摘要:

An approximately 7‐month solar cycle has been observed in the photoelectron current measured by the Langmuir probe on Pioneer Venus Orbiter over the time period from 1979 through 1987. The probe photoelectron current,Ipe, is obtained when the spacecraft is outside the Venus ionosphere, and the measured current is due to photoelectron emission caused by EUV solar radiation in the wavelength range from about 30 nm to 121.6 nm. About one half of theIpeis due to solar Lyman alpha emission. TheIpedata from mid‐1984 through 1987 are dominated by a 7‐month or 216‐day cycle. Spectral analysis of the 1980–1988 data shows that this cycle dominates the spectrum for periods less than 300 days; the second most dominant cycle is at 155 days, a 5‐month period. TheIpedata were spectrally analyzed with three different methods. The 2800‐MHz radio flux, observed from Earth over the same time range, exhibits a similar 7‐month cycle at about 234 days which is stronger than a 5‐month (158‐day) cycle. Solar Mesosphere Explorer (SME) Lyman alpha observations for the time period mid‐1981 through mid‐1988 also have cycles near 5 and 7 months. Since the 7‐month and 5‐month cycles are observed from Venus (Ipe) and from Earth (2800‐MHz radio flux and SME Lyman alpha), the observations suggest that they are intrinsic frequencies, independent of the orbital frequency of the observer. The 7‐ and 5‐month cycles are believed to be caused by long‐lived flux enhancements from nonlinear interactions of global oscillation modes in the Sun's convective envelope (rmod

ISSN:0148-0227    

DOI:10.1029/JA094iA07p08663

年代:1989

数据来源: WILEY

摘要:

Numerical models of corotating solar wind flows have enjoyed considerable success in simulating the evolution of shocks and corotating interaction regions (CIRs) in the region beyond 1 AU, but their performance with respect to stream fronts located nearer the Sun has been somewhat disappointing. In particular, they tend to predict erroneously that corotating shock pairs should occur relatively frequently within 1 AU, given the sort of sharp boundaries between slow and fast flows observed at stream fronts near 0.3 AU by Helios. We use an existing two‐dimensional MHD numerical model for corotating flow in the supersonic, superalfvenic solar wind to show that the predictions of premature shock pair formation are due to improper specification of flow conditions on the initial surface (inner boundary) used as the starting point in such models. This faulty initialization leads to the generation of a physically extraneous strong compression along the stream interface just outside the initial surface, which results in the appearance of evolutionary artifacts (like spurious discontinuities) further on in the solution. We describe an initialization scheme incorporating flow conditions more appropriate to stream fronts near the Sun and demonstrate that it produces the smooth initial behavior expected on physical grounds. Thus free of the evolutionary artifacts, we see that the shear flow at the stream interface approximately balances the kinematic steepening near the Sun, which for typical input conditions keeps the corotating shock pair from forming before about 1.5 AU. We describe the criteria for shock formation in terms of the interface dynamics and show that the steepening process cannot be treated even approximately with conventional kinematic techniques. In a subsequent paper we investigate how the three‐dimensional geometry of the stream front affects the dynamical evolution and the resulting CIR struct

ISSN:0148-0227    

DOI:10.1029/JA094iA07p08673

年代:1989

数据来源: WILEY

摘要:

The time evolution of the distribution function of newborn ions in the solar wind is investigated within the context of a quasi‐linear theory in which the level of intrinsic turbulence is assumed to be moderate and known. The initial distribution is taken to be a ring beam, which is approximated by delta functions in pitch angle and velocity, and it is assumed that the ions are created at a constant rate with a similar distribution. The long‐time asymptotic form of the distribution is obtained. This distribution is a mixture of ions created recently and ions generated throughout the entire process. The results obtained in the present analysis are found to be in good agreement with recent satellite observations. The time asymptotic distribution is also found to be unstable to low‐frequency hydromagnetic waves propagating parallel to the ambient magnetic

ISSN:0148-0227    

DOI:10.1029/JA094iA07p08685

年代:1989

数据来源: WILEY

摘要:

We investigate a stationary model of a rotating, axially symmetric “pole‐on” magnetosphere in MHD force balance. In this model both the planet's rotational and dipole axes are aligned with the magnetotail axis, which is the axis of symmetry in a cylindrical (r, ϕ,z) coordinate system. For mathematical convenience the magnetosphere is confined on the nightside by a cylindrical magnetopause with constant radius. On the sunward side, the magnetosphere is closed by an appropriate image dipole. Inside the magnetospheric cavity we assume isotropic thermal plasma pressure. We assume further that, in general, planetary rotation leads to differentially rotating magnetotail field lines causing field‐aligned Birkeland currents and a corresponding toroidal magneticBϕcomponent which leads to twisted magnetotail field lines. We calculate the deformation of magnetotail field lines under the influence of both thermal plasma pressure and centrifugal forces. We present “linear” (analytic) solutions to the Grad‐Shafranov equation which include the centrifugal force term. In the linear model, two free physical parameters,kand ω, measure the plasma thermal pressure and the ratio between plasma rotational and thermal energy densities, respectively. Low ω and highkvalues indicate the plasma‐dominated case. Conversely, lowkand high ω values indicate the rotation‐dominated case. One limiting case,k= ω = 0, generates a simple vacuum magnetic field of a dipole confined within the magnetospheric cavity. The nonrotational magnetosphere with hot thermal plasma leads to a field configuration without a toroidalBϕcomponent and without field‐aligned Birkeland currents. The other extreme, namely, a rapidly rotating magnetosphere with cold plasma, leads to a configuration in which the plasma must be confined within a thin disk in a plane where the radial magnetic field componentBrvanishes locally. Utmost stretched magnetotail configurations can be achieved by increasing either the plasma thermal pressure or the rotation frequency of the magnetosphere, or both. In the rotation‐dominated case we found the plasma sheet thinner and located closer to the magnetopause than in the purely plasma‐dominated case. This implies that hypothetical polar aurorae, under the influence of magnetospheric rotation, appear at higher magnetic latitudes on both the dayside and the nightside. This also implies that the size of the polar oval shrinks under the influence of magnetospheric rotation. A “pole‐on” magnetosphere can be expected at Uranus in the

ISSN:0148-0227    

DOI:10.1029/JA094iA07p08693

年代:1989

数据来源: WILEY

摘要:

A self‐consistent axisymmetric steady model of Jupiter's nightside magnetosphere is presented. Magnetospheric currents are assumed to be restricted to a thin equatorial disc and to magnetopause current sheets. The plasma velocity is assumed to be purely toroidal, and the magnetic field is assumed to be purely poloidal. Data from the Voyager missions are used as input for the solution of the equation determining the self‐consistent equilibrium magnetic structure. Several numerical computations are performed, varying parameters such as the hot plasma composition and the current disc cutoff radius. We find that, with sufficiently large discs, the magnetic configuration has anXline and anOline. TheXline is located between 37 and 45 Jovian radii, depending on the values chosen for the relevant parameters, while the position of theOline is less well determined. Excellent agreement between the magnetic field data in the tail and the results of the model is found. When the actual, nonaxisymmetric magnetosphere is considered, equilibrium with a purely azimuthal velocity is not possible beyond theXline, and an ordered tailward motion must develop. This could be identified either with the “magnetospheric wind” observed by the Voyager satellites or with the possible formation of plasmoids in the Jovian magnet

ISSN:0148-0227    

DOI:10.1029/JA094iA07p08707

年代:1989

数据来源: WILEY

摘要:

Phase space density profiles for protons with first invariants µ = 600 and 3000 MeV/G (integral invariantsK= 0.30 and 0.003 G½RS) previously derived from measurements by the University of Iowa detector G on Pioneer 11 during the 1979 Saturn encounter are analyzed using solutions of the time‐averaged radial diffusion equation in a dipolar planetary magnetic field. A series of loss models ranging from satellite absorption only to satellite plus ring E absorption and added distributed losses is assumed and the corresponding form of the time‐averaged radial diffusion coefficientD(L)(taken to be of the form D(L) =DoLn, wherenis an integer) is determined in each case by a minimum‐variance fit to the data‐derived phase space density profiles. The inferred forms forD(L)are characterized by a low‐orderL‐dependence (∼L³–L6) and a relatively high amplitude (Do≃ 10−9–10−8RS²s−1) in approximate agreement with earlier studies. Loss models that include only satellite absorption result in pronounced Dione macrosignatures in the model phase space density profiles that are not present in the data profiles, indicating a need for additional distributed losses. Absorption by ring E would be sufficient to explain the absence of a Dione macrosignature only if both the presently accepted maximum normal optical depth and mean ring particle size are increased by at least 1 order of magnitude. Charge exchange losses, estimated from observational upper limits on the concentrations of relevant neutral species, also appear to be only marginally important. Pitch angle scattering precipitation losses occurring within the orbit of Rhea at a rate of at least 0.01 times that of the strong pitch angle diffusion limit could in principle provide the

ISSN:0148-0227    

DOI:10.1029/JA094iA07p08721

年代:1989

数据来源: WILEY

摘要:

Energetic electron data obtained by the University of Iowa instrument during Pioneer 11's two‐way traversal of Saturn's inner magnetosphere in September 1979 are reviewed. There were substantial differences between inbound and outbound observations. It is argued that the inbound data are more likely to represent the time‐stationary state. Adopting these inbound data, we develop a quantitative model for the spatial and spectral distributions of relativistic electrons. The principal features of this model are as follows: (1) The radial dependence of omnidirectional intensity J in the equatorial plane is given by J = k′ exp(−1.05 x)[1 − exp(−6.5 x)] where k′ = 1.28 × 107(cm² s)−1and x = (r−2.30) with r the radial distance in units of the planet's radius 60,000 km. This distribution is applicable to the range 2.30 ≤ r ≤ 3.60. (2) The latitudinal dependence of J is derived from observed pitch‐angle distributions. These distributions are reasonably well represented by j(α) = j(90°)sin1.5α (where j is the unidirectional intensity and α is the pitch angle), if one ignores the relatively small depletion of intensity near α = 90°, such depletion being significant in other contexts. (3) As shown previously, the energy spectrum is a relatively narrow one with characteristic energies E in MeV at various r as follows: 0.69 at 5.0; 1.10 at 4.0; 1.89 at 3.0; 2.62 at 2.5; and 3.05 at 2.30. The synchrotron emission of the entire population of relativistic electrons in Saturn's inner magnetosphere is estimated to be about 1 kW with a spectral maximum at 720 kHz. It is shown that this radiation will be very difficult, if not impossible, to observe, even in the near vicinity of the planet. The paper includes a crude but instructive explication of the truly enormous differences between the synchrotron emissions of the inner magnetospheres of Jupiter and Saturn. Indeed, this comparison was an underlying motive fo

ISSN:0148-0227    

DOI:10.1029/JA094iA07p08731

年代:1989

数据来源: WILEY

摘要:

A theoretical model is developed here in order to determine an envelope for the average spectrum of the Saturnian kilometric radiation (SKR). The microscopic generation mechanism is supposed to be the so‐called synchrotron (or cyclotron) maser instability. As in recent works on the terrestrial kilometric radiation, the effect of the magnetic field inhomogeneity on the generation process must be taken into account. Then, assuming that the emission is nonlinearly saturated by trapping, our calculation allows us to put an upper limit on the SKR spectral intensity very simply: the maximum level of the wave electric field within the source region is indeed linked to a few macroscopic plasma parameters, which can be derived from the observations (the structure of the magnetospheric magnetic field, the cold plasma density, and the characteristic energy of the hot emitting electrons). We have used a dipolar magnetic field model, while the plasma distribution results from the superposition of two components: an ionospheric population and a plasma disc, whose scale heights have been roughly determined from Voyager measurements. The energetic electrons responsible for the emission are supposed to precipitate along the high‐latitude magnetic field lines where SKR emission is known to take place. The width of the source region can be self‐consistently estimated from the model. A very good agreement is obtained between the theoretical spectrum and the observational radio data. The calculated spectral intensities exceed the most intense observed intensities by up to 1 order of magnitude, suggesting that the SKR emission is only marginally saturated by nonlinear proc

