K. R. Pyle

Saturnian Trapped Radiation and Its Absorption by Satellites and Rings: The First Results from Pioneer 11

Electrons and protons accelerated and trapped in a Saturnian magnetic field have been found by the University of Chicago experiments on Pioneer 11 within 20 Saturn radii (Rs) of the planet. In the innermost regions, strong absorption effects due to satellites and ring material were observed, and from ∼ 4 Rs inwards to the outer edge of the A ring at 2.30 Rs (where the radiation is absorbed), the intensity distributions of protons (≥ 0.5 million electron volts) and electrons (2 to 20 million electron volts) were axially symmetric, consistent with a centered dipole aligned with the planetary rotation axis. The maximum fluxes observed for protons (> 35 million electron volts and for electrons < 3.4 million electron volts) were 3 x 104 and 3 x 106 per square centimeter per second, respectively. Absorption of radiation by Mimas provides a means of estimating the radial diffusion coefficient for charged particle transport. However, the rapid flux increases observed between absorption features raise new questions concerning the physics of charged particle transport and acceleration. An absorption feature near 2.5 Rs has led to the discovery of a previously unknown satellite with a diameter of ≈ 200 kilometers, semimajor axis of 2.51 Rs, and eccentricity of 0.013. Radiation absorption features that suggest a nonuniform distribution of matter around Saturn have also been found from 2.34 to 2.36 Rs, near the position of the F ring discovered by the Pioneer imaging experiment. Beneath the A, B, and C rings we continued to observe a low flux of high-energy electrons. We conclude that the inner Saturn magnetosphere, because of its near-axial symmetry and the many discrete radiation absorption regions, offers a unique opportunity to study the acceleration and transport of charged particles in a planetary magnetic field.

The galactic cosmic-ray radial intensity gradient and large-scale modulation in the heliosphere

Measurements of fluxes from 1 AU on the IMP8 satellite to 11 AU on the Pioneer 11 and to >24 AU on the Pioneer 10 spacecraft show that the nucleation integral gradient is a function of the level of the approx.11 year solar modulation cycle. The gradient decreases from approx.4.5% to 1.5% throughout the solar minimum of 1975--1977, and increases to approx.2.5--3.0% per AU near maximum modulation. Pioneer 10 at >24 AU is still deep in the modulation region, leading to estimates of 50--70 AU for the radial distance to the heliopause at both solar minimum and solar maximum modulation.

The solar latitude and radial dependence of the anomalous cosmic-ray helium component

We present observations of the spatial intensity gradients of 11--20 MeV per nucleon anomalous helium made with Pioneer 10 and Pioneer 11 over the radial range 1--11.3 AU for Pioneer 10 and to a heliographic latitude of 16/sup 0/ at 3.75 AU for Pioneer 11. We find evidence for a significant gradient in heliographic latitude, with flux increasing away from the equatorial plane at the rate of approx.2%--3% per degree of latitude. This result shows that the common assumption of spherical symmetry for solar modulation is incorrect. By comparison with gradients measured for 29--67 MeV per nucleon helium for 11--20 MeV and 29--67 MeV protons, we find that both the radial and latitude gradients of the low energy anomalous helium are the largest of those measured. We briefly discuss the implications of these results for the origin of the anomalous component and for solar modulation.

Changes in radial gradients of low-energy cosmic rays between solar minimum and maximum: - Observations from 1 to 31 AU

Observations of the fluxes and radial gradients of protons and helium (with energies of 10-70 Mev per n) between the period of minimum solar modulation ending in 1977 and the period of maximum modulation in 1981-1983 show that, over the radial range 1-31 AU, radial gradients decreased for both species from their solar minimum values (5-10 percent per AU for galactic protons and helium) to values of 2-4 percent per AU or less. For these low-energy cosmic rays it is found that the variation in modulation with the phase of the solar cycle is much stronger at radii of 20-30 AU than at 1 AU, and that at solar maximum, more than 99 percent of the total modulation takes place beyond 31 AU.

The heliospheric intensity gradients of the anomalous He4 and the galactic cosmic-ray components

Measurements with the Pioneer 10 and 11 spacecraft over the radial range from 1 to approx.25 AU during the period 1972--1981 show that at solar minimum (1972--1977) the radial gradient for the anomalous helium component (He(A)) in the energy range from 11 to 20 MeV n/sup -1/ is approx.14% AU/sup -1/, significantly smaller than predicted for either charge state (He/sup +/(A) or He/sup + +/(A)) of the component based on the observed spectrum and the assumptions of conventional solar modulation for particles propagating inward from the heliospheric boundary. It is shown that, although the He(A) dissapeared at 1 AU by 1979 as a result of increasing solar modulation, it remained present in the outer solar system (R> or approx. =20 AU) into 1980, when the solar polar magnetic fields were observed to reverse and solar activity was near its maximum levels. By 1981, He(A) had essentially dissapeared at Pioneer 10 (Rroughly-equal24 AU) and the radial gradient had decreased to levels consistent with those expected for galactic helium with a normal modulated spectrum.

Characteristics of Jovian Trapped Electrons and Protons for R ⪝ 20 R J and Their Interaction with Io

Discussion of the energetic particles associated with Jupiter divides naturally into discussion of three regions. These regions are (a) the inner core of Jupiter’s magnetosphere (R ⪝ 20 R J) where the highest intensity trapped radiation is to be found, and where trapping is almost certainly stable and long term; (b) the extended outer region of the magnetosphere (2 R J⪝ R ⪝ 100 R J) where the energetic particle fluxes are confined primarily to the magnetic equatorial plane, and where the particles are probably not stably trapped, and (c) interplanetary space, where electrons from Jupiter are observed as much as 1 AU upstream from Jupiter in the solar wind.