摘要:

Cosmic ray transport theories that incorporate the effect of adiabatic focusing into the usual pitch angle scattering formalism suggest that the commonly assumed first‐order (cosine of pitch angle) anisotropy should be replaced by an exponential function in which the scattering mean free path λ and the indexq, which characterizes the dependence of the scattering rate upon pitch angle, appear as parameters. The solar cosmic ray event of February 16, 1984, which exhibited an unusually large, persistent anisotropy, provides an ideal observational basis for testing this theoretical prediction. Analysis of energetic particle data from the International Cometary Explorer and from eight high‐latitude neutron monitors indicates that an exponential with λ>2 AU and withqin the range 1.3–1.7 does indeed provide a significantly better description of particle anisotropies than the first‐order form. Because the local interplanetary mean free path is determined purely from the form of the angular distribution, that is, without reference to the shape of the temporal density profile, the technique utilized here allows the relative influences of coronal versus interplanetary transport processes to be more cleanly separated. For the February 16 event the time profile was shaped primarily by coronal processes. Specifically, coronal diffusion with a rigidity‐independent diffusion coefficient ∼1018cm² s−1and with velocity‐dependent escape provides a good fit to both the ICE and the ne

ISSN:0148-0227    

DOI:10.1029/JA091iA08p08713

年代:1986

数据来源: WILEY

摘要:

The characteristics of directional discontinuities (DDs) in the interplanetary magnetic field have been studied using data from the Mariner 10 primary mission. The entire data set was surveyed using an automated procedure to identify DDs as changes in field direction of at least 30° in a 42‐s interval. This study yielded anr−1.3±0.4heliographic distance dependence for the daily average number of discontinuities per hour. In addition to this statistical survey, DDs were visually identified using 1.2‐s averages for three selected time intervals, and the corresponding 40‐ms data were studied in detail by means of the Sonnerup‐Cahill variance procedure. The three time intervals were November 4–17, 1973, February 5–12 and April 3–10, 1974, corresponding to the equally spaced heliocentric distances of 1.0, 0.72, and 0.46 AU. Editing the candidate DDs produced a total of 644 events. Two methods were used to estimate the ratio of the number of tangential discontinuities (TDs) to the number of rotational discontinuities (RDs). In the first approach those DDs with substantial normal components (Bn/〈B〉 ≥ 0.3) were interpreted as RDs, and the remainder were considered to be TDs, except that some RDs were eliminated on the basis of unacceptably large relative magnitude variances across their discontinuity zone. The second method considered the total number of RDs to be the sum of those DDs with substantial normal components (again excluding those with large variances) plus an estimated number of those DDs with small normal components. The estimate was based on the assumption that there is a uniform distribution of RDs per degree of discontinuity cone angle β (=cos−1|Bn|/B, whereBis the average magnitude of the field across the DD) for all β. Then all other DDs are assumed to be TDs. Both methods showed that the ratio of TDs to RDs decreased with decreasing radial distance, being 1.5, 1.2, and 0.86 for the distances 1.0, 0.72, and 0.46 AU, respectively, from the first method and 0.89, 0.66, and 0.43, respectively, from the second (preferred) method. A decrease in average discontinuity thickness of 41% was found between 1.0 and 0.72 AU and 56% between 1.0 and 0.46 AU, independent of type (TD or RD). This decrease in thickness for decreasingris in qualitative agreement with Pioneer 10 observations between 1 and 5 AU. When the individual DD thicknesses were normalized with respect to the estimated local proton gyroradius (RL), the average thickness at the three locations given above was nearly constant, 36±5RL. This held true for both RDs and TDs separately. Statistical distributions of these and other properties, such as normal directions and discontinuity plane angles (ω), are presented. No obvious relationship was found between ω and the thickness of either TDs or RDs when widely sep

ISSN:0148-0227    

DOI:10.1029/JA091iA08p08725

年代:1986

数据来源: WILEY

摘要:

