Large Radial Distances Research Articles

Spectral and time domain characteristics of single muscle fibre action potentials during continuous activity extracted from model considerations.

A model of the muscle fibre extracellular action potentials (ECAPs) calculation using experimentally recorded intracellular action potentials (ICAPs) has been applied to investigate the effect of repetitive stimulation on the electrical activity of isolated frog muscle fibres. The ECAPs were calculated both at small (0.01 mm) and at large (5 mm) radial distances to the fibre axis, and their relationship with the original ICAP parameters has been inferred. Fourier transformation of the calculated ECAPs in order to obtain the spectral characteristics and to trace out their behaviour during continuous fibre activity was performed. Stimulation frequency dependence on the ECAP time characteristics and on the shift of the maximum spectral density towards low frequencies at small and large radial distance were observed. The spectral density peak frequency is propagation velocity (PV)-dependent. The advantage of the presented method over the available experimental extracellular recording techniques from isolated muscle fibers is the possibility to show the effect of continuous muscle fibre activity on the parameters of the ECAPs and their spectral characteristics at large radial distance, which is not experimentally accessible. Our results are in agreement with those experimentally obtained. The results from the model prove the role of changes in PV of excitation along the muscle fibres (representing the last link in the complex organized motor system) in the development of fatigue.

Hydrodynamic Simulations of Counterrotating Accretion Disks

Hydrodynamic simulations have been used to study accretion disks consisting of counterrotating components with an intervening shear layer(s). Configurations of this type can arise from the accretion of newly supplied counterrotating matter onto an existing corotating disk. The grid-dependent numerical viscosity of our hydro code is used to simulate the influence of a turbulent viscosity of the disk. Firstly, we consider the case where the gas well above the disk midplane rotates with angular rate +\Omega(r) and that well below has the same properties but rotates with rate -\Omega(r). We find that there is angular momentum annihilation in a narrow equatorial boundary layer in which matter accretes supersonically with a velocity which approaches the free-fall velocity and the average accretion speed of the disk can be enormously larger than that for a conventional \alpha-disk rotating in one direction. Secondly, we consider the case of a corotating accretion disk for r r_t. In this case we observed, that matter from the annihilation layer lost its stability and propagated inward pushing matter of inner regions of the disk to accrete. Thirdly, we investigated the case where counterrotating matter inflowing from large radial distances encounters an existing corotating disk. Friction between the inflowing matter and the existing disk is found to lead to fast boundary layer accretion along the disk surfaces and to enhanced accretion in the main disk. These models are pertinent to the formation of counterrotating disks in galaxies and possibly in Active Galactic Nuclei and in X-ray pulsars in binary systems.

Simulation of dispersionless injections and drift echoes of energetic electrons associated with substorms

The term “dispersionless injection” refers to a class of events which show simultaneous enhancement (injection) of electrons and ions with different energies usually seen at or near geosynchronous orbit. We show that dispersionless injections can be understood as a consequence of changes in the electric and magnetic fields by modeling an electron injection event observed early on January 10, 1997 by means of a test‐particle simulation. The model background magnetic field is a basic dipole field made asymmetrical by a compressed dayside and a weakened nightside. The transient fields are modeled with only one component of the electric field which is westward and a consistent magnetic field. These fields are used to model the major features of a dipolarization process during a substorm onset. We follow the electrons using a relativistic guiding center code. Our simulation results, with an initial kappa electron energy flux spectrum, reproduce the observed electron injection and subsequent drift echoes and show that the energization of injected electrons is mainly due to betatron acceleration of the preexisting electron population at larger radial distances in the magnetotail by transient fields.

