摘要:

Analysis of white light coronagraph observations of a polar coronal hole led Munro and Jackson (1977) to infer large solar wind speeds in the coronal hole and a consequent need for substantial energy addition to the outflow well above the coronal base. Recent interplanetary Lyman α observations by Kumar and Broadfoot (1979) and Lallement et al. (1985) indicate that the solar wind mass flux from well‐developed polar coronal holes may be considerably less than that typically observed in situ near the ecliptic plane. A reexamination of the Munro and Jackson analysis, with attention given both to the inferences drawn from the interplanetary Lyman α observations and to the uncertainties inherent in the coronal white light observations, leads to conclusions significantly different from those drawn by Munro and Jackson. Specifically, the coronal observations are found to be consistent not only with the high flow speeds and substantial energy addition inferred by Munro and Jackson, but also with much lower flow speeds and the absence of any significant energy addition in the region observed. This removal of definite observational confirmation of substantial energy addition to the coronal expansion between 1Rsand 5Rshas important implications for our understanding of energy balance in the solar w

ISSN:0148-0227    

DOI:10.1029/JA091iA06p06751

年代:1986

数据来源: WILEY

摘要:

An analysis is presented of the magnetic field data obtained by the Pioneer 10 and 11 spacecraft in the outer heliosphere, with regard to radial and latitudinal gradients in the field components and magnitude. At first sight the field data appear to conform fairly well with the predictions of the Parker model. However, when the data obtained by Pioneer are compared with data from earth orbiting satellites, it is found that time variations are significantly affecting the field throughout the low‐latitude heliosphere. Removing these time variations allows a significant departure from the Parker model to emerge. The field magnitude and directions are found to be consistent with meridional flux transport from the equatorial plane towards higher heliographic latitudes. Such meridional transport could be driven by higher pressure near the solar equator. Enhanced pressure would be expected on the basis of the increased intensity of the spiral field in that region. However, another possible source of increased pressure is associated with the heating that occurs in compressive solar wind interaction regions and the large field magnitudes that occur in these regions both of which may be enhanced at low latitudes at least at the radial distances sampled thus far. Finally, evidence is presented which suggests a solar cycle dependence in the degree of meridional flux transpor

ISSN:0148-0227    

DOI:10.1029/JA091iA06p06760

年代:1986

数据来源: WILEY

摘要:

A critical analysis of the current interpretations of the antisolar motion of the Venus ionosphere is presented. It is argued that the pressure gradient forces present across the terminator are not sufficient to produce the observed acceleration of the plasma. A balance condition between the height‐integrated momentum flux of the ionospheric flow and the height‐integrated deficiency of momentum flux of the shocked solar wind in the vicinity of the terminator is formulated in terms of measurable quantities. It is found that the observed 2 to 4‐km/s flow velocities of the upper ionosphere can be readily inferred from such a relation. This result is consistent with the concept that an efficient transport of momentum takes place across the ionopause, and that the bulk of the kinetic energy required to accelerate the Venus ionosphere is derived from the shocked solar

ISSN:0148-0227    

DOI:10.1029/JA091iA06p06765

年代:1986

数据来源: WILEY

摘要:

The effects of pressure anisotropy and backstreaming heat flux on the fast and slow shock jump conditions are studied parametrically. The fast shock jump condition can be brought into agreement with earth bow shock observations by (1)q1

ISSN:0148-0227    

DOI:10.1029/JA091iA06p06771

年代:1986

数据来源: WILEY

摘要:

A computer model of a magnetosphere in which the magnetic dipole axis points into the solar wind has been developed. The axial symmetry implied by this physical situation admits a two‐dimensional treatment. The model features (1) a magnetopause in pressure balance with the solar wind, (2) an analytic asymptotic tail model to account for the distant magnetotail, and (3) a full accounting of the magnetohydrodynamic pressure in the magnetosphere. The plasma pressure is a function of the Euler potential, α (=rAϕ). The principal physical result is that with an increase in the plasma pressure from zero the magnetic field configuration changes slowly at first. As the pressure increases further, the field configuration changes more rapidly until X and O lines are for

ISSN:0148-0227    

DOI:10.1029/JA091iA06p06777

年代:1986

数据来源: WILEY

摘要:

A global computer simulation of the interaction of the solar wind with the earth's magnetosphere was executed by using a three‐dimensional magnetohydrodynamic model. As a result, we were able to reproduce quasi‐steady‐state magnetospheric configurations and a Birkeland field‐aligned current system which depend on the polarity of thezcomponent of the interplanetary magnetic field (IMF). Twin convection cells and a dawn to dusk electric potential of 30–100 kV appeared at the equator in the magnetosphere. Four types of field‐aligned currents were observed. Region 1 and 2 field‐aligned currents generated for all IMF conditions were 0.6–1.0×106A and 0.15–0.61×106A, respectively, in the total current. Region 1 currents at high latitudes are generated from the field‐aligned vorticity at the flanks through a viscous interaction and are strengthened by a twisting of open magnetic field lines in the tail region for southward IMF. On the other hand, the low‐latitude region 2 currents probably are generated mainly from the inner pressure gradient of the plasma sheet. The region 1 current obtained from the simulation was in good agreement with an estimate from our theoretical analysis of the localized Alfvén mode. The other two types of field‐aligned currents are the dayside magnetopause currents in the dayside cusp region, which increase for northward IMF, and the dayside cusp currents for southward IMF. The cusp currents are associated with a twisting of open magnetic field lines

