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Van Allen, J. A.

Publications and source records attributed to Van Allen, J. A..

At least 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Energetic electrons in the Jovian magnetosphere

Results are reported for a detailed analysis of Pioneer 10 data on energetic particle species in the magnetodisk region of Jupiter's magnetosphere. It is shown that the observed counting rates in the magnetodisk (beyond 20 Jupiter radii) were caused primarily by electrons with energies exceeding 0.06 MeV. Absolute omnidirectional electron intensities in the magnetodisk are presented for five integral energy ranges, and a model electron differential energy spectrum is found to fit the intensities throughout most of the encounter trajectory. It is suggested that the observed spectral shape results from losses of high-energy electrons by pitch-angle scattering. Observed equatorial energy spectra are used to compute distribution functions for several values of the first adiabatic invariant, mu. The radial profiles of the functions are found to have maxima at about 50 Jupiter radii inbound as well as at about 90 radii outbound and to diminish strongly for lesser radii. The large decreases in density are shown to require strong losses, and resonant electron whistler-mode pitch-angle scattering is suggested as a loss mechanism.

Baker, D. N.↗
NASA NTRS1975

Recirculation of energetic particles in Jupiter's magnetosphere

A significant new finding from analysis of Pioneer 11 observations in the magnetosphere of Jupiter is that there is net streaming of both electrons E above 40 keV and E above 560 keV and protons in the range from .61 to 3.41 MeV away from the planet along high-latitude field lines. This result is compatible with the recent suggestion of Nishida that energetic particles undergo trans-L shell diffusion at low altitudes without significant change of energy. This provides a plausible explanation for the remarkable pitch angle distributions near the equator in the range of L values from 12 to 25; the presence of particles of about 1 MeV energy at the outer edge of the magnetosphere; and hence, via conventional inward diffusion processes, the presence of those having magnetic moments of several hundred MeV per gauss in the inner magnetosphere. The recirculation of energetic particles emerges as an important dynamical feature of the Jovian magnetosphere.-

Sentman, D. D.↗
NASA NTRS1975

Pioneer 11 observations of energetic particles in the Jovian magnetosphere

A preliminary report is presented of energetic electrons and protons observed with the University of Iowa instrument on Pioneer 11. A graph shows absolute, spin-averaged unidirectional intensities of electrons and protons as a function of time during traversal of the central magnetosphere. Another graph shows the effects of the Jovian satellites Io and Amalthea on particle intensities. It is pointed out that a full analysis of satellite effects is the most promising technique for understanding the physical dynamics of the magnetosphere of Jupiter.

Van Allen, J. A.↗
NASA NTRS1974

Effects of interplanetary shock waves on energetic charged particles

Experimental data on the influence of interplanetary perpendicular and oblique shock waves on the ambient energetic protons are presented along with a theoretical analysis of the acceleration of particles in almost perpendicular shock waves. It was found that low-energy protons can be accelerated in perpendicular shock waves by repeated crossings of the shock front up to a maximum energy given by the product of their initial energy times the ratio of the magnetic fields. High-energy protons need to stay at the shock front for longer times than low-energy protons in order to reach the same relative energy gain. In the theoretical study of proton acceleration at almost perpendicular shock waves, it was found that protons reflected at shock waves with the angle between the upstream magnetic field and the shock normal greater than about 80 deg achieve large energy gains at the shock front. The larger this angle, the higher the energy gain. However, the reflection and energization of protons at these shock waves is not 'instantaneous', neither is it a one-step process: it is performed through repeated crossings of the shock front.

Sarris, E. T.↗
NASA NTRS1974

The magnetosphere of Jupiter as observed with Pioneer 10. I - Instrument and principal findings

Description of the first in situ observations of energetic electrons of energy greater than 0.06 MeV in the magnetosphere of Jupiter during November-December 1973. The outer magnetosphere has the form of a thin disklike quasi-trapping region extending from about 20 to over 100 Jovian radii. This magnetodisk is confined near the magnetic equatorial plane and has approximate axial symmetry about the magnetic axis of the planet. The observations inside a radial distance of 12 Jovian radii are well organized by a centered dipolar model of the planet's magnetic field with a tilt of 9.5 plus or minus 0.5 deg to the rotational axis and with pole at a system III longitude of 230 plus or minus 3 deg. Absolute omnidirectional intensities of electrons within the stable trapping region inside 20 Jovian radii are given for five energy ranges greater than 0.06, 0.55, 5.0, 21, and 31 MeV.

Van Allen, J. A.↗
NASA NTRS1974

Jupiter's magnetosphere as observed with Pioneer 10

During November and December 1973 the spacecraft Pioneer 10 provided the first in situ observations of energetic particles in the magnetosphere of Jupiter. Observations made with a University of Iowa instrument are reported. It is found that Jupiter's magnetosphere consists of two quite different parts. The outer magnetosphere has the form of a thin, disk-like, quasi-trapping region extending from about 20 to 100 planetary radii. The inner magnetosphere is characterized by a dipolar magnetic field and very high intensities of durably trapped energetic particles. Particle intensities throughout both regions are discussed, taking into account conditions at the orbits of Io, Europa, and Ganymede.

Van Allen, J. A.↗
NASA NTRS1974

Energetic electrons in the magnetosphere of Jupiter

Observations of energetic electrons (above 7 MeV) show that the outer magnetosphere of Jupiter consists of a thin disklike, quasi-trapping region extending from about 20 to 100 planetary radii. This magnetodisk is confined to the vicinity of the magnetic equatorial plane and appears to be an approximate figure of revolution about the magnetic axis of the planet. Hard trapping is observed within a radial distance of about 20 Jovian radii. The omnidirectional intensity of electrons with energy above 21 MeV between 3 and 20 Jovian radii is described by a provisional expression in terms of radial distance and magnetic latitude. There is tentative evidence for mild effects of the Galilean satellite Europa and possibly Io and Ganymede but not Callisto.

Van Allen, J. A.↗
NASA NTRS1974

Variability of intensity ratios of H to He and He to ions with Z not smaller than 3 in solar energetic particle events

Data from the solid-state detector on Explorer 35 are applied to a study of two intensity ratios in the sub-MeV per nucleon specific kinetic energy range for several energetic particle events. It is found that the intensity ratios vary markedly from event to event, particularly during the time history of the individual events. This implies that the ratios have no simple relationship to 'solar abundances' in the usual sense of the term. The pattern of the variability of each ratio is established; the ratio of He to ions with Z not smaller than 3 starts with a low value and increases as the event proceeds. The H/He ratio exhibits a qualitatively similar time history with marked relative enhancement of He early in an event. Differential diffusion of the various ionic species with differing magnetic rigidities is seen to be the dominant physical cause for the variabilities observed.

Van Allen, J. A.↗
NASA NTRS1973

Anisotropies in the interplanetary intensity of solar protons with energies greater than 0.3 MeV.

By using Explorer 35 interplanetary observations of solar protons with energies greater than 0.3 MeV during ten selected solar events (1967-1970) the tine dependence of intensity and of the angular distribution of intensity has been studied for the first time in the sub-MeV range of energy. The respective contributions of diffusive and convective transport are resolved. Results are qualitatively similar to those of McCracken et al. (1968, 1971) in the energy range from 7.5to 45 MeV; but, as was expected, convective transport is found to be relatively more important at the lower energies. The convective component of the anisotropy vector yields values of the solar wind velocity in good agreement with directly measured values.

Innanen, W. G.↗