INTERNAL EXTINCTION OF DISC GALAXIES

ABSTRACT Many techniques to correct for the extinction of light by dust in our own Galaxy have been developed but for many years it was believed that the extinction of starlight by dust in the interstellar media of external galaxies was negligible. Over the last ten years the evidence for low extinctions in galaxies has become very controversial and numerous papers indicating both high and low extinction have been published, throwing the situation into confusion. This has triggered a new interest in the subject as many other areas of astrophysics depend on its resolution. In this thesis a new technique for mapping the extinction of spiral galaxies at high resolution is presented. The technique combines a low resolution energy balance measurement of the extinction with an optical/near infrared colour map that shows relative extinction at high resolution. When combined correctly, an extinction map that has both high resolution and absolute normalisation can be obtained. An extinction map is computed for the nearby, face-on, Sc galaxy NGC 6946. The total extinction in NGC 6946 is found to be AB=0.45, considerably higher than previous estimates for Sc galaxies (AB=0.2). The high resolution map shows the extinction to be extremely patchy and to severely affect the appearance of the galaxy. When the extinction map is used to correct the observed image of NGC 6946 for the effects of dust it displays a markedly different morphology, the amplitude of the spiral arms is decreased and the bulge regions become more prominent. As well as the extinction map, a number of other related results were found during the analysis. The mass of dust in NGC 6946 was found to be ~ 4-10 times that estimated by previous measurements, bringing the gas-to-dust mass ratio in agreement with that of the Galaxy. Contrary to previous beliefs, the distribution of the dust was found to have a longer scale length than the stars, - cold dust therefore dominates the far infrared energy output at large radial distances. A comparison of optical and far infrar\\ ed luminosity functions is used to show that, on average, the B band light from\\ galaxies arises from regions with tauB ~ 1. Modelling of the extinction as a function of wavelength suggests that absorption at near infrared wavelengths may produce the largest fraction of the far infrared emission.

Helioradius Dependence of Interplanetary Carbon and Oxygen Abundances during 1991 Solar Activity

A comparison of the heliocentric dependence of the relative abundance of low energy (~1 MeV nucleon-1) carbon and oxygen on the Ulysses (~3-4 AU) and Voyager 1 and 2 (~35 and ~47 AU) spacecraft shows that the C/O abundance ratio is significantly smaller at the larger radial distances. This finding is attributed to a more abundant seed population for anomalous cosmic-ray oxygen than for carbon in the outer solar system, where the seed population can be accelerated by shocks in global merged interaction regions. The energy spectra of carbon and oxygen at the different locations are approximately the same.

Monte Carlo Simulation of Cometary Atmospheres: Application to Comet P/Halley at the Time of the Giotto Spacecraft Encounter. II. Axisymmetric Model

view Abstract Citations (49) References (69) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Monte Carlo Simulation of Cometary Atmospheres: Application to Comet P/Halley at the Time of the Giotto Spacecraft Encounter. II. Axisymmetric Model Xie, Xingfa ; Mumma, Michael J. Abstract An axisymmetric Monte Carlo model of a cometary atmosphere, composed of H2O and its daughter species H and OH, has been established and applied to comet 1P/Halley at the Giotto encounter. Results are presented for comae modeled with two gas density functions, represented by total water production rates Q0 and water angular distributions f(θ) On the inner boundary (r = 500 km). In case (1), Q0 = 6 x 1029 molecules s-1 and f(θ) = 1 + 0.5 cos θ, and in case (2), Q0 = 5 x 1029 molecules s-1 and f(θ) = 1 + 0.9 cos θ, respectively. Our simulations clearly show that anisotropy of water is maintained at least to a radial distance of 2 x 105 km, but that the gas density becomes increasingly isotropic at larger radial distances. For daughter species, changes of angular distribution in the coma also depend on the ejection velocity: H becomes more or less isotropic in the outer coma due to its large ejection velocities, while OH preserves the initial angular distribution to a greater degree during its outward expansion due to a small mean ejection velocity of 1 km s-1. The flattening of angular distributions leads to a reduced asymmetry in gas column density from the intermediate coma (r ∼a few thousand km from the nucleus) to the outer coma and could be partly responsible for reduced asymmetries seen in observed cometary data. We also show that (a) the outward expansion of H2O, H, and OH is mainly radial, with very small mean lateral velocities throughout the range from the inner boundary (r = 500 km) coma to the outer coma, and (b) the outflow velocities of H2O and OH increase with increasing local gas production rates, while that of H decreases with increasing local production rates. Furthermore, the gas production rate estimated from fitting the Giotto velocity measurements is a local one and does not represent the true total gas production rate. The retrieval of the latter requires information on the lag angle of outgassing and its angular distribution. Applications to trace volatiles (CH3OH, H2S) and to gaseous species having both direct and extended sources (CO, H2CO, CN) are discussed. Publication: The Astrophysical Journal Pub Date: June 1996 DOI: 10.1086/177336 Bibcode: 1996ApJ...464..457X Keywords: ACCELERATION OF PARTICLES; COMETS: INDIVIDUAL NAME: HALLEY full text sources ADS | Related Materials (1) Part 1: 1996ApJ...464..442X

