摘要:

Bidirectional, field‐aligned flows of low‐energy particles in interplanetary space have been proposed as a possible signature of large‐scale, looplike magnetic structures. We present the results of a survey of bidirectional anisotropy observations using data from the low‐energy proton and magnetometer experiments on board ISEE 3 covering a 45‐month period corresponding to the last solar maximum. During this period, 66 bidirectional flow (BDF) events have been identified, of which 48 could positively be associated with isolated magnetic field structures. We suggest that these structures are an interplanetary manifestation of coronal mass ejection (CME) events, and are able to group the BDF events into five classes according to the field signature of the related magnetic structure and the association or otherwise with an interplanetary shock. We conclude that the BDF events are an interplanetary signature not only of energetic, flare‐associated transients but also low energy, nonflare CMEs. From a comparison of anisotropy signatures at 35 and 620 keV we conclude that our observations are most consistent with the transient magnetic structures being detached bubbles comprising closed loops or tightly wound cylindrical helices rather than extended tonguelike loops attached to the Sun at the time of

ISSN:0148-0227    

DOI:10.1029/JA092iA10p11009

年代:1987

数据来源: WILEY

摘要:

The magnetic field and plasma data collected by the Voyager spacecraft between 1 and 11 AU are used to study the properties of interplanetary MHD fluctuations and to attempt to answer several related questions about the Alfvénicity of solar wind fluctuations: First, to what extent are magnetic and velocity fluctuations Alfvénic? Second, does the dominant propagation direction of Alfvénic fluctuations evolve with heliocentric distance? Third, is the presence of Alfvénic fluctuations correlated with large‐scale structures, such as stream interaction regions? In addition, we investigated the contributions of compressive modes to the interplanetary fluctuations. We find that near 1 AU at most 15% of the fluctuations at the scale of a few hours or less are purely Alfvénic and these usually propagate outward from the Sun. The propagation direction becomes more inward on average with increasing heliocentric distance. Although it is commonly supposed that compression regions are not generally Alfvénic, we found that the wave propagation direction is only slightly more mixed in compression regions than in the corresponding rarefaction regions. Moreover, the evolution in propagation direction is not directly due to the growth of large‐scale compression regions. There is a tendency for magnetic fluctuations to be larger than velocity fluctuations at scales less than a day, while the reverse is true, due to the dominance of stream energy, at larger scales. While smaller‐scale density fluctuations are uniformly small (≃ 0.1), they are usually negatively correlated with field magnitude variations even on time scales that preclude contributions from tangential discontinuities. In general the solar wind cannot be in a static state of superposed large‐amplitude waves. Fluctuations must be produced outside the Alfvénic critical point, perhaps generated by stream shear with subsequent no

ISSN:0148-0227    

DOI:10.1029/JA092iA10p11021

年代:1987

数据来源: WILEY

摘要:

Interstellar atoms penetrate the heliosphere, are ionized by solar UV radiation or charge exchange with solar wind ions, and are “picked up” by the solar wind onto a ring distribution in velocity space. The ring distribution is unstable to the generation of hydromagnetic waves with growth time scales which are small compared with those of the continual pickup process and convection with the solar wind. First, the growth rates for parallel propagation are derived, presented, and compared with previous work. Then the spatially homogeneous quasi‐linear equations describing the subsequent hydromagnetic wave excitation and pickup ion velocity diffusion in pitch angle and energy are presented under the assumption of wave propagation parallel to the ambient magnetic field. Neglecting ion energy changes, analytical expressions for the time‐asymptotic wave spectra accompanying the time‐asymptotic isotropic ion distribution are derived. The results indicate that pickup helium has a very small (unobservable) effect on the solar wind wave spectrum, but that pickup hydrogen results in substantial modifications at cyclotron resonant frequencies (∼10−2Hz at ∼7 AU). Finally, based on the previous expressions, the radial evolution of the pickup‐hydrogen‐modified wave spectra for both polarizations and propagation directions is computed analytically including the degradation of the wave power in the divergent solar wind. The predicted modifications beyond ∼5 AU are substantial and could be observable at spacecraft frequencies greater than ∼5 × 10−3Hz if not degraded by turbulent wave‐wave interactions or stochastic ion acceleration. The predicted enhancement by a factor of order 10 at ∼7 AU involves ∼10% of the power in the ambient field, is left‐polarized (Alfvénic as opposed to magnetosonic) in the solar wind frame, is unpolarized in the spacecraft frame, and includes an equal

