摘要:

Pioneer 11, later named Pioneer Saturn, reached its point of closest approach to Saturn on September 1, 1979. Pioneer Saturn carried 11 operating scientific instruments. The 2.293‐GHz telemetry carrier was also used for a radio occultation experiment and a celestial mechanics experiment. The Pioneer Saturn scientific instruments, objectives, and principal investigators are listed in Table 1.Pioneer Saturn was launched April 5, 1973. The objectives of Pioneer Saturn and its companion spacecraft, Pioneer 10, were to penetrate the asteroid belt and to explore the environment of Jupiter. Following the successful encounter with Jupiter by Pioneer 10, Pioneer Saturn was targeted to encounter Jupiter on a trajectory that propelled the spacecraft back across the solar system to be the first spacecraft ever to encounter Saturn, nearly 5 years late

ISSN:0148-0227    

DOI:10.1029/JA085iA11p05651

年代:1980

数据来源: WILEY

摘要:

Pioneer 11 vector helium magnetometer observations of Saturn's planetary magnetic field, magnetosphere, magnetopause, and bow shock are presented. Models based on spherical harmonic analyses of measurements inside 8Rsreveal that the planetary field has a high degree of symmetry about the rotation axis. The vector dipole moment of 0.2 GRs³ has a tilt angle less than 1° and is offset along the polar axis 0.04±0.02Rs. Equatorial offsets derived from the models show substantial variability and could be consistent with a very small offset. Beyond 10Rs, near the noon meridian, the field topology is characteristic of a dipole field being compressed by high‐speed solar wind. There is no evidence of plasma outflow, i.e., a planetary wind. Near the dawn meridian the field lines in the outer magnetosphere are stretched‐out into a nearly equatorial orientation. Crossings of a thin current sheet are observed, apparently caused by motions driven from outside the magnetosphere. The field above and below the current sheet spirals out of the magnetic meridian plane at large distances to point tailward and parallel to the magnetopause. The location of the magnetopause is consistent with a shape that is similar to that of the earth but perhaps more blunt, as suggested by the attitude of the magnetopause near dawn. Near both the noon and dawn magnetopause the field in the magnetosheath equals or exceeds the field in the magnetosphere. The noon observations suggest a piling‐up of magnetosheath field lines adjacent to the magnetopause. Large impulsive field compressions are observed in the magnetosheath near noon. Multiple crossings of the bow shock are observed, and the absence of significant changes in field direction shows that it is quasi‐perpendicular. The speeds of motion of the shock toward and away from Saturn are estimated to be 150 and 50 km/s, respectively. A shock thickness of ∼2000 km

ISSN:0148-0227    

DOI:10.1029/JA085iA11p05655

年代:1980

数据来源: WILEY

摘要:

Analysis of magnetic field observations by the Goddard Space Flight Center high‐field flux gate magnetometer on the Pioneer 11 spacecraft during Saturn encounter yields estimates of the planetary field. The field is mainly dipolar but rather weaker than expected, with a moment equal to 0.20 GRs³ or 4.3 × 1028G cm³, opposite in polarity to earth's. Surprisingly, the field appears to be axially symmetric but with a small (0.04Rs) offset to the north so that N (S) polar field intensities are 0.6 (0.4) G, respectively. The deduced polar offset appears not to be an artifact of the limited spatial extent of the observations or the presence of fields of external origin. The average stand‐off distance of the magnetopause is expected to be 20RS, i.e., at the orbit of Titan, so that this largest of solar system satellites is immersed not only in the Saturnian magnetosphere but also at times in its magnetosheath and sometimes even in the interplanetary

ISSN:0148-0227    

DOI:10.1029/JA085iA11p05675

年代:1980

数据来源: WILEY

摘要:

This paper reports the results of continuing analysis and interpretation of energetic particle observations obtained by the University of Iowa instrument on Pioneer 11 during traversal of Saturn's magnetosphere in August–September 1979. On the basis of the radial dependence of the phase space density of very energetic protons (Ep>80 MeV) and estimates of the necessary source strength, it is reasonably certain that cosmic ray neutron albedo from the planet's atmosphere and Rings A and B is the source of such particles. At radial distancer=2.7Rsand forEp>80 MeV, the ratio of source strength�� to radial diffusion coefficientDis ∼2 × 10−24cm−5. A reasonable pair of values is�� ∼7 × 10−15cm−3s−1andD∼1 × 10−10Rs² s−1. The spectrum of electrons 0.040

ISSN:0148-0227    

DOI:10.1029/JA085iA11p05679

年代:1980

数据来源: WILEY

摘要:

The solar wind plasma analyzer on board Pioneer 11 provides first observations of low‐energy positive ions in the magnetosphere of Saturn. Measurable intensities of ions within the energy per unit charge (E/Q) range 100 eV to 8 keV are present over the planetocentric radial distance range ∼4–16Rsin the dayside magnetosphere. The plasmas are found to be rigidly corotating with the planet out to distances of at least 10Rs. At radial distances beyond 10Rs, the bulk flows appear to be in the corotation direction but with lesser speeds than those expected from rigid corotation. At radial distances beyond the orbit of Rhea at 8.8Rs, the dominant ions are most likely protons, and the corresponding typical densities and temperatures are 0.5 cm−3and 106°K, respectively, with substantial fluctuations. Identification of the mass per unit charge (M/Q) of the dominant ion species is possible in certain regions of Saturn’s magnetosphere via the angular distributions of positive ions. A large torus of oxygen ions is located inside the orbit of Rhea, and the densities are>10 cm−3over the radial distance range ∼4–7.5Rs. Density maxima appear at the orbits of Dione and Tethys where oxygen ion densities are ∼50 cm−3. The dominant oxygen charge states are O2+and O3+in the radial distance ranges ∼4–7Rsand 7–8Rs, respectively. The observations are suggestive of a decrease of ion energies to values less than the instrument energy threshold ofE/Q=100 eV at the apparent inward edge of the torus at 4Rs. Ion temperatures increase rapidly from ∼2 × 105°K at 4Rsto 5 × 106°K at 7.3Rs. It is concluded that the most likely source of these plasmas is the photodissociation of water frost on the surface of the ring material with subsequent ionization of the products and radially outward diffusion. The sources associated with the satellites Dione and Tethys are probably of lesser strength. The presence of this plasma torus is expected to have a large influence on the dynamics of Saturn's magnetosphere, since the pressure ratio β of these plasmas approaches unity at radial distances as close to the planet as 6.5Rs. On the basis of these observational evidences it is anticipated that quasi‐periodic outward flows of plasma, accompanied by a reconfiguration of the magnetosphere beyond ∼6.5Rs, will occur in the local night sector in order to relieve the plasma pressure fr

ISSN:0148-0227    

DOI:10.1029/JA085iA11p05695

年代:1980

数据来源: WILEY

摘要:

Data are presented for a 1‐min dip in electron intensity that was observed coherently by four different University of Iowa detectors at 1546∶51 earth received time on day 244 of 1979 as Pioneer 11 crossed the orbit of Mimas inbound during its encounter with the Saturn system. By a detailed analysis we show that (1) this absorption microsignature in electron intensity is plausibly attributable to the particle sweeping effect of Mimas; (2) the radial width of the signature is caused primarily by energy dispersion in the longitudinal drift rate of electrons and not by radial diffusion; (3) the spectrum of trapped electrons at Mimas is nearly monoenergetic, centered at kinetic energyE=1.59 MeV with a spread δE∼0.1 MeV; (4) this narrow spectrum, which is unique in magnetospheric physics, is caused by the ‘band‐pass filtering’ action of the successive inner satellites, most importantly Enceladus, on an inward diffusing population of electrons; and (5) the radial diffusion coefficientDof electronsE= 1.0 MeV is 1.0 × 10−10Rs² s−1atL=4 and probably in the range 3.7 × 10−11to 8.

ISSN:0148-0227    

DOI:10.1029/JA085iA11p05709

年代:1980

数据来源: WILEY

摘要:

When a spacecraft detector measures particle flux as a function of look direction in a plane (the scan plane) anisotropy is often seen. This anisotropy is caused by spatial gradients, byE × Bparticle drift, and by various spectral and geometric effects. This paper treats all of these effects systematically, starting from the nonrelativistic Vlasov equation. The general analysis is applied to a simple model of an anisotropic distribution to give a relation between theE × Bdrift, the gradient and the experimentally observed first, second, and third harmonics of the flux as a function of angle in the scan plane. Even with an assumed model, anisotropy observations in one plane alone do not suffice to determine theE × Bdrift velocity and the spatial gradient independently. If one is assumed, the other follows. If theE × Bvelocity is assumed (e.g., the corotational velocity in a rotating planetary magnetosphere), the spatial gradient may be deduced, and from it the time rate of change of flux in a nonrotating frame of refere

ISSN:0148-0227    

DOI:10.1029/JA085iA11p05719

年代:1980

数据来源: WILEY

摘要:

The theory and technique of Northrop and Thomsen [1980] are applied to observations of energy spectra and directional anisotropies of 0.61‐ to 3.41‐MeV protons in Saturn's magnetosphere. The observations were made by the Goddard Space Flight Center/University of New Hampshire and University of Iowa instruments aboard Pioneer 11 during the Pioneer encounter with Saturn in August–September 1979. Fourier fits to 15‐min intervals of data are combined with spectral indices to yield information about theE × Bconvection velocity and temporal changes in the particle population. There is a fundamental inability to distinguish unambiguously between the two, but if one can be assumed, the other then follows from these calculations. It is found that although these data do not by themselves allow an unambiguous determination of the extent of corotation in Saturn's outer magnetosphere, they are consistent with exact corotation at the nominal rotation period in the presence of significant but not unreasonable temporal variations in the energetic proton po