A transport model for the diffusive shock acceleration and modulation of anomalous cosmic rays in the heliosphere

Standard transport theory is used to investigate the acceleration and modulation of anomalous cosmic rays during solar minimum periods. With the developed anomalous cosmic ray transport model, pickup ions are transformed into anomalous cosmic rays by diffusive shock acceleration at the heliospheric termination shock. The emphasis is on explaining the 1987 cosmic ray spectra at large radial distances in the equatorial plane. Energy density calculations of cosmic ray spectra at the termination shock suggest that the termination shock might be strongly mediated by anomalous cosmic rays. Therefore, in the spirit of nonlinear shock acceleration theory, the termination shock is modeled as a hyperbolic tangent. Calculations show that realistic anomalous cosmic ray intensity radial gradients can only be achieved, within the framework of our assumptions, for a termination shock structure with a compression ratio of 3.2 <s <4.0 and a scale length of ∼1.2 AU. In addition, simulations of anomalous and galactic cosmic ray spectra revealed as follows: (1) For radial distances of less than at least 24 AU from the Sun it should be generally difficult, although not impossible, to identify anomalous protons, for example, in the observed 1987 proton spectra. (2) Anomalous protons should be more easily detectable, however, in the proton spectra seen at 42 AU by Pioneer 10 during 1987 and should be clearly visible at larger radial distances. (3) The best way to identify anomalous protons in observed proton spectra is to look for a clear flattening in the spectral slopes in the energy interval of ∼20–100 MeV. Recent 1994 observations by Pioneer 10 at ∼60 AU and by Voyager 1 and Voyager 2 support the model prediction of very flat proton spectra in the above mentioned energy interval at large radial distances. Within the framework of our assumptions this might be an indication that the average termination shock position during 1994 was in the vicinity of 80 AU as assumed in the model.

Extracellular action potentials of skeletal muscle fibre affected by 4-aminopyridine: a model study.

A new specially designed analytical function approximating the intracellular action potentials (ICAPs) for calculation of the extracellular potentials (ECAPs) at various radial and axial distances from the active fibre is proposed. 4-Aminopyridine (4-AP) was used to obtain ICAPs with a prolonged repolarization phase in order to investigate the influence of changes in ICAP shape on the ECAPs. From the experimentally recorded ICAPs before and after treatment of frog skeletal muscle fibres with 4-AP, approximated by the new function, the ECAPs were calculated applying the line-source model in a finite fibre. Using this function allowed calculation of the ECAPs at distances not accessible for the experimental recordings. The total ionic current (Ii) during the action potential was calculated using the cable equation. Our results showed that the ratio of the first positive to the negative phases of the ECAPs of treated fibres increased at large radial distances (3000 microns and more) and the terminal positive phase was asymmetric with an abrupt initial deflection followed by a slow inverse deflection. The calculated ECAPs at various axial distances from the fibre end (cylindrical and conical part) and at radial distances from the fibre membrane ranging from 0 to 5000 microns, corresponded in shape to the experimentally recorded potentials of untreated and 4-AP-treated muscle fibres.

