Several previously proposed techniques for determining the radial diffusion coefficient from the observed effects of the inner Jovian satellites on the energetic particle fluxes are discussed, and important shortcomings are pointed out. A new method is proposed which avoids the most important shortcoming by dealing with data from regions somewhat removed from the actual sweeping region. The new technique is applied to data obtained at the orbit of Io by the University of Iowa proton detector on Pioneer 11 and to a published electron phase space density profile constructed from data obtained (also at Io's orbit) by the University of California at San Diego instrument on Pioneer 10. If satellite sweeping with an effective satellite radius equal to the geometric radius is assumed to be the only loss process operating, the resulting diffusion coefficient for protons with • --1.7 MeV/G at L --6 is D -• 3 x 10 -s Rj•s -•, and that for electrons with • --1 x 104 MeV/G is D --4 x 10 -? Rj•s -•. A possible alternative to satellite sweepup as an explanation for the large proton losses across Io's L shell is proposed. This alternative consists of enhanced precipitation of protons near Io's L shell due to resonant interaction with ion cyclotron waves. The region of enhanced precipitation is localized to the region near Io's orbit becausethat is a region of increased plasma density. Some consequences of such a hypothesis are discussed. One important consequence is that the radial diffusion coefficient for the protons has an upper limit of about 2x 10 -? Rj • s-• at L --6. This upper limit corresponds to the case in which the pitch angle scattering occurs at the strong diffusion limit. Finally, the effects which other potentially significant processes, such as injection or additional loss mechanisms, might have on a determination of the diffusion coefficient from observed satellite effects are discussed. It is pointed out that until such processes can be either dealt with or dismissed, the diffusion coefficient cannot be reliably deduced from observed satellite effects. 8w-N MeV ß ß ß ß ß ß ß '--INBOUND ß-" OUTBOUND ß _ ß ß ß _
Introduction—One of the most interesting geophysical discoveries of recent years was that made with the early United States satellites Explorer I (satellite 1958α) and Explorer III (satellite 1958γ). It was found [Van Allen, 1958; Van Allen and others, 1958] that an immense region around the earth is occupied by a very high intensity of charged particles (protons and electrons), temporarily trapped in the geomagnetic field. The detailed study of this radiation has been a major endeavor of the past year and a half by a group at the State University of Iowa in the United States, and by IGY workers in the Soviet Union. Important additional information at relatively low altitudes has been obtained in rocket experiments flown by other workers.
The low-energy charged particle (LECP) instrument on Voyager 2 measured within the magnetosphere of Neptune energetic electrons (22 kiloelectron volts = E = 20 megaelectron volts) and ions (28 keV = E = 150 MeV) in several energy channels, including compositional information at higher (>/=0.5 MeV per nucleon) energies, using an array of solid-state detectors in various configurations. The results obtained so far may be summarized as follows: (i) A variety of intensity, spectral, and anisotropy features suggest that the satellite Triton is important in controlling the outer regions of the Neptunian magnetosphere. These features include the absence of higher energy (>/=150 keV) ions or electrons outside 14.4 R(N) (where R(N) = radius of Neptune), a relative peak in the spectral index of low-energy electrons at Triton's radial distance, and a change of the proton spectrum from a power law with gamma >/= 3.8 outside, to a hot Maxwellian (kT [unknown] 55 keV) inside the satellite's orbit. (ii) Intensities decrease sharply at all energies near the time of closest approach, the decreases being most extended in time at the highest energies, reminiscent of a spacecraft's traversal of Earth's polar regions at low altitudes; simultaneously, several spikes of spectrally soft electrons and protons were seen (power input approximately 5 x 10(-4) ergs cm(-2) s(-1)) suggestive of auroral processes at Neptune. (iii) Composition measurements revealed the presence of H, H(2), and He(4), with relative abundances of 1300:1:0.1, suggesting a Neptunian ionospheric source for the trapped particle population. (iv) Plasma pressures at E >/= 28 keV are maximum at the magnetic equator with beta approximately 0.2, suggestive of a relatively empty magnetosphere, similar to that of Uranus. (v) A potential signature of satellite 1989N1 was seen, both inbound and outbound; other possible signatures of the moons and rings are evident in the data but cannot be positively identified in the absence of an accurate magnetic-field model close to the planet. Other results indude the absence of upstream ion increases or energetic neutrals [particle intensity (j) < 2.8 x 10(-3) cm(-2) s(-1) keV(-1) near 35 keV, at approximately 40 R(N)] implying an upper limit to the volume-averaged atomic H density at R = 6 R(N) of = 20 cm(-3); and an estimate of the rate of darkening of methane ice at the location of 1989N1 ranging from approximately 10(5) years (1-micrometer depth) to approximately 2 x 10(6) years (10-micrometers depth). Finally, the electron fluxes at the orbit of Triton represent a power input of approximately 10(9) W into its atmosphere, apparently accounting for the observed ultraviolet auroral emission; by contrast, the precipitating electron (>22 keV) input on Neptune is approximately 3 x 10(7) W, surprisingly small when compared to energy input into the atmosphere of Jupiter, Saturn, and Uranus.
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