ISSN:0148-0227    

DOI:10.1029/JA092iA10p11041

年代:1987

数据来源: WILEY

摘要:

Theoretical studies of waves and instabilities are often performed for an infinite, homogeneous plasma, and the results are given in terms of eigenmodes (plane waves) extending over all space. Real plasmas are, however, always finite and inhomogeneous, and observations of waves in space are normally presented as frequency power spectra which depend on the time and place of observation. The purpose of this study is to reconcile these apparently disparate descriptions of plasma waves. A model describing inhomogeneous spectral densities in a homogeneous plasma is first discussed in some detail. When this model is generalized to inhomogeneous plasmas, it is found that the incompressibility of the spectral densities inkspace results in a remarkably simple transfer equation. A method which allows the observable frequency power spectrum to be calculated from a space and time dependent dispersion relation and appropriate boundary conditions is also outlined.

ISSN:0148-0227    

DOI:10.1029/JA092iA10p11053

年代:1987

数据来源: WILEY

摘要:

As part of the Active Magnetospheric Particle Tracer Explorers (AMPTE) mission, releases of ∼1025atoms of lithium, and later barium, were made in the solar wind and magnetosheath. Initial photoionization produced a release plasma of sufficient densities that a diamagnetic cavity was seen to form at the release center, disrupting the ambient field and flow over a localized region (approximately tens of kilometers for Li, approximately hundreds of kilometers for Ba). Since the gyroradii of both the oncoming solar wind protons and the release ions were either greater than or of the order of the scale size of this perturbation while the electron gyroradii remained small, a hybrid description, where the ions are treated as particles, and the electrons as a charge‐neutralizing fluid, is appropriate. Here we present the results of one‐dimensional simulations for lithium using a hybrid description in order to investigate the process of momentum transfer between the two ion species that occurs locally over the boundary layer which forms between them. We find that a well‐defined field and plasma structure evolves in which the bulk of the lithium ions are moved en masse by a "snowplough" type process, those remaining either being accelerated to higher speeds to produce “taillike” structures or remaining upstream to form part of the snowplough momentum balance. Field and ion density structures found in the simulations are in good gross agreement with observations. We also obtain an analytical estimate of the snowplough speed which is in good agreement with that obtained computationally. When set in the context of the three‐dimensional release geometry, features resolved by our one‐dimensional simulation are found to have clear parallels in

ISSN:0148-0227    

DOI:10.1029/JA092iA10p11059

年代:1987

数据来源: WILEY

摘要:

We examine intense hydromagnetic waves at comet Giacobini‐Zinner to investigate the mode and direction of wave propagation and thereby provide important constraints on potential mechanisms for wave origin in the vicinity of the comet. The character of the wave polarization changes from basically elliptical far from the comet to almost linear followed by either a rapid partial rotation (less than 360°) or multiple rotations (high‐frequency wave packets) in the vicinity of the bow wave. Simultaneous high‐resolution measurements of electron density fluctuations demonstrate that the long‐period (∼100 s) waves are propagating in the magnetosonic (fast MHD) mode. Principal axis analyses of the long‐period waves and accompanying partial rotations show that the sum of the wave phase rotations is 360°. This indicates that both are parts of the same wave. The change in polarization characteristics near the comet is simply a consequence of wave steepening. From the time sequence of the steepened waveforms observed by ICE, we demonstrate that the waves must propagate in the general direction along the magnetic field toward the Sun with phase velocities less than the solar wind speed. They are consequently blown back across the spacecraft and observed with a left‐hand sense of rotation. All available observations are therefore consistent with wave generation by the right‐hand resonant ion ring instability which predicts waves propagating in the ion beam (solar) direction. Based on the Wu‐Davidson linear growth rate expression, the total convective gain is shown to exhibit a weak (logarithmic) dependence on distance from the comet reaching a maximum of 30e‐folding in the vicinity of the bow wave. Conditions necessary for the origin of multiple rotations currently are not well understood. Arguments for their being standing whistler waves consisting of a partial rotation plus multiples of 360° rotations are presented. Models of such emissions should be able to explain them as integral parts of the steepened magnetosonic waves. The large amplitudes, ΔB/|B| ∼O(1), and small‐scale sizes (rotational discontinuities) of the cometary waves suggest that rapid pitch angle scattering and energy transfer with energetic ions should occur. Since the waves are highly compressive, Δ|B|/|B| ≃O(0.5), one can also anticipate possible first‐order Fermi

ISSN:0148-0227    

DOI:10.1029/JA092iA10p11074

年代:1987

数据来源: WILEY

摘要:

Experiments on the Voyager 1 and 2 spacecraft and observations made by the International Ultraviolet Explorer (IUE) have provided evidence for the existence of energetic particle precipitation into the upper atmosphere of Jupiter from the magnetosphere. This auroral precipitation has been shown to generate large ionization and dissociation rates, to excite auroral emissions, and also to vibrationally excite molecular hydrogen. A theoretical model of vibrationally excited H2in the upper atmosphere of Jupiter is presented in this paper. Models are considered for both the auroral region and also for lower latitudes, where H2is vibrationally excited owing to processes associated with the absorption of solar ultraviolet radiation. The ground electronic state (X¹Σg+) of molecular hydrogen can be vibrationally excited in a variety of ways, such as by direct electron impact excitation, by dissociative recombination of H3+, and by excitation of the Lyman and Werner bands. Vibrationally excited H2can react with either H2or H via vibration‐translation (VT)interchange collisions, or via vibration‐vibration (VV) interchange collisions with other H2molecules. Reaction with H+ions is possible for vibrational levels with υ ≥ 4. Molecular and eddy diffusion of the vibrationally excited H2is also taken into account in these model calculations. The reaction of H+ions with vibrationally excited molecular hydrogen is shown to be a very important loss process for this ion and reduces the electron densities theoretically calculated for the auroral region by about a factor of 10. The theoretical ionosphere calculations are coupled with the vibrationally excited H2calculations in a self‐consist

ISSN:0148-0227    

DOI:10.1029/JA092iA10p11083

年代:1987

数据来源: WILEY

摘要:

We have reduced the general magnetostatic equilibrium problem for the geomagnetic tail to the solution of ordinary differential equations and ordinary integrals. The theory allows the integration of the self‐consistent magnetotail equilibrium field from the knowledge of four functions of two space variables,xandy: the neutral sheet locationzo(x,y), the total pressure , the magnetic field strengthBo(x,y), and thezcomponent of the magnetic fieldBzo(x,y)at the neutral sheet. The self‐consistency of the equilibrium problem thus reduces the effort to construct a tail model from empirical knowledge of four functions (Bx, By, Bz, andp) of three space coordinates to four functions of two coordinates only. All functions can be (and are in fact to a large extent) obtained from observations. The basic assumption of the quasi‐static theory with isotropic pressure is that spatial variations in thexandydirections can be treated as small in comparison with variations in thezdirection perpendicular to the plasma sheet. This assumption restricts the validity of the theory to average, quiet configurations of the interior tail excluding, in particular, regions of fast flow and strongygradients such as the plasma mantle and the low‐latitude boundary layer. The theory allows one to include the effects of field‐aligned currents and distortions due to an average cross‐tailByfield (in addition to an antisymmetricByof a flaring tail) not present in ear

