fields should naturally also gain energy due to a stochastic second-order Fermi type of acceleration. This link between diffusion and reacceleration is based on general physical principles, and we can show that the cosmic-ray data can be explained in this 'Diffusive Reacceleration Model.' As a consequence, the escape length lambdaesc(E), which defines the particles path in the galaxy, becomes a power-law in rigidity with an exponent of -1/3, which would indicate a Kolmogorov-type spectrum of magnetic turbulences. The increase of lambdaesc(E) combined with the increase of the amount of reacceleration to low energies in then interpreted as a decrease of the diffusion coefficient or an increase of the frequencies of scattering center. Publication: The Astrophysical Journal Pub Date: March 1995 DOI: 10.1086/175350 Bibcode: 1995ApJ...441..209H Keywords: Astronomical Models; Cosmic Rays; Diffusion; Particle Acceleration; Turbulence; Kolmogorov Theory; Magnetohydrodynamics; Mathematical Models; Radiative Transfer; Stochastic Processes; Transport Theory; Astrophysics; ACCELERATION OF PARTICLES; DIFFUSION; ISM: COSMIC RAYS; MAGNETOHYDRODYNAMICS: MHD; TURBULENCE full text sources ADS |

Fluctuations of Polarized Radiation in a Random Magnetoplasma

This paper analyses fourth-order moments of polarized radiation passing through magnetoactive plasma with random irregularities both in electron density and magnetic field. We consider the new propagation effect arising from chiral properties of random magnetoplasma. That is, the lenses formed by the same irregularities of magnetic field may be characterized by refractive properties of opposite sense vis-a-vis the rotation of wave polarization vector. This produces an appearance of slight circular polarization fluctuations arising from initially nonpolarised radiation. Analysis of the polarized radiation fluctuations may allow the spatial spectrum of magnetic field irregularities to be detected. The enchanced level of circularly polarized component, and the share of fluctuations owing to magnetic field irregularities, can be readily observed at low frequencies only, say in the radiation passing through the solar chromosphere or Jovian and terrestrial ionosphere (magnitosphere).

Some relativistic effects in short timescale variations of compact radio sources: AApS 13/4 (1993) 314–322

Some relativistic effects are discussed for the synhrotron radiation from rela-tivistic shock propagating in jet and illuminating its irregularities of electron density and magnetic field. The dependence of the, timescale and amplitude of the flux variations on the shock's thickness and the stale of the irregularities is investigated. It is shown that the difference of the shock-thickness for optical emission and radio emission can give rise to a time delay of the radio emission relative to the optical emission, and this may provide an explanation for some observed phenomena, for example in BL Lac object 0716 + 714.

On statistical acceleration of particles near Venus

Various gasdynamic and kinematic problems of the solar wind interaction with the ionosphere of Venus on the dayside are investigated. On the assumption that in the transition region between the bow shock and ionopause there exists an irregular magnetic field of B ≈ 20 nT, we estimate typical plasma parameters such as: mean free path, internal scale of inhomogeneities, viscosities, plasma conductivity and dissipated energy. A statistical acceleration mechanism in the transition region is investigated. We find that protons are accelerated in time at a rate proportional to t 2 3 up to an energy of approximately keV, and their distribution function is n(E) ∝ E 1 2 ; n(ν) ∝ ν 2 . The acceleration of electrons up to relativistic energies is proportional to the acceleration time. Heavy ions are also subject to acceleration and the mechanism appears able to explain the observed phenomena.

Requirements for accuracy of superconducting coils in the Large Helical Device

Irregular magnetic fields resonate with the rational surface of the magnetic confinement systems, form magnetic islands and ergodic layers, and destruct the plasma confinement. To avoid this confinement destruction the requirement of an accuracy of 10 −4 in the magnetic field is adopted as the magnetic-accuracy design criterion for the LHD machine. Following this criterion the width of the undesirable magnetic island is kept less than one tenth of the plasma radius. The irregular magnetic field from the superconducting (SC) helical and poloidal coils is produced by winding irregularity, installing irregularity, cooling-down deformations and electromagnetic deformations. The local irregularities such as feeders, layer connections, adjacent-conductor connections of the coils also produce an error field. The eddy currents on the supporting shell structure of SC coils, the cryostat, etc. are also evaluated. All irregular effects are analyzed using Fourier decomposition and field mapping methods for the LHD design, and it is confirmed that the present design of the superconducting coil system satisfies the design criterion for these field irregularities.

On the interpretation of the correlated optical and radio outbursts in AO0235+164

A detailed analysis of the spectral properties of the correlated optical and radio outbursts occurred in the BL Lac object AO02235+164 is presented, taking the outburst in Oct.–Nov. 1975 as an example. The observed features can be interpreted reasonably in the frame of relativistic shock-jet models; (1) The background jet spectrum is determined and its structure is analysed on the basis of Königl's jet model. The distributions of electron density and magnetic field along the jet and other parameters are discussed. (2) With the background jet spectrum subtracted, the spectrum of the outburst is determined. It is shown that it can be represented by a uniform synchrotron radiation source with the optically thin spectral index α ∼ 0.25, the break frequency v b 3 × 10 12Hz and the high frequency cutoff v ̃ h ∼ 2 × 10 14 Hz . (3) The adiabatic acceleration of particles and amplification of magnetic field, and the parameters of the emitting regions (i.e. the post-shock regions) are discussed. (4) The simultaneity of the optical and radio outbursts is considered to be due to the optical thinness of the emitting region. The optically thin spectral index in radio band ( α ∼ 0.25) is smaller than that of the background jet spectrum ( α s2 ≅ 0.5) at similar frequencies (∼ 10 12 Hz >), and this can be used to explain the much larger amplitude of variations in optical band. (5) It is suggested that the difference between the spike profile of optical outburst and the broad profile of radio outburst (as an example, for the correlated optical radio outburst in 1979) could be explained by shock propagating through irregularities of energetic electrons and magnetic field along the jet, that is, the scale of the optical emission region along the jet might be much smaller than that of the radio emission region. In summary, the results given here show that the correlated optical-radio outbursts in AO0235+164 may be intrinsic to the source, not due to gravitational micro-lensing effects.