摘要:

Dust grains in the ring systems and rapidly rotating magnetospheres of the outer planets such as Jupiter and Saturn may be sufficiently charged that the magnetic and electric forces on them are comparable with the gravitational force. The adiabatic theory of charged particle motion has previously been applied to electrons and atomic size particles. But it is also applicable to these charged dust grains in the micrometer and smaller size range. We derive here the guiding center equation of motion, drift velocity, and parallel equation of motion for these grains in a rotating magnetosphere. The effects of periodic grain charge‐discharge have not been treated previously and have been included in this analysis. Grain charge is affected by the surrounding plasma properties and by the grain plasma velocity (among other factors), both of which may vary over the gyrocircle. The resulting charge‐discharge process at the gyrofrequency destroys the invariance of the magnetic moment and causes a grain to move radially. The magnetic moment may increase or decrease, depending on the gyrophase of the charge variation. If it decreases, the motion is always toward synchronous radius for an equatorial grain. But the orbit becomes circular before the grain reaches synchronous radius, a conclusion that follows from an exact constant of the motion. This circularization can be viewed as a consequence of the gradual reduction in the magnetic moment. This circularization also suggests that dust grains leaving Io could not reach the region of the Jovian ring, but several effects could change that conclusion. Excellent qualitative and quantitative agreement is obtained between adiabatic theory and detailed numerical orbit integrati

ISSN:0148-0227    

DOI:10.1029/JA088iA01p00001

年代:1983

数据来源: WILEY

摘要:

The MHD equilibrium of thin, rotating plasma disks is considered. An axisymmetric steady state is assumed. A corotation lag is included, as is a nonzero toroidal field withBф∝rBr. Rigorous expressions are obtained for a characteristic Alfvén Mach number and temperature, both of which involve vertical averages. If there is strict corotation, the temperature always diverges at large radii. If there is a corotation lag, the temperature need not diverge, and the magnetodisk solutions can extend to arbitrarily large radii. With weak additional assumptions, it is shown thatMA<21/2for a mainly poloidal field andMA<1 for a mainly toroidal field. For the Jovian magnetodisk,MA= 0.8 from Voyager 1 inbound data, and the observation of a temperature nearly constant or slowly decreasing withrfrom 20RJto 80RJon the outbound pass implies a corotation lag th

ISSN:0148-0227    

DOI:10.1029/JA088iA01p00013

年代:1983

数据来源: WILEY

摘要:

This study utilizes gas dynamic calculations in conjunction with observational bow shock models to investigate the solar wind flow patterns about the terrestrial planets. Average dayside bow shock position could be predicted for the earth by theory with an error of only ∼2%, given the observed shape and location of the magnetopause. Accordingly, our findings confirm the validity of the single‐fluid gas dynamic approximation for describing this major aspect of solar wind flow past the earth. Modeled using gas dynamic theory, the solar wind interactions with Venus and Mars exhibit very significant differences. At Mars the mean inferred altitude of the solar wind‐obstacle interface varies from 510 km at the stagnation point to almost 1000 km near the terminator. The effective magnetic moment required to produce a magnetosphere of this size for average solar wind dynamic pressures and terrestrial‐type internal current systems is 1.4 ± 0.6×1022G cm³. Gas dynamic modeling of the January 21, 1972, Mars 3 and July 20, 1976, Viking 1 lander particles and fields observations supports the conclusion that the Martian obstacle to the solar wind lies at altitudes too high for it to be associated with only an ionospheric or atmospheric interaction. In contrast with Mars, our modeling of the Venus observations has found that the bow wave is closer to the planet than would be expected for a purely ionospheric obstacle. The subsolar width of the Venus ionosheath in the Venera and PVO measurements is only 60% and 90%, respectively, of that predicted by the gas dynamic model. This result is attributed to the presence of solar wind‐neutral atmosphere interactions in the lower ionosheath that are not included in the gas

ISSN:0148-0227    

DOI:10.1029/JA088iA01p00019

年代:1983

数据来源: WILEY

摘要:

