摘要:

A comprehensive review is presented of the mathematical methods used to represent magnetic fields in the Earth's magnetosphere, of the way existing data‐based models use these methods and of the associated problems and concepts. The magnetic field has five main components: the internal field, the magnetopause, the ring current, the tail and Birkeland currents. Methods of representing separately each of these are discussed, as is the deformation of magnetic fields; Appendix B traces the connection between deformations and the Cauchy integral. A summary section lists the uses of data‐based models and their likely future evolution, and Appendix A supplements the text with a set of probl

ISSN:0148-0227    

DOI:10.1029/94JA01239

年代:1994

数据来源: WILEY

摘要:

Magnetic field data measured by the MAGMA instrument in the Martian magnetotail lobes are compared with the ram pressure of the upstream solar wind observed by the TAUS instrument in the circular orbits of the Phobos 2 spacecraft. High correlation was found between the magnetic field intensity in the Martian magnetotail lobes and the solar wind ram pressure. From this relationship the average flaring angle of the Martian magnetotail was determined as ∼13°, and the average magnetosonic Mach number was estimated as ∼5. The observed relationship between the Martian magnetotail magnetic field intensity and the solar wind magnetic field reflects the correlation of the solar wind magnetic field to the ram pressure providing a value of ∼7 for the average Alfvenic Mach number. The flaring angle obtained for the Martian magnetotail was found to be an intermediate value between the flaring angle of the magnetotail of the Earth and that of Venus at comparable dis

ISSN:0148-0227    

DOI:10.1029/94JA00946

年代:1994

数据来源: WILEY

摘要:

Using Ulysses radio wave data taken during the 1992 Jupiter encounter, we conclude that there are significant large and small spatial scale azimuthal asymmetries at high latitudes in the Io plasma torus. During a period of time near perijove when the spacecraft motion was predominantly in the azimuthal direction and was relatively fixed in both latitude and radial distance, inferred electron densities depart significantly from the common assumption of longitudinal symmetry. Specifically, electron plasma concentrations near 0° system III longitude (and 0400 LT) are greater than those near 180° (and 0000 LT). Superposed on this large‐scale variation are regularly spaced density depletions, 30‐50% in magnitude, and having a spatial periodicity of about 17°. Some of these depletions may drive various known radio and plasma wave sources by means of largeBparallel electric potentials. The observations are compared with recent models and with the in situ Voyager observ

ISSN:0148-0227    

DOI:10.1029/94JA01615

年代:1994

数据来源: WILEY

摘要:

We use a combination of analytical and numerical methods to investigate the dynamics of charged dust grains in planetary magnetospheres. Our emphasis is on obtaining results valid for particles that are not necessarily dominated either by gravitational or electromagnetic forces. A Hamiltonian formulation of the problem yields exact results, for all values of charge‐to‐mass ratio, when we introduce two constraints: particles remain in the equatorial plane and the magnetic field is taken as axially symmetric. In particular, we obtain locations of equilibrium points, the frequencies of stable periodic orbits, the topology of separatrices in phase space, and the rate of longitudinal drift. These results are significant for specific applications: motion in the nearly aligned dipolar field of Saturn, and the trajectories of arbitrarily charged particles in complex magnetic fields for limited periods of time after ejection from parent bodies. Since the model is restrictive, we also use numerical integrations of the full three‐dimensional equations of motion and illustrate under what conditions the constrained problem yields reasonable results. We show that a large fraction of the intermediately charged and highly charged (gyrating) particles will always be lost to a planet’s atmosphere within a few hundred hours, for motion through tilted‐dipole magnetic fields. We find that grains must have a very high charge‐to‐mass ratio in order to be mirrored back to the ring plane. Thus, except perhaps at Saturn where the dipole tilt is very small, the likely inhabitants of the dusty ring systems are those particles that are either nearly Keplerian (weakly charged) grains or grains whose charges place them in the lower end of the intermediate charge zone. Finally, we demonstrate the effect of plasma drag on the orbits of gyrating particles to be a rapid decrease in gyroradius followed by a slow radial evolution of the g

ISSN:0148-0227    

DOI:10.1029/94JA01231

年代:1994

数据来源: WILEY

摘要:

We present an analytic, self‐consistent model of time‐dependent solar‐wind‐driven convection in the magnetosphere of Uranus. Because of the unusual orientation of the planetary rotation and magnetic dipole axes, magnetic merging on the dayside magnetopause varies as a function of planetary spin, in response to the changing orientation of the planetary magnetic field relative to the upstream interplanetary magnetic field, which is assumed to have a fixed direction for many planetary rotations. Therefore the magnitude of the solar‐wind driven convection electric field varies sinusoidally in time with the 17.2‐hour planetary spin period, even though the field direction is fixed in the corotating frame in a direction analogous to the dawn‐to‐dusk direction in the Earth's magnetosphere. We assume that the “hot” (keV) protons observed by the Voyager 2 plasma science instrument in the inner magnetosphere convect sunward from a source in the near tail and form a ring current shielding layer nearL= 5. The shielding process requires a time‐dependent model because the convection timescale (∼20 days) is much larger than the 17‐hour period of variation of the convection field. The time‐averaged part of the imposed electric field is strongly attenuated inside the shielding layer, but the sinusoidally varying part of the imposed field penetrates the layer without significant attenuation because the shielding timescale (∼30 hours) is longer than the 17‐hour oscillation period. A fraction of the hot plasma is thereby “scattered” onto closed drift orbits to form a trapped ring current population. This trapped ring current population is sufficiently long‐lived to undergo charge exchange and inelastic collisions with the widely distributed neutral hydrogen corona, resulting in the energy degradation of the “hot” component and the simultaneous appearance of the “intermediate” (∼ 100 eV) and "warm" (∼ 10 eV) components evident in the Voyager 2 plasma science measurements betweenL= 5 andL= 7. The Birkeland current system is concentrated near the ring current shielding layer, consistent with the relatively low latitude of the auroral emissions o

ISSN:0148-0227    

DOI:10.1029/94JA00870

年代:1994

数据来源: WILEY

摘要:

We consider nonlinear effects associated with the propagation of a quasi‐monochromatic whistler wave packet along the inhomogeneous geomagnetic field. We show that under the specific conditions of the Siple experiment the whistler packet is subject to the modulational instability, resulting in longitudinal and transverse modulation of the envelope amplitude and the excitation of quasi‐neutral density perturbations akin to ion acoustic waves. In particular, we examine the behavior of the wave packet in the vicinity of the region where the group velocity of the packet is a maximum. In general, the wave packet is subject to the competing effects of dispersive broadening and the nonlinear tendency to steepen. In the vicinity of the region of interest the dispersive broadening is small, and the tendency for steepening prevails. This results in the creation of anomolously narrow solitary waves. We analyze the situation analytically using a modification of the nonlinear Schrodinger equation. We evaluate the conditions required for the onset of the modulational instability and find its typical growth rate. We then construct nonlinear traveling wave solutions. These take the form of solitons, which move toward the rear edge of the wave packet. Because the dispersive effects are small, these solitons are narrow structures with steep boundaries. We then analyze the possibility that these solitons act as drivers for triggered emission. The interaction of resonant particles with the wave at the soliton leads to particle bunching in velocity. The resulting current serves as a source term for monochromatic radiation, the frequency of which changes with time as the bunches move in the inhomogeneous geomagnetic fi

ISSN:0148-0227    

DOI:10.1029/94JA00718

年代:1994

数据来源: WILEY

摘要:

Unducted VLF signals produced by lightning activity can form a population of magnetospherically reflected (MR) whistlers in the inner magnetosphere. It has been suggested recently that in the absence of significant attenuation such waves could merge into a broadband continuum with sufficient intensity to account for plasmaspheric hiss. To test this conjecture we have evaluated the path‐integrated attenuation of MR whistlers along representative ray paths using the HOTRAY code. Using a realistic plasma distribution modeled on in situ data, we find that the majority of MR waves experience significant damping after a few transits across the equator. This is primarily due to Landau resonance with suprathermal (0.1‐1 keV) electrons. The attenuation is most pronounced for waves that propagate through the outer plasmasphere; this can readily account for the infrequent occurrence of multiple‐hop MR waves forL≥ 3.5. Selected waves that originate at intermediate latitudes (15° ≤ λ ≤ 35°) and whose ray paths are confined to the inner plasmasphere may experience up to 10 magnetospheric reflections before substantial attenuation occurs. These waves should form the population of observed MR waves. Wave attenuation becomes more pronounced at higher frequencies; this can account for the absence of multiple‐hop waves above 5 kHz. Weakly attenuated MR waves tend to migrate outward to theLshell, where their frequency is comparable to the equatorial lower hybrid frequency. The enhanced concentration of waves due to a merging of ray paths would produce a spectral feature that rises in frequency at lowerL. This is quite distinct from the reported properties of plasmaspheric hiss, which maintains a constant frequency band throughout the entire plasmasphere. Furthermore, in the absence of mode conversion, waves below 500 Hz, which often form an important if not dominant part of the spectral properties of hiss, are unable to escape from the topside ionosphere in the whistler mode. Consequently, we conclude that unducted lightning signals cannot account for the origin of pl

ISSN:0148-0227    

DOI:10.1029/94JA01006

年代:1994

数据来源: WILEY

摘要:

The path‐integrated linear growth of electromagnetic ion cyclotron waves in the outer (L≥ 7) magnetosphere is investigated using a realistic thermal plasma distribution with an additional anisotropic energetic ring current H+to provide free energy for instability. The results provide a realistic simulation of the recent AMPTE observations. For conditions typical of the dayside magnetosphere, high plasma beta effects reduce the group velocity and significantly increase the spatial growth rates for left‐hand polarized instabilities just below the helium gyrofrequency ΩHe+, and on the unguided mode above Ω>He+but below the cross over frequency ωcr. Relatively high densities, typical of the afternoon local time sector, favor these low group velocity effects for predominantly field‐aligned waves. Lower densities, typical of those found in the early morning local time sector, increase the group velocity but allow strong convective instabilities at high normalized frequencies well above ΩHe+. These waves are reflected in the magnetosphere and can exist for several equatorial transits without significant damping. They are left‐hand polarized only on the first equatorial crossing and become linearly polarized for the remainder of the ray path. Consequently, these waves should be observed with basically linear polarization at all frequencies and all latitudes in the early morning local time sector. Wave growth below ΩHe+is severely limited owing to the narrow bandwidth for instability and the small resonant path lengths. In the afternoon sector, where plasma densities can exceed 107m−3, intense convective amplification is possible both above and below ΩHe+. Waves below ΩHe+are not subject to reflection when the O+concentration is small and therefore should be observed with left‐hand polarization near the equator and essentially linear polarization at higher latitudes. Since the He+concentration is usually large in the afternoon sector, guided mode waves above ΩHe+reflect to form a background distribution with basically linear polarization. We suggest that the strong left‐hand polarized emissions observed by AMPTE in the afternoon sector near the equator are probably due to strongly growing low group velocity waves at frequencies just below ΩHe+, and on the

ISSN:0148-0227    

DOI:10.1029/94JA01259

年代:1994

数据来源: WILEY

摘要:

Electromagnetic ion cyclotron (EMIC) waves in the frequency range below the helium gyrofrequency can be excited in the equatorial region of the outer magnetosphere by cyclotron resonant instability with anisotropic ring current H+ions. As the unducted waves propagate to higher latitudes, the wave normal should become highly inclined to the ambient magnetic field. Under such conditions, wave energy can be absorbed by cyclotron resonant interactions with ambient O+, leading to ion heating perpendicular to the ambient magnetic field. Resonant wave absorption peaks in the vicinity of the bi‐ion frequency and the second harmonic of the O+gyrofrequency. This absorption should mainly occur at latitudes between 10° and 30° along auroral field lines (L≥ 7) in the postnoon sector. The concomitant ion heating perpendicular to the ambient magnetic field can contribute to the isotropization and geomagnetic trapping of collapsed O+ion conics (or beams) that originate from a low‐altitude ionospheric source region. During geomagnetic storms when the O+content of the magnetosphere is significantly enhanced, the absorption of EMIC waves should become more efficient, and it may contribute to the observed acceleration of O+ions of ionospheric origin up to ring current e

ISSN:0148-0227    

DOI:10.1029/94JA01007

年代:1994

数据来源: WILEY

摘要:

On rare occasions, satellites at geosynchronous altitude enter the magnetospheric lobe, characterized by extremely low ion fluxes between 1 eV and 40 keV and electron fluxes above a few hundred eV. One year of plasma observations from two simultaneously operating spacecraft at synchronous orbit is surveyed for lobe encounters. A total of 34 full encounters and 56 apparent near encounters are identified, corresponding to ∼0.06% of the total observation time. Unlike energetic particle (E>40 keV) dropouts studied earlier, there is a strong tendency for the lobe encounters to occur postmidnight, as late as 07 local time. The two spacecraft encounter the lobe with different rates and in different seasons. These occurrence properties are not simply explicable in terms of the orbital geometry in either the solar magnetic or the geocentric solar magnetospheric coordinate system. A composite coordinate system which previously organized more energetic particle dropouts is somewhat more successful in organizing the lobe encounters, suggesting that solar wind distortion of the magnetic equatorial plane away from the dipole location and toward the antisolar direction may be largely responsible for these dropouts. Our results further suggest that this distortion persists even sunward of the dawn‐dusk terminator. However, a simple dawn‐dusk symmetric distortion does not fully account for all the seasonal and local time asymmetries in the occurrence of the lobe encounters; thus there is probably an additional dawn‐dusk asymmetry in the distorted field. The lobe encounters are strongly associated with magnetospheric activity and tend to occur in association with rare magnetosheath encounters at synchronous orbit. It thus appears that the presence of the lobe at geosynchronous orbit is the result of major, probably asymmetric modifications of the magnetospheric field geometry in times of strong dist

ISSN:0148-0227    

DOI:10.1029/94JA00423

年代:1994

数据来源: WILEY