ISSN:0148-0227    

DOI:10.1029/JA091iA06p06791

年代:1986

数据来源: WILEY

摘要:

In an earlier paper we suggested that the multiple X line reconnection process may occur at the dayside magnetopause and lead to the occurrence of flux transfer events. In a series of papers we shall examine in detail various aspects of the multiple X line reconnection processes. In the present paper, the first of the series, we attempt to obtain a criterion for the transition from the classical single X line reconnection to the multiple X line reconnection based on a two‐dimensional simulation model. It is found that the development of tearing instability in the diffusion region of the reconnection configuration may lead to the formation of magnetic islands and hence to the occurrence of multiple X line reconnection. The criterion for the tearing instability is found to bel/δ>7 coth (8/S0.5), whereSis the Lundquist number based on the width of the diffusion region,lis the half‐length, and δ the half‐width of the diffusion region. Furthermore, a formula is obtained for the dependence of the ratiol/δ on the system lengthLz, the imposed reconnection rateR0, and the resistivity η. The ratiol/δ is found to increase with an increasing system lengthLz, an increasing imposed reconnection rateR0, and a decreasing resistivity η. Therefore the multiple X line reconnection process tends to occur when the system length is long, the reconnection rate is high, or the resistivi

ISSN:0148-0227    

DOI:10.1029/JA091iA06p06807

年代:1986

数据来源: WILEY

摘要:

The stability of a finite amplitude circularly polarized Alfvén wave of wave numberk0is studied by using the two‐fluid isentropic equations. Linear perturbation analysis, involving two sideband transverse waves having wave numbersk0±kand a longitudinal wave with wave numberk, is used to find the exact sixth‐order dispersion relation. The analysis is then limited to the case wherek≪k0. The resulting fourth‐order dispersion relation is examined analytically and numerically, and a surface is found that separates stable and unstable regions in parameter space. This surface describes the boundary between stable and unstable regions not only fork≪k0but for the entire branch of the dispersion relation which extends tok= 0. We refer to this branch as the modulation branch and the corresponding instability as a modulation instability. A sufficient condition for modulation stability is found to be υϕ0csfor right‐hand polarized waves, where υϕ0andcsare the phase velocity of the unperturbed wave and the unperturbed sound speed, respectively. Modulation wave amplitudes and g

ISSN:0148-0227    

DOI:10.1029/JA091iA06p06816

年代:1986

数据来源: WILEY

摘要:

The linear and nonlinear behavior of the right‐hand resonant and nonresonant electromagnetic ion beam instabilities driven by cool, β ∼ 1 ion beams is investigated as a function of beam mass and density. When the heavy ion beam is the less dense component, the maximum growth rate of the resonant mode and the threshold velocity of the nonresonant mode decrease with beam mass, while the maximum growth rate of the nonresonant mode and the threshold velocity of the resonant mode are nearly independent of the mass of the beam. When the heavy ion beam is the more dense component, a reversal of roles occurs, with the properties of the instabilities being determined by the sparser core ions. The nonlinear properties obtained from simulation show a similar behavior. At low beam densities the magnetic field fluctuation level increases with beam mass; at higher beam density it depends on the core mass. Very large magnetic fluctuations (δB/B0∼ 3), and smaller but still sizeable density fluctuations that do not correlate with the magnetic fluctuations, are observed and shown to agree well with those derived from theory. Applications to comets and Venus are su

ISSN:0148-0227    

DOI:10.1029/JA091iA06p06825

年代:1986

数据来源: WILEY

摘要:

Linear theory analysis and particle simulation studies were carried out in an attempt to further our understanding of the mechanisms generating broadband electrostatic noise and the effects of BEN on particles in the geomagnetic tail region. The plasma sheet particle population was modeled by counter‐streaming ion beams as well as by warm ions and electrons. When the beam temperature,Tb, is much smaller than that of the warm plasma sheet electrons,Te, both beam‐ion acoustics and ion‐ion two‐stream instabilities grow. The frequency of the ion acoustic instability is given by ω = ±(k(nb/ne)1/2cs−k∥U) ≃ ±k∥U≤ ωpewherecs= (Te/mi)1/2is the ion sound speed andUis the ion beam speed relative to the warm plasma sheet electrons. When the beam ion temperature is comparable to the temperature of the warm plasma sheet particles, electrostatic ion cyclotron instabilities become unstable, giving rise to low frequency noise at ω ≲nΩiwhere Ωiis the ion gyrofrequency. The simulation results confirm the presence of broadband electrostatic noise extending from ω = 0 to ω =k · U≲ ωpe, with an amplitude ofE∼ mV/m, which is in good agreement with the satellite data. The observed wave intensity also peaks near perpendicular propagation, which is also in

ISSN:0148-0227    

DOI:10.1029/JA091iA06p06833

年代:1986

数据来源: WILEY