During its flyby of Jupiter in 1974, Pioneer 11 passed through a region in which low‐energy (∼1 MeV) trapped protons had been exposed to absorption by Amalthea (L≃ 2.5) only ∼5 hours earlier. In passing through this region, a brief (∼10 s duration) decrease was observed in the counting rate of ∼1‐MeV protons measured by the University of Chicago charged particle instrument. We interpret this decrease as a transient signature of the recent absorption of the proton flux by Amalthea. No such clear effects were observed in fluxes of electrons (E≥ 3.4 MeV) or high‐energy nuclei (Z>5,E⪞ 70 MeV/nucleon) at any crossing of Amalthea's orbit. These observations place an upper limit on the radial diffusion coefficient, κ, for ∼1‐MeV protons at the orbit of Amalthea of 2×10−10RJ²/s. By comparison with diffusion coefficients derived from absorption effects by Io (L≃ 6), this limit requires that if the radial diffusion coefficient betweenL≃ 2.5 andL≃ 6 varies withLasLn, thenn≥ 5, whereas diffusion driven by atmospheric neutral winds would predict κ ∝L³ for low‐energy protons. Our results are consistent with diffusion driven by fluctuating elect

ISSN:0148-0227    

DOI:10.1029/JA088iA01p00036

年代:1983

数据来源: WILEY

摘要:

Pioneer 10 and 11, during their encounters with Jupiter in 1973 and 1974, penetrated toLvalues of 2.8 and 1.7, respectively. During these encounters, at severalLvalues, decreases in the intensity of energetic trapped particles were observed, some of which could be explained as due to absorption by the known moons Io and Amalthea; however, some decreases insideL=4 could not be explained. The recent Voyager 1 and 2 optical discoveries of several new moons and a ring in this region has led us to reexamine our particle data, and we summarize results in this report. We report observations in three channels: protons 0.5–8.7 MeV; electrons>3.4 MeV; and medium‐Znuclei, probably oxygen and sulfur>70 MeV/nucleon. We find that with the additional moons and the ring, all observed intensity features in the stably trapped radiation are accounted for by satellite and ring absorpt

ISSN:0148-0227    

DOI:10.1029/JA088iA01p00045

年代:1983

数据来源: WILEY

摘要:

Titan and Venus are both unmagnetized bodies enveloped by induced magnetospheres created within the flowing plasmas in which they are embedded. When Saturn's magnetosphere extends beyond the position of Titan, as during the Voyager 1 flyby on November 12, 1980, the plasma incident at Titan contains a steady magnetic field approximately perpendicular to the plasma flow velocity. At Venus the incident plasma usually contains a time‐varying magnetic field, but occasionally, the interplanetary conditions remain steady for the few hours required for the Pioneer Venus orbiter to pass through the induced magnetosphere. In particular, on February 28, 1979, the interplanetary field appears to have been stable and transverse to the solar wind velocity during the wake pass which was selected for a comparative Venus‐Titan study. For both wakes, symmetry properties of the measured magnetic fields were used to infer the directions of ambient plasma flow. In each case the flow direction differed from nominal, with the flow at Titan aberrated about 27° radially inward of the corotation direction and the flow at Venus aberrated about 10° relative to the heliocentric radial direction. Corroborating evidence for the aberrated flows was provided by plasma observations. The wake field magnitudes were nonuniform, and in both cases, reduced magnitudes were found on field lines linked to the illuminated (dayside) ionospheres. Finally, both induced magnetotails contained regions in which the projection of the magnetic field onto the incident magnetic field was negative. This observation may indicate that in an induced magnetosphere the plasma flows toward the mid‐tail current sheet in the near wake, thus producing a field configuration that corresponds to a reconnection g

ISSN:0148-0227    

DOI:10.1029/JA088iA01p00049

年代:1983

数据来源: WILEY

摘要:

In this paper we examine the detailed magnetic structure of ropes and quantitatively compare it with a general flux rope model. First, the seemingly chaotic ionospheric magnetic signature is shown to be ordered in a local coordinate system unique to each rope. This signature is also consistent with that expected of a flux rope. We have selected a special subset of the flux rope data that provides detailed information on rope magnetic structure. This subset, referred to as the small impact parameter subset, consists of flux rope traversals on which the spacecraft passed through the center of the structure. We have formulated a model of flux rope structure and have fit it successfully to cases from the small impact parameter subset. From these modeled cases we have inferred that flux ropes are comprised of primarily field‐aligned currents with maximum current densities of a few tens of microamps/m² and that parallel electric fields of less than a microvolt/m can drive these currents given the background ionospheric conductivity. We have also found that Joule heating rates within flux ropes are much smaller than ambient photoelectron heating. These modeled cases also suggest that flux ropes are stable to pinch‐related instabilities like the sausage and planar kink modes, but are unstable to the long‐wavelength helical kin