ETS-VI Magnetic Field Observations of the Near-Earth Magnetotail during Substorms

The magnetic field experiment on board ETS-VI (the Engineering Test Satellite-VI) and initial results from the experiment are presented. ETS-VI was launched on August 28, 1994 and has been placed in anear equatorial orbit with a perigee of 2.3Re, an apogee of 7.1Re, an inclination of 13.4°, and an orbital period of 14.4 hours. The spacecraft is three-axis stabilized. The magnetic field experiment on board the satellite consists of a triaxial fluxgate magnetometer. The magnetometer has a sampling interval of 3 s, and except near perigee, it is operated in the mode having a ±256 nT dynamic range and a 0.125 nT resolution. We have examined the magnetometer data obtained at radial distances from 5.0 to 7.1Re, magnetic latitudes from-10° to 25°, and local times from 14 MLT through midnight to 04 MLT, to study magnetic field variations associated with substorms. Substorm-associated field variations are easily seen at larger radial distances and at local times later than 19 MLT. Using 92 substorm events that occurred in the 21-01 MLT sector, we have constructed the average field configuration during the growth phase. The magnetic field becomes highly taillike and its intensity increases at high latitudes (> 10°), while the field intensity decreases at lower latitudes (<10°). The field configuration suggests that the inner edge of the growth phase current system is located near the synchronous altitude at the end of the growth phase.

Kinetics of Surface Coagulation

A theory is developed for the surface coagulation of particles resulting from collisions between suspended flowing particles and particles deposited on a surface analogous to the classical Smoluchowski model for orthokinetic coagulation, with and without taking into account colloidal and hydrodynamic interactions. The theory predicts an increase in the capture frequency with an increase in the radial distance from the stagnation point. Experimental observations on the surface coagulation of calcium carbonate particles done in a well-defined hydrodynamic flow field created by an impinging jet are in agreement with theory for small radial distances but show a decrease in capture frequency for large radial distances, possibly due to surface roughness of the colliding particles.

Radial energy transport by magnetospheric ULF waves: Effects of magnetic curvature and plasma pressure

The “radial” transport of energy by internal ULF waves, stimulated by dayside magnetospheric boundary oscillations, is analyzed in the framework of one‐fluid magnetohydrodynamics. (The term radial is used here to denote the direction orthogonal to geomagnetic flux surfaces.) The model for the inhomogeneous magnetospheric plasma and background magnetic field is axisymmetric and includes radial and parallel variations in the magnetic field, magnetic curvature, plasma density, and low but finite plasma pressure. The radial mode structure of the coupled fast and intermediate MHD waves is determined by numerical solution of the inhomogeneous wave equation; the parallel mode structure is characterized by a WKB approximation. Ionospheric dissipation is modeled by allowing the parallel wave number to be complex. For boundary oscillations with frequencies in the range from 10 to 48 mHz, and using a dipole model for the background magnetic field, the combined effects of magnetic curvature and finite plasma pressure are shown to (1) enhance the amplitude of field line resonances by as much as a factor of 2 relative to values obtained in a cold plasma or box‐model approximation for the dayside magnetosphere; (2) increase the energy flux delivered to a given resonance by a factor of 2–4; and (3) broaden the spectral width of the resonance by a factor of 2–3. The effects are attributed to the existence of an “Alfvén buoyancy oscillation,” which approaches the usual shear mode Alfvén wave at resonance, but unlike the shear Alfvén mode, it is dispersive at short perpendicular wavelengths. The form of dispersion is analogous to that of an internal atmospheric gravity wave, with the magnetic tension of the curved background field providing the restoring force and allowing radial propagation of the mode. For nominal dayside parameters, the propagation band of the Alfvén buoyancy wave occurs between the location of its (field line) resonance and that of the fast mode cutoff that exists at larger radial distances.