ISSN:0148-0227    

DOI:10.1029/JA092iA10p11101

年代:1987

数据来源: WILEY

摘要:

Electric field measurements at oblique, subcritical bow shock crossings are presented from the ISEE 1 University of California, Berkeley, double‐probe electric field experiment. The measurements averaged over the 3‐s spin period of the spacecraft provide the first observations of the large‐scale (100 km) laminar oscillations in the longitudinal component of the electric field associated with the whistler precursor which is characteristic of these dispersive shocks. The amplitude of the oscillations increases from ∼0.5 mV/m to a maximum of 6 mV/m across the magnetic ramp of the shock (directed along the shock normal). The calculated electric potential drops across the shocks varied from 340 to 550 volts, which is 40–60% of the observed loss of kinetic energy associated with the bulk flow of the ions. These measurements suggest that at these shocks the additional deceleration of incident ions is due to the Lorentz force. The contributions to the normal component of the large‐scale electric field at the shock due to the parallel and perpendicular components (relative to the magnetic field) of the electric field are evaluated. It is shown that the perpendicular component of the electric field dominates, accounting for most of the cross‐shock potential, but that there is a nonnegligible parallel component. This large‐scale parallel component has a magnitude of 1–2 mV/m which sometimes results in a potential well for electrons with a depth of ≈150 eV. It is experimentally demonstrated that the dominance of the perpendicular over the parallel component of the electric field resulted in a correlation between the longitudinal component of the large‐scale electric field and the fluctuations in the magnetic field component perpendicular to the coplanarity plane. This paper also presents, for the first time, high time resolution (dc to 32 Hz) measurements of the electric field waveform in the current layer of a collisionless shock. The measurements in this region reveal the presence of intense electric field spikes with amplitudes ranging up to 100 mV/m lasting about 0.1 s superimposed on the large‐scale laminar electric field wave train described earlier. The period of these spikes is consistent with lower hybrid or ion acoustic waves which have been strongly Doppler shifted as a consequence of the solar wind motion. These electric field structures can have significant components along the shock normal and may play a variety of roles in the microphysi

ISSN:0148-0227    

DOI:10.1029/JA092iA10p11109

年代:1987

数据来源: WILEY

摘要:

Coincident ground‐based and satellite observations are presented of a premidnight auroral surge over Amundsen‐Scott South Pole station. The set of near‐simultaneous measurements provides an excellent opportunity to gain a more quantitative understanding of the nature of premidnight substorm activity at high geomagnetic latitudes. The surge produced a rapid onset of cosmic radio noise absorption at the station. On the polar‐orbiting DMSP F6 spacecraft, intense X ray emissions with E>2 keV energy were imaged 1° to 2° magnetically equatorward of South Pole approximately 1 min prior to the peak of the absorption event. The spectrum of precipitating electrons determined from the X ray measurements could be characterized by an e‐folding energy of ∼11 keV and is found to be adequate to account for the cosmic noise absorption and maximum auroral luminosity recorded at South Pole. Photometer, all‐sky camera, riometer, and magnetometer data are used to estimate the velocity of motion and spatial extent of the auroral precipitation and the ionospheric currents associated with the surge. The electron precipitation region is deduced to have a latitudinal scale size of2 keV) exceeded 200 ergs/cm² s and contributed to a vertical current density of ∼0.017 A/cm. This current density is comparable, to within a factor of ∼2, with the horizontal ionospheric current density (∼0.028 A/cm) inferred from the ground‐based magnetometer measurements at South Pole station. A contribution of a large flux of electrons with E<2 keV to the vertical current density is discounted as an explanation for the difference on the basis that the expected 630‐nm auroral luminosity would exceed by about an order of magnitude the less than 1 kR luminosity tha

ISSN:0148-0227    

DOI:10.1029/JA092iA10p11123

年代:1987

数据来源: WILEY