ISSN:0148-0227    

DOI:10.1029/JA088iA01p00058

年代:1983

数据来源: WILEY

摘要:

Direct measurements of neutral CO2, O, CO, N2, He, and N densities from the Pioneer Venus Orbiter Neutral Mass Spectrometer are described in terms of a spherical harmonic representation (latitude and local time coordinates) of exospheric temperature and number densities at 150 km, using modified Bates temperature profiles. The exospheric temperatures are determined from the altitude variations of atomic oxygen. A global average temperature of 228°K is derived with a first harmonic variation of 57%. The altitude profiles are extended downward to 100 km by using empirical formulas to provide a transition through the turbopause region (simulating the effect of eddy diffusion and vertical flows) and matching entry probe density data. The model reflects the observed variations of temperature and density with the 10.7 cm radio flux index. For a given change in flux at the planet, the exospheric temperature on Venus changes by only 10% of the change seen in the terrestrial thermosphere

ISSN:0148-0227    

DOI:10.1029/JA088iA01p00073

年代:1983

数据来源: WILEY

摘要:

We present three examples of correlated observations of 35‐ to 1600‐keV upstream ions and low‐frequency waves measured on ISEE 3, located between ∼76 and ∼200REfrom the earth's bow shock. The observations show that when no upstream waves are present, the particle anisotropies are beamlike, aligned along the magnetic field after transformation into the drift velocity reference frame. When waves are present, the pitch angle distributions are broadened, the amount of broadening being correlated with the amount of wave activity. These waves are transverse, circularly polarized, and have a period of ∼30 s. From our observations we conclude that during long‐lasting events an equilibrium is reached where the region between the spacecraft and the bow shock contains a significant amount of low‐frequency wave activity, together with significant pitch angle scattering. The amount of scattering decreases with upstream distance, ranging from strong scattering at the bow shock to weak scatt

ISSN:0148-0227    

DOI:10.1029/JA088iA01p00085

年代:1983

数据来源: WILEY

摘要:

A survey of two‐dimensional electron velocity distributions, ƒ(V), measured near the earth's bow shock using Los Alamos/Garching plasma instrumentation aboard ISEE 2 is presented. This survey provides clues to the mechanisms of electron thermalization within the shock and the relaxation of both the upstream and downstream velocity distributions. First, near the foreshock boundary, fluxes of electrons having a power law shape at high energies backstream from the shock. Although most often they appear as a monotonically decreasing extension of solar wind distributions in the backward hemisphere along the magnetic field direction, , they occasionally appear as a resolved peak in energy. Within the interior of the foreshock, in addition to the hot, isotropic electrons at higher energies, field‐aligned depressions in ƒ(V) are observed at the lowest energies (E≲ 15 eV) and twin angular peaks centered on are observed at intermediate energies (15 eV ≲E≲ 45 eV). Such distributions are associated closely with 1‐Hz whistler waves. Second, within the shock, cuts through ƒ(V) along ƒ(V∥), often show single maxima offset toward the magnetosheath by speeds comparable to, but larger than, the upstream thermal speed. When sequences of such distributions are observed in a single shock transition, offset speeds increase and peak heights of ƒ(V∥) decrease with increasing penetration toward the downstream (magnetosheath) side. Third, magnetosheath distributions generally have flat tops out to an energy,E0, with maxima substantially lower than that in the solar wind. Occasionally, cuts through ƒ(V) along show one and sometimes two small peaks at the edge of the flat tops making them appear concave upward. The magnetosheath distributions often have strong angular anisotropies which depend on energy. For energies less thanE0, ƒ(V∥)>ƒ(V⊥) at constantE, whereas forE>E0, ƒ(V∥)<ƒ(V⊥). The electron distributions characteristic of these three regions are interpreted as arising from the effects of macroscopic (scale size comparable to or larger than the shock width) electric and magnetic fields and the subsequent effects of microscopic (scale size small in comparison with the shock width) fields. In particular, our results suggest that field‐aligned instabilities are likely to

ISSN:0148-0227    

DOI:10.1029/JA088iA01p00096

年代:1983

数据来源: WILEY