On the symmetric circulation of a moving hurricane

The evolution of the symmetric circulation of a moving hurricane-scale vortex on a beta-plane is investigated and interpretations of this investigation are given in terms of vorticity fluxes. the study, which is fundamental to understanding vortex motion, is based largely on numerical integrations of the barotropic vorticity equation, using a finite-difference method. In the absence of any large-scale flow, the north-westward drift of an initially symmetric, cyclonic vortex in the northern hemisphere is accompanied by a decrease in the tangential circulation at most radii, and a consequent deceleration of the azimuthally averaged tangential velocity. This behaviour is not simply a consequence of the increase in the Coriolis parameter over the domain of the cyclone as is often supposed, but may be explained as the sum of three effects: the outwards radial flux of relative vorticity associated with the asymmetric component of flow, the corresponding flux of planetary vorticity, and the rate of change in planetary vorticity due to the meridional displacement of the vortex. In particular, the net rate of change in absolute vorticity, the sum of the last two effects, makes the largest contribution to the circulation changes. the flux of absolute vorticity is associated, inter alia, with the generation of Rossby waves by the vortex. the behaviour is different from that in recent calculations by Carr and Williams in which the flux of planetary vorticity was omitted. It is pointed out that in the analytic theory for vortex motion developed by the first author and co-workers, an initially symmetric vortex with zero net relative circulation at large radial distances subsequently develops a finite negative relative circulation whose strength increases linearly with time; this is because the net flux of planetary vorticity at large radial distances is finite. In contrast, in the numerical model the flux is close to zero so that the symmetric circulation decays more rapidly with radius than the 1/(radius) decay rate in the analytic model. At inner radii there is good qualitative agreement between the predictions of the two methods, even though there are quantitative differences between them. It is noted that the occurrence of the finite circulation in the analytic theory violates a theorem of Flierl et al. and that it represents a limitation of the long-term validity of the theory.

On the symmetric circulation of a moving hurricane

AbstractThe evolution of the symmetric circulation of a moving hurricane‐scale vortex on a beta‐plane is investigated and interpretations of this investigation are given in terms of vorticity fluxes. the study, which is fundamental to understanding vortex motion, is based largely on numerical integrations of the barotropic vorticity equation, using a finite‐difference method. In the absence of any large‐scale flow, the north‐westward drift of an initially symmetric, cyclonic vortex in the northern hemisphere is accompanied by a decrease in the tangential circulation at most radii, and a consequent deceleration of the azimuthally averaged tangential velocity. This behaviour is not simply a consequence of the increase in the Coriolis parameter over the domain of the cyclone as is often supposed, but may be explained as the sum of three effects: the outwards radial flux of relative vorticity associated with the asymmetric component of flow, the corresponding flux of planetary vorticity, and the rate of change in planetary vorticity due to the meridional displacement of the vortex. In particular, the net rate of change in absolute vorticity, the sum of the last two effects, makes the largest contribution to the circulation changes. the flux of absolute vorticity is associated, inter alia, with the generation of Rossby waves by the vortex. the behaviour is different from that in recent calculations by Carr and Williams in which the flux of planetary vorticity was omitted.It is pointed out that in the analytic theory for vortex motion developed by the first author and co‐workers, an initially symmetric vortex with zero net relative circulation at large radial distances subsequently develops a finite negative relative circulation whose strength increases linearly with time; this is because the net flux of planetary vorticity at large radial distances is finite. In contrast, in the numerical model the flux is close to zero so that the symmetric circulation decays more rapidly with radius than the 1/(radius) decay rate in the analytic model. At inner radii there is good qualitative agreement between the predictions of the two methods, even though there are quantitative differences between them. It is noted that the occurrence of the finite circulation in the analytic theory violates a theorem of Flierl et al. and that it represents a limitation of the long‐term validity of the theory.

Galactic cosmic-ray mediation of a spherical solar wind flow. 1: The steady state cold gas hydrodynamical approximation

view Abstract Citations (8) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Galactic Cosmic-Ray Mediation of a Spherical Solar Wind Flow. I. The Steady State Cold Gas Hydrodynamical Approximation Le Roux, J. A. ; Ptuskin, V. S. Abstract Realistic models of the outer heliosphere should consider that the interstellar cosmic-ray pressure becomes comparable to pressures in the solar wind at distances more than 100 AU from the Sun. The cosmic-ray pressure dynamically affects solar wind flow through deceleration. This effect, which occurs over a scale length of the order of the effective diffusion length at large radial distances, has important implications for cosmic-ray modulation and acceleration. As a first step toward solution of this nonlinear problem, a steady state numerical model was developed for a relatively cold spherical solar wind flow which encounters the confining isotropic pressure of the surrounding Galactic medium. This pressure is assumed to be dominated by energetic particles (Galactic cosmic rays). The system of equations, which are solved self-consistently, includes the relevant hydrodynamical equations for the solar wind flow and the spherical cosmic-ray transport equation. To avoid the closure parameter problem of the two-fluid model, the latter equation is solved for the energy-dependent cosmic-ray distribution function. Publication: The Astrophysical Journal Pub Date: January 1995 DOI: 10.1086/175086 Bibcode: 1995ApJ...438..427L Keywords: Astronomical Models; Cold Gas; Galactic Cosmic Rays; Hydrodynamics; Solar Wind; Steady State; Cosmology; Deceleration; Heliosphere; Mathematical Models; Radiation Pressure; Space Radiation; ACCELERATION OF PARTICLES; HYDRODYNAMICS; ISM: COSMIC RAYS; SUN: SOLAR WIND full text sources ADS | Related Materials (1) Part 2: 1995ApJ...452..423L

