摘要:

It is shown that the basic features of the neutral line configuration on the source surface (a concentric sphere of 2.5 solar radii) and their variations during a sunspot cycle can be represented fairly well in terms of an axial main dipole at the center of the Sun and two (or at most three during a very limited time period) antipodal dipoles on the photosphere. This representation is not unique, but may be physically more meaningful than the harmonic analysis results. The nature of the photospheric dipoles is also discussed. The results may have some application to the Earth's magnetic field and its variations.

ISSN:0148-0227    

DOI:10.1029/JA094iA11p14993

年代:1989

数据来源: WILEY

摘要:

Ring‐type ion distributions, such as are found in the vicinity of comets, are shown to drive mirror waves unstable. Major differences are found between mirror waves driven by anisotropic bi‐Maxwellian distributions and mirror waves driven by ring‐type distributions; in particular, for pure ring distributions the maximum mirror wave growth rate occurs for quasiparallel wave propagation. The growth rate for ring‐type driven mirror waves is approximated analytically and calculated numerically. For pure ring ion distributions, the analytic approximations are quite good. For ring‐beam distributions, the mirror waves appear with a real frequency approximately equal to the ring ion Doppler frequency; the analytic approximations agree with the numerical results when the real frequency is small compared to the proton gyrofrequency so that there is little proton Landau damping. The analytic approximations provide a simple explanation for the growth dependences on propagation angle and wavelength, and lead to a simple (approximate) expression for the most unstable wavelength. Resonant instabilities can also be driven by ring‐type distributions; growth rates for the nonresonant mirror wave instability are comparable to growth rates for resonant instabilities. In the presence of a ring‐type distribution, the mirror waves remain nonoscillatory (ωr= 0) when viewed in the frame of th

ISSN:0148-0227    

DOI:10.1029/JA094iA11p15001

年代:1989

数据来源: WILEY

摘要:

Our two‐dimensional (one velocity space and one spatial dimension), time‐dependent model calculations indicate that an interplay between velocity and spatial diffusion may be responsible for the acceleration of implanted heavy ions in the cometary preshock region. Velocity diffusion (second‐order Fermi acceleration) accelerates the pickup ions to moderate energies, thus creating a seed population for the more efficient diffusive‐compressive shock acceleration. Solar wind convection limits the time available for diffusive‐compressive acceleration, therefore the resulting energy spectrum above the pickup energy is a combination of an exponential decrease at lower energies (up to about 100 keV) and a power law spectrum at higher energies. The calculated energy spectra constitute a surprisingly good fit to the published VEGA and Giotto energy spectra above the pick

ISSN:0148-0227    

DOI:10.1029/JA094iA11p15011

年代:1989

数据来源: WILEY

摘要:

The magnetometer onboard the Giotto spacecraft observed a diamagnetic cavity surrounding the nucleus of comet Halley. The location of the boundary of this diamagnetic cavity is determined by a balance between an inward magnetic pressure gradient force and an outward ion‐neutral drag force, associated with collisions between the outwardly flowing neutrals and the stagnated ions. A one‐dimensional, time‐dependent, magnetohydrodynamical model has been developed for the inner coma of comet Halley, and includes ion‐neutral collisions, photochemical production and loss of plasma, and finite conductivity. This model is used to investigate the plasma dynamics in the vicinity of the diamagnetic cavity boundary surface. A narrow transition layer with enhanced plasma density is shown to exist just inside the boundary, although a full understanding of this layer will require a two‐ or three‐dimensional MHD model. The flux of cometary ions into this shocklike layer is removed by electron‐ion recombination. The thickness of this layer is determined by the Mach number of the

ISSN:0148-0227    

DOI:10.1029/JA094iA11p15025

年代:1989

数据来源: WILEY

摘要:

The model of Pontius et al. (1986) for plasma transport in a corotation‐dominated magnetosphere is modified to satisfy observational constraints introduced by Richardson and McNutt (1987). As in the earlier model, small discrete flux tubes whose plasma content differs significantly from the longitudinally averaged value (transient flux tubes) move steadily under the influence of the centrifugal force. We consider the electric fields surrounding a transient flux tube of elliptical cross section and derive expressions for the translational velocity and cross‐sectional distortion of such flux tubes. To account for the observed radial decrease of flux shell content at Jupiter, we assume that plasma can pass between transient flux tubes and the background by means of single‐particle microdiffusion. The background plasma distribution is affected by the passage of a transient in two ways: direct mass loading through microdiffusion, and radial displacement to conserve magnetic flux. Upon adopting a simple form to describe the microdiffusion process, we find that there exists a particular slope of flux shell plasma content for which these effects cancel and the background maintains a steady state distrib

ISSN:0148-0227    

DOI:10.1029/JA094iA11p15041

年代:1989

数据来源: WILEY

摘要:

A spherical harmonic model of Jupiter's planetary magnetic field is combined with a self‐consistent model of the Jovian magnetodisc. The sets of parameters of both models are determined simultaneously by using a generalized inverse technique. Assuming that the pressurePin the middle (and outer) magnetosphere is related to the unit flux tube volumeVthroughPVγ= const, the fit yields a value of 0.88 for γ. If the hot (30 keV) plasma is transported adiabatically inward under the interchange instability triggered by the centrifugal force of the heavy torus ions, losses are not sufficient to account for such a low value of γ beyondL= 10 (as compared to γ ≈ 5/3 that would be expected for adiabatic transport of monoatomic gas). Closer to the planet, as the outer edge of the Io plasma torus is approached (at distances between 7 and 9RJfrom Jupiter),PVγis found to decrease inward, as expected from the particle measurements, which identified an inner boundary of the particle fluxes in that region. At the magnetic equator, our pressure estimates were compared with the ones obtained from direct particle measurements (low‐energy charged particle (LECP) experiment). Assuming a mixture of O+and H+at the same temperature (or with the same spectral power law exponent), consistency between those two independent determinations of the pressure would require that the pressure produced by H+constitute 18–36% (at most) of the total pressure, at distances between 13 and 21RJ. Finally, as concerns the internal field coefficients, despite an overall consistency, a slight variability is found between the present estimates and previous ones derived from the same data set, which puts some limitation on the accuracy with which internal coefficients can be determined from the Voyager 1 enc

ISSN:0148-0227    

DOI:10.1029/JA094iA11p15055

年代:1989

数据来源: WILEY

摘要:

Geomagnetic pulsations are one of the dominant features of the dynamics of the Earth's solar wind‐magnetosphere‐ionosphere coupling system. Whether such ultralow‐frequency waves are also excited within the Jovian magnetosphere has been the subject of a close inspection of Voyager 1 magnetic field observations during its close encounter with Jupiter. These observations clearly indicate the existence and an increase of ultralow‐frequency wave activity and indicate that the activity becomes more regular as soon as Voyager 1 entered the Io plasma torus at around 0700 spacecraft event time on March 5, 1979. In particular, periodic transverse and compressional magnetic field fluctuations with periods of about 1200 s and 800 s, respectively, are observed with the different periods pointing toward a decoupling between these two different types of oscillations. The coincidence between the increase in wave activity and the entry into the Io plasma torus is in support of treating the torus as a low Alfvén velocity region and thus as a hydromagnetic waveguide. A first theoretical treatment of hydromagnetic wave propagation within the torus suggests that decoupling of toroidal and poloidal type oscillations can occur under the condition of axisymmetry of the wave field. Numerical calculations of the fundamental mode toroidal and first harmonic poloidal eigenperiods for a model Jovian magnetosphere give values quite in accord with the observed periods. We thus conclude that nearly axisymmetric, decoupled toroidal and poloidal mode eigenoscillations of the Io plasma torus are observed, indicating a large‐scale source mechanism for the magnetic field fluctuations detected. The total electromagnetic energy of the observed torus oscillations is of the order of 1013J and is thus significant with respect to the energetics of the Io pl

ISSN:0148-0227    

DOI:10.1029/JA094iA11p15063

年代:1989

数据来源: WILEY

摘要:

Phase space density profiles for protons and electrons with first invariants µ≤100 MeV/G (integral invariantsK= 0.3 and 0.6 G1/2RU) previously derived from measurements by the low energy charged particle (LECP) detector on Voyager 2 during the 1986 Uranus encounter are analyzed using solutions of the time‐averaged radial diffusion equation in a dipolar magnetic field. These profiles are selected for their consistency with an absence of local sources of particles. A loss model consisting of absorption by the major inner satellites Miranda, Ariel, and Umbriel is assumed and the corresponding form of the time‐averaged radial diffusion coefficientD(L)(taken to be of the formD(L)=DoLn, wherenis an integer) is determined by a minimum‐variance fit to the phase space density profiles. Satellite macrosignatures present in the experimentally derived profiles are approximately reproduced in several cases lending credence to the loss model and indicating that magnetospheric distributed losses are not as rapid as satellite absorption near the minimum satelliteLshells for these particles. The latter inference implies an upper limit of approximately 10 cm−3for the neutral hydrogen number density near the orbit of Ariel, based on a comparison of charge exchange lifetimes with the calculated satellite absorption lifetimes. The preferred forms forD(L)are characterized by a low‐orderLdependence (∼L³–L4) and an amplitude (Do≃ 10−11–10−10RU² s−1). The inferredLdependence is least consistent with “terrestrial‐type” diffusion mechanisms including magnetic impulses and electrostatic field fluctuations of purely magnetospheric origin. “Jovian‐type” mechanisms including the ionospheric dynamo electric field mechanism and the centrifugal interchange instability in regions of strong negative radial plasma density gradients predictLdependences that are more consistent with the inferred form. Of these, the ionospheric d

ISSN:0148-0227    

DOI:10.1029/JA094iA11p15077

年代:1989

数据来源: WILEY

摘要:

Under certain conditions electrons can be reflected and effectively energized at quasi‐perpendicular shocks. This process is most prominent close to the point where the upstream magnetic field is tangent to the curved shock. The energization process has been observed at the Earth's bow shock, but is also considered to occur elsewhere in space. A theoretical explanation of the underlying physical mechanism has been proposed which assumes conservation of magnetic moment and a static, simplified shock profile. We perform test particle calculations of the electron reflection process in order to examine the results of the theoretical analysis without imposing these restrictive conditions. A one‐dimensional hybrid simulation code generates the characteristic field variations across the shock. Special emphasis is placed on the spatial and temporal length scales involved in the mirroring process. The simulation results agree generally well with the predictions from adiabatic theory. The effects of the cross‐shock potential and unsteadiness are quantified, and the influence of field fluctuations on the reflection process is disc

ISSN:0148-0227    

DOI:10.1029/JA094iA11p15089

年代:1989

数据来源: WILEY

摘要:

Using a two‐dimensional compressible MHD code we have numerically studied the reconnection process at an interface where (1) the magnetic field is higher on one side than on the other and total pressure is in balance by a higher density on the weak field side, and where (2) the magnetic field is identical on both sides, but the temperature is higher on one side than on the other. Reconnection is caused by applying a localized resistivity in the center of the current sheet. The plasma is allowed to enter and exit freely at the boundaries. By using a system size with a large extension parallel to the current sheet, we are able to study time‐dependent as well as steady state effects. Denoting the weak magnetic field or low temperature region as magnetosheath and the high magnetic field or high temperature region as magnetosphere we find the following. Sudden onset of reconnection causes the development of a bulge in the current layer. The bulge moves with about 70% of the magnetosheath Alfvén speed along the magnetopause away from the reconnection region. The growth of the bulge normal to the magnetopause is proportional to the reconnection rate. For a ratio of magnetospheric to magnetosheath magnetic field larger than about 1.5 the bulge is almost exclusively on the magnetosheath side of the magnetopause current layer. The motion of the bulge along the magnetopause leads to signatures in the normal magnetic field component similar to the ones observed in flux transfer events. When the bulge has moved far enough from the reconnection site, stationary reconnection proceeds at the X line. If there exists initially a magnetic field difference across the magnetopause current layer, a slow mode shock is formed in the magnetosphere and possibly a strong intermediate shock is formed in the magnetosheath. The magnetosheath plasma is accelerated to about twice the magnetosheath Alfvén speed and constitutes a boundary layer on the magnetospheric side of the magnetopause current

ISSN:0148-0227    

DOI:10.1029/JA094iA11p15099

年代:1989

数据来源: WILEY