ISSN:0148-0227    

DOI:10.1029/JA094iA07p08739

年代:1989

数据来源: WILEY

摘要:

We present a method for deriving the radio spectrum of electrical discharges from the properties of the time series of charges crossing the discharge gap. For the simple case considered, the resulting spectrum is the product of the power spectrum of emission from a single charge crossing the gap and of the power spectrum of the time series of charges crossing a given point in the gap. This result is applied to the observed spectra of both terrestrial lightning and Saturn electrical discharge(s) (SED). These observations are discussed, particularly in the case of SED. SED occurrence and power density are shown to have subtle, yet important, differences from these observables as they have been described in the last 5 years. It is demonstrated that throughout the episode of Voyager 1's (V1) closest approach to Saturn, SED probably occurred continuously in frequency upward at least from the upper limit of Saturn kilometric radiation at about 800 kHz. This is so despite the fact that in the dynamic spectra a strip in time and frequency in which SED do not occur extends in frequency from 1.3 MHz up to the oft‐discussed “lower limit” of SED in the leading edge of the episode of closest approach. This cutoff band can be well interpreted through the combined effects of lower receiver sensitivity resulting from spacecraft interference and the increasing efficiency of power transfer from a given small antenna as frequency increases up to about the quarter‐wave value (above 7 MHz for the planetary radio astronomy antennas). The greater power in SED that occurred after V1 closest approach is emphasized; it is shown to be consistent with the lower frequency of the maximum in their power spectra. This is a result of the greater length of the discharge gap as it generates SED after closest approach. The variable gap length factor is also invoked to explain the variable frequency cutoff in the range 5–15 MHz of the episodes before closest approach. The SED source moved along a single arc defining both preencounter and postencounter events. The discharge gap lengths were a continuous function of position along this arc, with the shortest gaps lying about 5° west (as seen from the spacecraft) of the noon meridian of Saturn and the longest gaps lying on the nightside of

ISSN:0148-0227    

DOI:10.1029/JA094iA07p08757

年代:1989

数据来源: WILEY

摘要:

There are two fundamentally different approaches in calculating the mean free path of energetic charged particles. The first approach is concerned with the particle data. Here measured time intensity profiles are fitted to theoretical models and a best fit value for the mean free path is obtained. For undisturbed propagation of solar particles this theoretical model usually is a numerical solution of a Fokker‐Planck equation. In the case of quasi‐parallel interplanetary shocks the mean free path can be determined from the exponential increase of the particles intensity in the upstream region of the shock. The second approach starts with the magnetic field properties. One often‐applied model assumes that the scattering of the particles is due to Alfvén waves that propagate along the guiding field. Here the particles are scattered resonantly by the components of the field fluctuations perpendicular to the guiding field. A second model assumes that a different type of magnetohydrodynamic wave, the magnetosonic wave, in addition to Alfvén waves plays an important role in particle scattering. The scattering by magnetosonic waves is a nonresonant interaction. We discuss the time intensity profiles of low‐energy protons (35–1600 keV) associated with interplanetary shocks observed by ISEE 3 during the period August 1978 to December 1981. For five shock‐associated particle intensity increases, we calculate the amount of scattering upstream of the shock by fitting the intensity increase as an exponential in time. The anisotropy in the solar wind frame is analyzed. We hereby test the validity of our diffusion model. We compare the mean free paths obtained from the particle data with mean free paths derived from magnetic field data. Here we first calculate the amount of resonance scattering from the power density spectrum of the magnetic field fluctuations and then include the effect of magnetosonic waves. The main difference between these two models lies in the predicted rigidity dependence of the mean free path. For all five events, pure resonance scattering predicts mean free paths that increase with particle rigidity; this disagrees with the observed particle behavior. Quite good agreement is obtained by the model of a mixture of Alfvén waves and magnetosonic waves; here a more complicated rigidity dependence is possible. Therefore we conclude that magnetosonic waves together with Alfvén waves are responsible for scattering of particles, at least in the upstream region of interp

ISSN:0148-0227    

DOI:10.1029/JA094iA07p08769

年代:1989

数据来源: WILEY