Implosive accretion and outbursts of active galactic nuclei

view Abstract Citations (106) References (54) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Implosive Accretion and Outbursts of Active Galactic Nuclei Lovelace, R. V. E. ; Romanova, M. M. ; Newman, W. I. Abstract A model and simulation code have been developed for time-dependent axisymmetric disk accretion onto a compact object including for the first time the influence of an ordered magnetic field. The accretion rate and radiative luminosity of the disk are naturally coupled to the rate of outflow of energy and angular momentum in magnetically driven (+/-z) winds. The magnetic field of the wind is treated in a phenomenological way suggested by self-consistent wind solutions. The radial accretion speed u(r,t) of the disk matter is shown to be the sum of the usual viscous contribution and a magnetic contribution is proportional to r^3/2^B_p_^2^/σ, where B_p_(r, t) is the poloidal field threading the disk and σ(r, t) is the disk's surface mass density. An enhancement or variation in B_p_ at a large radial distance leads to the formation of a soliton-like structure in the disk density, temperature, and B-field which propagates implosively inward. The implosion gives a burst in the power output in winds or jets and a simultaneous burst in the disk radiation. The model is pertinent to the formation of discrete fast-moving components in jets observed by very long baseline interferometry. These components appear to originate at times of optical outbursts of the active galactic nucleus. Publication: The Astrophysical Journal Pub Date: December 1994 DOI: 10.1086/174981 Bibcode: 1994ApJ...437..136L Keywords: Accretion Disks; Active Galactic Nuclei; Astronomical Models; Interstellar Magnetic Fields; Plasma Jets; Stellar Winds; Computerized Simulation; Mathematical Models; Astrophysics; GALAXIES: ACTIVE; ACCRETION; ACCRETION DISKS; GALAXIES: JETS; GALAXIES: NUCLEI full text sources ADS | data products SIMBAD (2)

Alfvénic disturbances in the equatorial solar wind with a spiral magnetic field

We study a two‐dimensional propagation of Alfvénic perturbations within the solar equatorial plane (SEP) in the solar wind with a spiral magnetic field. Analytical solutions within the entire frequency range are derived at large radial distance r. Numerical solutions which smoothly pass through the Alfvén critical point rA are also obtained. Since the spiral magnetic field caused by the solar rotation scales as ∼r−1 at large r, the velocity and magnetic field perturbations, which are perpendicular to the SEP, remain small relative to the wind parameters in the entire radial range. The frequency criterion for the non‐WKB radial scalings of the perturbation variables to appear is (f + mf⊙)² &lt; fc², where f is the perturbation frequency, f⊙ is the solar rotation frequency, m is the azimuthal wavenumber, and fc is the well‐known characteristic frequency determined by the wind parameters. We demonstrate that the process of continuous reflection in a spiral magnetic field and the effect of superposing Alfvénic perturbations with various f and m can lead to variations of the relative magnetic field fluctuation, the normalized cross helicity σc, the Alfvén ratio ra, the magnetic perturbation energy density Eb, and the kinetic perturbation energy density Eυ, which differ from the predictions of the conventional WKB theory. In particular, the non‐WKB radial scalings of the perturbation variables can appear at any timescales with appropriate values of m. The relevance of these results to data analyses on interplanetary fluctuations is discussed, and we emphasize the importance of obtaining fluctuation spectrum with respect to both f and m in the outer heliosphere.

