摘要:

This paper deals with a possible cause of the large plasma flow velocities parallel to the magnetic field observed near the boundary of the plasma sheet in the earth's magnetotail. For a large class of steady state configurations with typical cases involving magnetic reconnection we first show qualitatively that high parallel flow velocities can be expected to exist on field lines connecting to a region of weak magnetic field. For reconnection configurations this weak field region contains the diffusion region. The maximum value of the parallel flow velocity is sensitive to the lowest magnetic field magnitude just outside the diffusion region. The physical mechanism causing large values of the parallel velocity component υ∥can be visualized as a strong imbalance of perpendicular mass flux into and out of magnetic flux tubes passing through regions where the magnetic field is weak and inhomogeneous. In a steady state the unbalanced perpendicular flow requires large parallel flow to establish mass conservation. The presence of a parallel velocity is not unusual in MHD systems. The new (generic) aspect is that near the separatrix, parallel flow is the dominant form of plasma transport. For a quantitative evaluation we consider an MHD model specialized for the domains where the inertia force can be neglected. By a self‐consistent treatment we evaluate υ∥and find that |υ∥| can substantially exceed the perpendicular velocity υ⊥=E/B; in a typical example with stretched magnetic field lines we obtain |υ∥| ≈ 40υ⊥. We apply our results to the earth's magnetotail and conclude that this mechanism is able to explain the parallel flow velocities near the boundary of the plasma sheet in the range of several hundreds of

ISSN:0148-0227    

DOI:10.1029/JA092iA01p00095

年代:1987

数据来源: WILEY

摘要:

The interchange instability is reviewed with particular emphasis on its relevance to planetary magnetospheric processes. Results of earlier work are reviewed and drawn together by the derivation of a general stability condition, based on a small perturbation approach, for a plasma with isotropic pressure contained in a curved magnetic field in the presence of gravity. The results are not restricted to low pressure (low β) or special field geometry, and the straight field result is recovered in the appropriate limit. We use a small perturbation approach for the instability analysis. Conditions under which an effective gravity may be introduced to simulate rotational effects are discussed. Recent criticism of the Jovian ring current impoundment hypothesis which was based on the straight field line results is shown to be ill founded

ISSN:0148-0227    

DOI:10.1029/JA092iA01p00109

年代:1987

数据来源: WILEY

摘要:

The linear theory of electromagnetic ion beam instabilities for arbitrary angles of propagation is studied, with emphasis on the conditions necessary to generate unstable modes at low harmonics of the ion cyclotron resonance condition. The present results extend the analysis of Smith et al. (1985). That paper considered only the plasma parameters at a time during which harmonic wave modes were observed in the earth's foreshock. The parameters of that paper are used as the basis of parametric variations here to establish the range of beam properties which may give rise to observable harmonic spectra. We show that the growth rates of both left‐ and right‐hand cyclotron harmonic instabilities are enhanced by an increase in the beam temperature anisotropy and/or the beam speed. Decreases in the beam density and/or the core ion beta reduce the overall growth of the cyclotron harmonic instabilities but favor the growth of these modes over the growth of the nonresonant instability and thereby enhance the observability of the harmon

ISSN:0148-0227    

DOI:10.1029/JA092iA01p00117

年代:1987

数据来源: WILEY

摘要:

A test particle approach is used to compare gyroresonant pitch angle scattering of energetic electrons by coherent versus incoherent whistler mode waves, for the case in which the coherent wave amplitude is below the nonlinear phase trapping threshold. Wave packets of 400 ms duration propagating along the magnetic field atL= 4 within the plasmasphere are considered, and the wave‐induced pitch angle scattering along the propagation path from one hemisphere to the other and the resulting precipitation flux are computed. An incoherent wave spectrum is simulated by random modulation of the wave frequency at intervals of 1 ms, thereby generating signals with nearly constant power spectral density over a bandwidth of 2 kHz centered at 5.5 kHz. The associated pitch angle scattering is compared with that of a monochromatic 5.5‐kHz signal of 400 ms duration. Results of the test particle analysis are compared with those expected on the basis of a classical diffusion treatment, and an expression is derived for an effective “diffusion” coefficient for pitch angle scattering by coherent waves. The trajectory followed by a particle when interacting with incoherent waves essentially represents a random walk in velocity space, while for coherent waves the pitch angle of the particle varies in a well‐defined manner. In spite of the fact that individual particle scatterings are typically larger for coherent waves, the peak precipitation fluxes induced by incoherent waves are found to be approximately the same as those for coherent waves having the same total power. This results from the fact that incoherent waves interact with particles over a wider range of energies. As a consequence, the energy spectrum and the temporal extent of transient precipitation pulses due to incoherent wave packets are broader than those for equivalent cohe

ISSN:0148-0227    

DOI:10.1029/JA092iA01p00127

年代:1987

数据来源: WILEY

摘要:

Pc 1 waves observed at synchronous orbit are studied using magnetic field data from the flux gate magnetometer on board ATS 6. The data were obtained in the interval of June to September in 1975 when the satellite was located at the geographic equator at 36° east longitude near the geomagnetic equator (geomagnetic latitude ∼−3°). It is found that spectral structures of Pc 1 waves are significantly affected by variations of the magnitude of the local magnetic field near the geomagnetic equator. Spectral bands of Pc 1 waves are usually confined to the frequency range below the helium gyrofrequency (fHe+) when Pc 1 waves appear on the afternoon side accompanied by a decrease in the ambient magnetic field during substorms. In the daytime, Pc 1 waves often occur in association with an increase in the ambient magnetic field caused by a magnetospheric compression. In such cases, Pc 1 waves tend to appear in the frequency range above fHe+. The sense of polarization is left‐handed for most waves as was noted in previous studies. It is suggested from the linear growth rate analysis that the absence of Pc 1 waves in the frequency range above the helium gyrofrequency during substorms is mainly due to an increase in the energetic prot

ISSN:0148-0227    

DOI:10.1029/JA092iA01p00143

年代:1987

数据来源: WILEY

摘要:

Observations of the February 4, 1983, storm sudden commencement (ssc) by the STARE and SABRE radar systems are presented. The observing stations were in the dusk sector, where the ssc was accompanied by a preliminary reverse impulse (PRI). The radar data show the PRI to be a consequence of the ssc compression wave producing an initial antisunward flow. The polarization of the ssc electric field in the ionosphere is seen to have both longitudinal and latitudinal structure. The observations are discussed in terms of the propagation of the ssc disturbance throughout the magnetosphere.

ISSN:0148-0227    

DOI:10.1029/JA092iA01p00159

年代:1987

数据来源: WILEY

摘要:

Energy densities are computed for geomagnetic field fluctuations in the Pc 3–4 range usingHcomponent magnetometer data recorded at South Pole station over a three‐month period from June 3 to September 4, 1982. Hourly values of the energy densities in the Pc 3 and Pc 4 period bands are found to be highly correlated during geomagnetic local daytime hours. The results of multivariate analyses between geomagnetic energies and upstream solar wind quantities show that the most important quantity in controlling the magnitude of the field fluctuations is the solar wind speed, with the IMFBzcomponent being of next importance. We conclude that the Kelvin‐Helmholtz instability at the dayside magnetopause is, statistically, the dominant energy source that contributes to ULF hydromagnetic wave activity at cusp lati

ISSN:0148-0227    

DOI:10.1029/JA092iA01p00168

年代:1987

数据来源: WILEY

摘要:

Recent observations by the Dynamics Explorer 1 satellite over the dayside polar cap magnetosphere have indicated downward flows of heavy ions (O+, O++, N+, N++) with flow velocities of the order 1 km/s (Lockwood et al., 1985b). These downward flows were interpreted as the result of “parabolic” flow of these heavy ionospheric ions from a source region associated with the polar cleft topside ionosphere. Here we utilize a two‐dimensional kinetic model to elicit features of the transport of very low energy O+ions from the cleft ionosphere. Bulk parameter (density, flux, thermal energies, etc.) distributions in the noon‐midnight meridian plane illustrate the effects of varying convection electric fields and source energies. The results illustrate that particularly under conditions of weak convection electric fields and weak ion heating in the cleft region, much of the intermediate altitude polar cap magnetosphere may be populated by downward flowing heavy ions. It is further shown how two‐dimensional transport effects may alter the characteristic vertical profiles of densities and fluxes from ordinary profiles computed in one‐dimensional steady s

ISSN:0148-0227    

DOI:10.1029/JA092iA01p00175

年代:1987

数据来源: WILEY

摘要:

The current‐voltage relation within narrow auroral current sheets is examined through the use of high‐resolution data from the high‐altitude Dynamics Explorer 1 satellite. The north‐south perpendicular electric field and the east‐west magnetic field are shown for three cases in which there are large amplitude, oppositely directed paired electric fields which are confined to a region less than 20 km wide. The magnetic field variations are found to be proportional to the second integral of the high‐altitude perpendicular electric field. It is shown that at the small‐scale limit, this relationship between ΔBandEis consistent with a linear “Ohm's law” relationship between the current density and the parallel potential drop along the magnetic field line. This linear relationship had previously been verified for large‐scale auroral formations greater than 20 km wide at the ionosphere. The evidence shown here extends our knowledge down to the scale size of d

ISSN:0148-0227    

DOI:10.1029/JA092iA01p00187

年代:1987

数据来源: WILEY

摘要:

During periods of southward interplanetary magnetic field (IMF) we have examined the relationship between magnetic variations in the central polar cap and the IMFByandBzcomponents. The geomagnetic polar cap indexPCthat can be used as a measure of the flow across the polar cap has been derived using data from Thule in the IMS period. The results have been compared with IMP 8 measurements of the IMF and the solar wind velocity. The statistical analysis shows that the absolute value of the azimuthal component |By| contributes to the cross‐polar cap flow in the same manner as the southward componentBs. The relative contributions of |By| andBzhave been examined and compared with the theoretical expression υBTsin² θ/2 for the merging electric field. It is found that the contribution of |By| compared toBzis only half as big in the observations as in the theoretical expression. TheByeffect onPCis compared to an earlier reported effect ofByon the geomagnetic indexAL(Murayama et al., 1980) and found to be quite different from this. This is discussed in relation to interpretations in terms of merging site asymm

ISSN:0148-0227    

DOI:10.1029/JA092iA01p00195

年代:1987

数据来源: WILEY