ISSN:0148-0227    

DOI:10.1029/JA085iA11p05725

年代:1980

数据来源: WILEY

摘要:

The Pioneer 11 spacecraft encounter with Saturn (closest approach September 1, 1979) has resulted in the discovery of a fully developed magnetosphere with high‐energy trapped radiation around Saturn, as reported inScience, 207, 400–453, 1980, by several investigators with charged‐particle instruments on the spacecraft. The present paper contains in detail the final energetic charged‐particle measurements and new observations obtained from the University of Chicago instrumentation on Pioneer 11, including the overall characteristics of the trapped electron, proton, and helium radiation, which was found to lie inside ∼20 Saturn radii (Rs) from the planet, and the regions extending outward to beyond the planetary bow shocks and into the interplanetary medium. For analytical purposes we divided the magnetosphere into an inner magnetosphere (<5Rs), where the intensity profiles displayed the near‐axial symmetry characteristics of the dipole magnetic field alignment with the spin axis, and an outer magnetosphere whose characteristics on the sunward side inbound were significantly different from the dawn side outbound, indicative of a possible magnetotail but with no dramatic evidence in the charged‐particle data for an equatorial current sheet, as observed at Jupiter. The intensities and energy ranges of the protons and electrons were intermediate between the levels found previously at Jupiter by Pioneer 10 and 11 and at earth. Energy spectra for protons and electrons and relative abundances of protons and helium nuclei are presented along with the average characteristics of particle anisotropies. At the time of encounter the magnetosphere was immersed in intense fluxes of electrons, protons, and helium nuclei of solar flare origin which are shown to penetrate from 1Rsto 10Rsinto the magnetosphere, where they dominated the flux levels in the far outer magnetosphere. A corotation anisotropy has been measured at the proton energy ∼1 MeV in the rotating magnetosphere after correcting the observed unidirectional anisotropy for the radial gradient of the proton flux. The principal focus of the paper is on the analysis of the trapped radiation in the inner magnetosphere, where the radiation reaches high intensity, and has a high degree of symmetry in theLshells around Saturn. Consequently, the absorption signatures in the radiation intensity profiles produced by rings and moons of Saturn can be analyzed quantitatively. Among other results the observation of the charged‐particle absorption features have led to the discovery of satellite 1979 S2 atL=2.53, which corresponds with the optically detected 1979 S1, a concentration of matter probably located at a Lagrangian point in the orbital range of Mimas, and the identification of narrow rings of matter and one or more satellites inside the radial range of the F ring discovered by the optical‐imaging investigators. It is pointed out that these discoveries will provide important tests for models of accretion of matter, satellite formation and the stability of narrow rings near planets. From the discrete character of the absorption symmetries it is shown that except for two extremely narrow intervals of Saturn's longitude, the equatorial offset of the dipole magnetic moment must be ≲0.01Rs. The inward diffusion coefficients for protons and electrons have been determined from the above absorption regions, especially at Mimas. Some questions are considered which may be resolved by the forthcoming Voyager e

ISSN:0148-0227    

DOI:10.1029/JA085iA11p05731

年代:1980

数据来源: WILEY

摘要:

We report observations of anisotropies and pitch angle distributions for 0.5–1.8 MeV protons, 7–17 MeV electrons and ⪞3.4 MeV electrons in Saturn's magnetosphere made with the University of Chicago experiments on Pioneer 11. In the outer magnetosphere (L>6) there is clear evidence for corotation of the proton flux, and the proton pitch angle distribution shows maximum flux perpendicular to the magnetic field (‘pancake’ distribution). Observed changes in the amplitude and shape of the pitch angle distributions suggest the existence of significant temporal variations in the outer magnetosphere. FromL=6 toL=4, the proton intensity decreased by more than two orders of magnitude, and the pitch angle distribution shifted to a ‘dumbbell’ form (maximum flux parallel to magnetic field). The shift in pitch angle distribution most likely results from preferential absorption of large pitch angle particles by the tenuous E‐ ring found in the equatorial plane out to at leastR=5Rs. ForL<4, the proton intensity increased inwards, implying an inner edge for the E ring atR≈4Rs. Except for regions where the flux was reduced by satellite absorption, the pitch angle distributions remained dumbbell. In absorption regions, pancake distributions were found. The observations are consistent with the suggestion by McKibben and Simpson (this issue) that inward diffusion and acceleration at Saturn may proceed primarily via large, infrequent disturbances. Electron anisotropies were measurable only forL⪝4.5, and the pitch angle distributions were found to be pancake for the entire regionL<4.5, suggesting that the absorbing particles in the E ring have radii that lie between the range of ∼1 MeV proton and ∼10 MeV electrons, or of t

ISSN:0148-0227    

DOI:10.1029/JA085iA11p05763

年代:1980

数据来源: WILEY