Solar Wind Motion near the Sun Derived from Simultaneous Interplanetary Scintillation Observations at 2 GHz and 8 GHz.

Single-station observations of interplanetary scintillation (IPS) at two frequencies; 2 GHz and 8 GHz, have been made simultaneously between 1990 and 1992 at the Kashima Space Research Center. The solar wind velocity near the sun is derived from these IPS observations by means of a co-spectrum technique. Derived velocities clearly reveal that solar wind acceleration occurs at radial distances between 12 and 80 Rs (solar radii), although the velocity structure on the solar surface occasionally influences the radial variation of our IPS data. A close association between high-speed winds and the coronal hole is demonstrated by comparing our IPS data with He (1083 nm) observations. Our velocity estimates are then compared with speeds obtained from three-station IPS measurements at 327 MHz. Consequently, a good agreement is found with IPS data taken in 1992, whereas the correlation is lost with IPS data from 1991 owing to the systematic bias between two measurements. It is shown that our velocity estimates are slightly higher than three-station estimates at large radial distances, and that this relation is reversed within 30 Rs. While the cause of this radial dependence is not fully understood, some possible effects are discussed here.

Three-dimensional Propagation of Transverse Magnetohydrodynamic Perturbations in Relativistic Winds

Two major classes of astrophysical systems are believed to involve magnetized relativistic outflows, viz., pulsar winds from pulsars and extragalactic radio jets from active galactic nuclei (AGNs). We study here some basic properties of a three-dimensional propagation of transverse incompressible perturbations and the associated second-order nonlinear effect in a relativistic magnetohydrodynamic (RMHD) radial outflow with spherical symmetry. Analytical solutions are derived in the high- and low-frequency limits, and in the asymptotic regime of large radial distance r, respectively. Numerical solutions are obtained to complement the analytical analysis. In essence, these perturbations are Alfvenic fluctuations. Several major results are as follows

Alfvenic fluctuations in a relativistic wind with a spiral magnetic field and random magnetic field structures in the Crab Nebula

The Crab Nebula is powered by the central pulsar. The appearance of lumpy and patchy structures within the nebula results from inhomogeneous distributions of magnetic field and of relativistic electron density. In this paper, we focus on the primary origin of inhomogeneous magnetic field structures. For that purpose, we develop a theoretical model for a two-dimensional propagation of Alfvenic perturbations within the equatorial plane of an axisymmetric relativistic magnetohydrodynamic (RMHD) wind embedded with a spiral magnetic field caused by the rotation of a central source. We obtain analytical solutions within the entire frequency range for asymptotic Alfvenic perturbations at a large radial distance r

Three‐dimensional steady compressible perturbations in the magnetohydrodynamic solar wind

We present a steady three‐dimensional magnetohydrodynamic (MHD) perturbation formulation for inhomogeneous solar wind stream structures. We derive analytical solutions for asymptotic behaviors of three‐dimensional compressible MHD perturbations at a large radial distance r in a background radial wind of spherical symmetry. There are three independent perturbation solutions at large r. In contrast with a three‐dimensional propagation of Alfvénic perturbations in the same kind of background wind, the magnetic field perturbation associated with a steady compressible MHD perturbation does not dominate the background radial magnetic field at large r. Since the three independent compressible MHD perturbation solutions are related by the two regularity conditions at the MHD slow and fast critical points, the leading asymptotic radial scalings of the relevant compressible MHD perturbation variables are the same as those of the background variables at large r. In other words, while the radial variations of compressible MHD perturbations gradually merge into the background MHD wind at large r, the distinct angular variations of the perturbations persist throughout the wind. To complement these analytical analyses, we obtain complete numerical solutions in the entire radial range, which smoothly pass through the MHD slow and fast critical points. We provide numerical examples in which the perturbation increase (or decrease) of the radial wind speed can be as large as ∼ 100 km s−1 due to the perturbation increase (or decrease) of the temperature above the coronal base while the solar wind mass flux variation remains sufficiently small. This is possible because of our three‐dimensional approach and because of the regulation mechanism of the magnetic field in the wind. We also present an axisymmetric model for steady MHD solar wind structures in the presence of solar differential rotation.