A qualitative model of substorm processes in the Mercury magnetosphere is presented based on Mariner 10 observations obtained in 1974–1975. The model is predicated on close analogies observed with the terrestrial case. Particular emphasis is given to energetic particle phenomena as observed by Mariner on March 29, 1974. We support the suggestion that energetic particles up to ∼500 keV are produced by strong induced electric fields at 3 ∼ 6RMin the Hermean tail in association with substorm neutral line formation. The bursts of energetic particles produced are, in this model, subsequently confined on closed field lines near Mercury and drift adiabatically on quasi‐trapped orbits for many tens of seconds. Such gradient and curvature drift of the particles can explain prominent periodicities of 5–10 s seen in the Mariner>170‐keV electron fl

ISSN:0148-0227    

DOI:10.1029/JA091iA08p08742

年代:1986

数据来源: WILEY

摘要:

The inner satellites of Saturn are icy bodies imbedded in a plasma environment in which they are continuously bombarded by energetic ions, corotating plasma, and solar radiation. Laboratory sputtering experiments indicate that this should result in the injection of substantial amounts of neutral H, H2, OH, H2O, and O2into the magnetosphere. We model the atomic processes affecting these neutrals and the neutrals and ions formed from them and calculate the steady state neutral and ion densities expected in the plasma tori of Enceladus, Dione‐Tethys, and Rhea. Comparison with observations shows that recombination can limit the Enceladus and Dione‐Tethys tori to the observed densities but that transport rates of at least 4×10−8Rs²/s are required to limit torus densities at Rhea to the observed

ISSN:0148-0227    

DOI:10.1029/JA091iA08p08749

年代:1986

数据来源: WILEY

摘要:

Extreme ultraviolet observations of Saturn from Voyager 1 and 2 are analyzed. The Lyman alpha and H2band emissions extend throughout the upper atmosphere, from the hydrocarbon homopause to well above the exobase. Analysis of the Lyman alpha emissions with a radiative transfer model indicates that the Lyman alpha source temperature is very high. This suggests that energetic protons or hydrogen atoms are responsible for a fraction of the emissions. Calculation of the solar‐scattered component of the emissions based on the neutral atmosphere of Smith et al. (1983) reveals that only 1–2 kR out of a total of 3.5 kR of the observed Lyman alpha intensity is due to solar scatter for the Voyager 2 disc observations; the remainder of the Lyman alpha emissions are collisionally excited. The Lyman alpha and H2bands are constant in longitude but decrease in local time by a factor of 2 from dawn to dusk. This correlation of the Lyman alpha and H2band intensities is further evidence that most of the Lyman alpha is collisionally exci

ISSN:0148-0227    

DOI:10.1029/JA091iA08p08756

年代:1986

数据来源: WILEY

摘要:

We present four years of observations of the disk‐averaged H Ly α emission from Uranus performed with the IUE Observatory. A detailed analysis of the uncertainties of these measurements is discussed, based both on known calibration uncertainties and on a new analysis of the uncertainty in our customized extraction procedure. On the basis of roughly 30 observations we derive an average brightness of 1400 Rayleighs. The larger data base now available has allowed us to perform a more detailed analysis of the character of this emission and its functional relationship with other parameters. The observed extent and time scales of the variability of the emission are presented, and no evidence for correlation with the solar H Ly α variations is found, implying a largely self‐excited emission. Limits are derived from the minimum observed brightness and from a modeling of the atmosphere of Uranus for the possible contribution by reflected solar H Ly α emission, which we estimate to be roughly 200 Rayleighs. We therefore interpret the remaining self‐excited emission as being produced by charged particle excitation, i.e., an aurora. Studies of possible correlations between the self‐excited component of the H Ly α emission and the density and velocity of the local solar wind are presented, based on comparisons with solar wind measurements performed in the vicinity of Uranus from the Voyager 2 and Pioneer 11 spacecraft. No evidence is found for any correlation between the solar wind density and the H Ly α brightness. We estimate an upper limit to the energy of the precipitating particles based on the lack of observed H2band emission (which sets a lower limit to the ratio H Ly α/H2) and by analogy to the auroral precipitation on Jupiter. Finally, an estimate of the total power in the precipitating particles is on the order of 1012watts (comparable to the aurora on Saturn), and the disturbance of the upper atmosphere by the deposited energy

ISSN:0148-0227    

DOI:10.1029/JA091iA08p08771

年代:1986

数据来源: WILEY

摘要:

We have analyzed the stability of a contact surface located at the ram pressure balance position in a comet coma ionosphere in the presence of collisions between ions and neutrals and finite compressibility. We find that if the surface is taken to be a tangential discontinuity which excludes the interplanetary magnetic field from the ionosphere, this ionopause will be violently unstable to MHD modes. We therefore expect the large‐scale magnetic field to permeate the entire coma, even during periods of quiet solar activity. As a result, a stable cometary ionopause may form at a much greater distance from the nucleu

ISSN:0148-0227    

DOI:10.1029/JA091iA08p08782

年代:1986

数据来源: WILEY

摘要:

Numerical simulations of electromagnetic instabilities, driven by a cool tenuous ion beam propagating along an ambient magnetic field, have been conducted in one and two spatial dimensions. The calculations employ particle ions, fluid electrons, and a predictor corrector scheme for solving the electromagnetic field in two dimensions that is described in some detail. While the principal features of the one‐dimensional calculations (which reproduce previous work) are retained, the two‐dimensional simulations show some reduction of the overall level of the magnetic field fluctuations. Enhancement of the heating of the beam ions at the expense of the core ions also occurs in the case where the beam density is sufficiently large that the right‐hand nonresonant instability dominates. Implications of the results for modeling of the ion foreshock and quasi‐parallel shocks are di

ISSN:0148-0227    

DOI:10.1029/JA091iA08p08789

年代:1986

数据来源: WILEY

摘要:

We analyze the perturbation of the solar wind in the earth's foreshock. The foreshock is modeled as a planar magnetic flux tube having a 15REhalf width. Within the flux tube the upstream energetic particle pressure is assumed to fall monotonically to zero at the flux tube boundary and decline in the upstream direction with a scale length of 8RE. The incident solar wind is assumed to flow uniformly with a velocity of 400 km s−1, a density of 8 cm−3, a pressure of 50 eV cm−3, and a magnetic field of 4γ directed parallel to the flow. The solar wind density, velocity, and magnetic field within the foreshock are described by the steady state ideal MHD equations. We find that (1) the vector solar wind velocity perturbation rotates from the sunward to the transverse direction with increasing distance from the axis of the flux tube, (2) the peak solar wind deflection is located ∼3REwithin the flux tube boundary, (3) a central upstream pressure of 200 eV cm−3produces a maximum deceleration of 6 km s−1and a maximum deflection of 1.3°, (4) a central upstream pressure of 600 eV cm−3produces a maximum deceleration of 19 km s−1and a maximum deflection of 3.6°, and (5) the deflection and deceleration are accompanied by perturbations of the solar wind density and magnetic field. These perturbations are largest near the flux tube boundary where both form spikes having a width of ∼2RE. For a 600 eV cm−3central pressure those spikes have amplitudes of 2 cm−3and 1γ, respectively. We have analyzed the linearized flow problem analytically and reduced the solutions to quadrature. These solutions are found to be good approximations to the numerical nonlinear solutions for moderate values of the u

ISSN:0148-0227    

DOI:10.1029/JA091iA08p08798

年代:1986

数据来源: WILEY

摘要:

The structure of high Mach number perpendicular shocks is examined using a simulation code which treats the ions as macroparticles and the electrons as a resistive massless fluid. It is shown that stable, stationary shock solutions can be found for Alfvén Mach numbersMAbetween 5 and 60 for upstream plasmaβ= 1, whereβ is defined as the ratio of the plasma pressure to the magnetic pressure, provided that the upstream resistive diffusion length is much smaller than the ion inertial lengthc/ωpi. If the resistive diffusion length is larger than 0.4c/ωpi, then the magnetic field overshoot is damped, and an imbalance in the electron momentum equation results in a periodic fluctuation of the fraction of reflected ions. In the limitMA≤ 10, the magnetic field overshoot and the fraction of reflected ions increase with increasingMA, consistent with earlier results. At higher Mach numbers the fraction of reflected ions peaks at approximately 40% and the magnetic field overshoot increases at a much slower rate. These results are consistent with simple scaling laws derived from the electron momentum equation. The importance of upstreamβ and electron inertial effects are discussed.

ISSN:0148-0227    

DOI:10.1029/JA091iA08p08805

年代:1986

数据来源: WILEY