摘要:

Voyager 1 and 2 magnetic field and plasma data are presented which demonstrate the existence of large scale, corotating, nonlinear pressure waves between 2 AU and 4 AU that are not accompanied by fast streams. The pressure waves are presumed to be generated by the interaction between fast corotating streams and slower flows near the sun. For two of the three pressure waves that are discussed, the absence of a relatively fast stream is probably a real, physical effect, viz, a consequence of deceleration of the stream and acceleration of the ambient flow by the associated compression wave. For the third pressure wave, the apparent absence of a stream may be a geometrical effect; it is likely that the stream was at latitudes just above those of the spacecraft, while the associated shocks and compression wave extended over a broader range of latitudes so that they could be observed by the spacecraft. It is suggested that the development of large‐scale, nonlinear pressure waves at the expense of part of the kinetic energy of streams produces a qualitative change in the solar wind in the outer heliosphere. Within a few astronomical units the quasi‐stationary solar wind structure is determined by corotating streams whose structure is determined by the boundary conditions near the sun. Beyond several astronomical units there is a zone in which the solar wind structure is determined by nonlinear pressure waves without streams, in which memory of the source conditions has largely been erased. Far from the sun (≳25 AU), these pressure waves should interact extensively with one another producing a zone which is more homogenous on a large scale yet more disordered on a smaller scale, where a statistical description may be more appropriate than deterministic models. This new view of heliospheric structure should provide a better foundation on which to interpret retrospectively prior observations and to analyze future data with respect to basic physical proc

ISSN:0148-0227    

DOI:10.1029/JA088iA08p06085

年代:1983

数据来源: WILEY

摘要:

The interaction of circularly‐polarized Alfvén waves with the surrounding plasma in high speed solar wind streams is investigated. Alfvén wave modulational instability is discussed and nonlinear envelope soliton solutions of the magnetohydrodynamic equations are introduced. The characteristics of these Alfvén solitons are compared with observational results obtained from Helios I and II. A model of the expected turbulent spectrum due to a collection of such solitons is constructed and its radial dependence is investigated, again along with comparison to Helios

ISSN:0148-0227    

DOI:10.1029/JA088iA08p06095

年代:1983

数据来源: WILEY

摘要:

The sharp, 90‐km wide transition from an optical depth of 0.2 in the C ring to 1 in the B ring begins at 91,970 km from Saturn's center. This radius is found to be almost exactly at the inward stability limit of charged particles launched in the ring plane at the local Kepler velocity, provided these particles have large charge to mass ratio. The zonal harmonic models of Saturn's magnetic field from the Voyager data and the gravitational field model from Pioneer data are essential to get the very close agreement between theory and observation. The theoretical stability limits are 91,973±145 km from Voyager 1 magnetic field data and 91,991±145 km from Voyager 2 magnetic data. The zonal harmonic magnetic field lines are not perpendicular to the ring plane. Therefore, in addition to the magnetic mirror, gravitational, and centrigual forces, an unknown force must be postulated to produce equilibrium in the ring plane and make the stability calculation meaning

ISSN:0148-0227    

DOI:10.1029/JA088iA08p06102

年代:1983

数据来源: WILEY

摘要:

A self‐consistent theory is presented for the excitation of hydromagnetic waves and the acceleration of ions upstream of interplanetary traveling shocks. The waves are excited by the ions by virtue of ion streaming relative to the solar wind; the ions are accelerated by being coupled to the compression of the shock via pitch angle scattering on the upstream waves and the downstream turbulence. Diffusion equations describing the ion transport and wave kinetic equations describing the hydromagnetic wave transport are solved self‐consistently to yield analytical expressions for the differential wave intensity spectrum as a function of wave numberkand distancezupstream of the shock and for the ion omnidirectional distribution functions and anisotropies as functions of energyEandz. In quantitative agreement with observations the theory predicts, for example, (1) power law ion spectra at the shock αE−rwith 2 ≲ Γ ≲ 3, (2) a decrease in intensity and hardening of the ion spectra with increasingz, (3) upstream ion anisotropies (∼0.3 fo 30‐keV protons) away from the shock front in the frame of the solar wind, (4) an unpolarized enhanced wave intensity spectrum in the wave number range corresponding to 0.4–3×10−2Hz in the spacecraft frame, and (5) a decrease in the wave intensity spect

ISSN:0148-0227    

DOI:10.1029/JA088iA08p06109

年代:1983

数据来源: WILEY

摘要:

Ion velocity space distributions measured with Los Alamos/Garching instruments on ISEE 1 and 2 across the earth's bow shock have revealed that the nearly perpendicular shock reflects some fraction of the incident solar wind ions over a wide range of Mach numbers (MA≃ 2.0–12.4). The relative density of the reflected ions varies from 1–3% of the solar wind density atMA≃ 2 to 15–25% atMA= 8–12, in qualitative agreement with computer simulations and with predictions that indicate that such ions are needed to provide essential dissipation at the shock. These ions gyrate aboutBin the plasma rest frame with a speed of ∼2 υin, where υinis the normal component of the solar wind flow, and their density increases by ∼2 orders of magnitude at the forward end of the ’foot‘ of the shock profile ∼0.7 ion gyroradii upstream from the shock ramp. This is substantial evidence that the bulk of these ions have been reflected at the shock in a nearly specular fashion. Depending on the shock geometry, some low numbers of reflected‐gyrating ions are sometimes observed a few gyroradii ahead of the outer edge of the foot. Model calculations suggest that these additional ions are produced by non‐specular reflection of a much smaller fraction of the incident solar wind. Downstream of the shock ramp, the secondary ions gyrate with a slightly higher speed, υg≃ 2.15 υin, in the downstream rest frame. Individual groups of gyrating ions rapidly disperse in velocity space to form a high‐energy torus centered at the bulk velocity of the distributions. Downstream ion toruses are most clearly seen, sometimes persisting for many minutes, when the shock Mach number is low. Downstream of highly supercritical shocks (M≫ 3), the distributions are more turbulent. In either case, the distributions retain some non‐Maxwellian features for a long time downstream from the ramp, in accordance with earlier observations. Reflected‐gyrating ions have also been identified in a number of crossings with more oblique field orientations (θnB≳ 45°, i.e., for quasi‐perpendicular shocks). Specularly reflected ions play an important role in producing down‐stream ion thermalization for all θnB≳ 45°, being rela

ISSN:0148-0227    

DOI:10.1029/JA088iA08p06121

年代:1983

数据来源: WILEY

摘要:

The possibility that the partial dropout of the 10‐MeV electrons observed at Voyager 1 encounter with Io might be explained by a forbidden zone is investigated. Because the convection velocity in the vicinity of Io decreases, gradBdrifts of energetic electrons opposing the convection result in a forbidden region around Io. It is found that the location and magnitude of the 10‐MeV electron depletion calculated on this basis agrees well with the location found by mapping along draped field lines from the Voyager trajectory to the location of Io. The depletion of 10‐MeV electrons thus should not be construed as evidence for a local dipole magnetic field at Io. If the forbidden zone explanation for the 10‐MeV dropout observed by Voyager is correct, then no electrons in this energy range would be lost to the surface of Io and there should not be a wake extending behind Io in which electrons of energies 10 MeV are depleted. Such a depletion of electrons near 9 MeV was observed by Pioneer, an apparent discrepancy. If more of the Jovian convection electric field had penetrated onto Io connected field (higher ratio of Alfvén wave conductance to Io conductance) during the Pioneer period than during the Voyager encounter, particle losses observed by Pioneer could be explained by precipitation at Io. Losses for both energetic electrons and protons at Io are calculated for different amounts of electric field shieldi

ISSN:0148-0227    

DOI:10.1029/JA088iA08p06137

年代:1983

数据来源: WILEY

摘要:

Voyager ultraviolet spectrometer (UVS) measurements provided the first unassailable evidence for particle precipitation in the Jovian atmosphere. Strong Lyman and Werner band emissions at high latitudes indicate particle precipitation energy fluxes of about 10 ergs cm−2s−1. On the other hand dayglow Lyman and Werner emissions at mid‐ and low‐latitudes may indicate additional particle precipitation fluxes on the order of 0.3 ergs cm−2s−1at all latitudes. Such particle precipitation can have significant aeronomical effects on the Jovian thermsophere and ionosphere. A one‐dimensional theoretical model is used to study these effects for the case of electron precipitation, although ion precipitation produces similar effects. Diffusion equations are solved for all the major neutral species and for H+, and photochemical solutions are given for the short lived ions. These neutral and ionospheric components of the model are coupled with the electron and ion energy equations and a two‐stream electron transport code that calculates the energy deposition of precipitating electrons (considered to be the precipitating particles) and photoelectrons. An independent calculation of the vertical neutral temperature is also obtained. The results of the model calculations can be broadly categorized as effects of electron precipitation (1) on the neutral composition and temperature of the thermosphere, and (2) on the composition and structure of the ionosphere. Auroral electron precipitation by 10‐keV electrons with a total energy flux of 10 ergs cm−2s−1produces 4.7×1011H atoms cm−2s−1and 5 ergs cm−2s−1of heat, over 2 orders of magnitude larger than solar EUV processes that produce 3.3×109H atoms cm−2s−1and 0.03 ergs cm−2s−1of heat. Thus, Jovian auroral H production coupled with aurorally driven meridional winds in the thermosphere can possibly explain the high concentration of atomic hydrogen in the low‐latitude Jovian upper atmosphere. Furthermore, aurorally produced changes in composition can create important feedback which affects the relative airglow efficiencies and heating rates in the high‐latitude thermosphere. In addition, ionization and vibrational heating of H2from precipitation processes appear to play a central role in determining the structure of the high‐latitude ionosphere. Theoretical fits to the Voyager radio occultation electron density profiles at high latitudes suggest a 10‐keV electron aurora with an energy flux of 10 ergs cm−2s−1coupled with a height‐dependent H2vibrational temp

ISSN:0148-0227    

DOI:10.1029/JA088iA08p06143

年代:1983

数据来源: WILEY

摘要:

New evidence supporting the widely accepted concept that certain of the Jovian decametric radio waves originate as northern hemisphere extraordinary mode cyclotron emissions (possibly from the Io flux tube) has been found in the Voyager radio observations. Shortly after the closest approach to Jupiter, the wave signals received by Voyager 1 near 10 MHz exhibited cusps in the fringe pattern which is attributed to Faraday rotation in the Io plasma torus. The diminished Faraday rotation indicated by these cusps implies that the wave path had become perpendicular to the magnetic field in the torus. At nearly the same time, the wave polarization near 1 MHz (where Faraday rotation is absent) exhibited a sudden reversal of its rotation sense, indicating that the wave path for those frequencies had also become perpendicular to the magnetic field at the spacecraft. For these two events it was possible to project the wave paths back toward Jupiter and determine to some extent where the waves originated. It was found that the waves came from the northern hemisphere, at progressively lower altitudes with increasing frequency, and if the source is assumed to be associated with anL= 6 field line, the emission seems to have occurred near the source cyclotron frequency somewhere in the local midnight sector. This suggests that the source could be at the Io flux tube, since Io was then also near local midnight. Since the polarization rotation sense was apparently right handed with respect to the source magnetic field, the emitted wave mode must have been extraordinary. Furthermore, in order for the torus to produce a detectable Faraday rotation, the emitted wave polarization must have been substantially noncircular, and that would require a low plasma density near the source, much like that which occurs with auroral kilometric radiation at the earth.

ISSN:0148-0227    

DOI:10.1029/JA088iA08p06165

年代:1983

数据来源: WILEY

摘要:

The relevant parameters of the magnetospheres of Jupiter and earth are studied from the point of view of wave‐particle resonant interactions that are believed to be responsible for the generation of VLF chorus emissions observed on Voyager‐1. Using existing models of the cold and energetic plasma distributions in the Jovian magnetosphere, expressions for the wave‐particle interaction length (LI) and the nonlinearity parameter (ρ) are derived. Values of these parameters are compared with those computed for the earth's magnetosphere. It is found that the typical interaction lengths are at least 2–5 times larger in the Jovian than in the terrestrial magnetosphere. Also, the wave intensity necessary to reach the threshold of nonlinearity in the Jovian magnetosphere was found to be up to 5–100 times lower. The Voyager 1 measurements show, however, that the inferred wave magnetic field intensities of the Jovian chorus are in the range of reported intensities for terrestrial chorus. This is attributed to fact that the fluxes of few keV resonant particles found in the Jovian magnetosphere were typically two orders of magnitude higher. In this case, it is predicted that the temporal growth rates of Jovian chorus bursts should be higher than for the earth. Growth rate measurements on Voyager 1 broadband wave data are used to confirm this

ISSN:0148-0227    

DOI:10.1029/JA088iA08p06171

年代:1983

数据来源: WILEY

摘要:

Gyroresonance and Landau resonance interactions between unducted low‐frequency whistler waves and trapped electrons in the earth's plasmasphere have been studied. Ray paths for waves launched near the plasmapause have been traced. In agreement with recent findings by Thorne et al. (1979), we have found waves which return through the equatorial zone with field‐aligned wave normal angles. However, when the growth along the ray path is calculated for such waves, assuming an electron distribution function of the formE−nsinmα, we find that for all the waves considered, the local growth rate becomes negative before plasmapause reflection, limiting the total gain to small values. Most waves reach zero gain before reflection. This is the result of Landau damping at oblique propagation angles, which necessarily occurs before reflection can take place. We conclude that the concept of cyclic ray paths does not provide an explanation for the generation of unguided plasmaspheri

ISSN:0148-0227    

DOI:10.1029/JA088iA08p06181

年代:1983

数据来源: WILEY