J. A. Van Allen

Saturn's Magnetosphere, Rings, and Inner Satellites

Our 31 August to 5 September 1979 observations together with those of the other Pioneer 11 investigators provide the first credible discovery of the magnetosphere of Saturn and many detailed characteristics thereof. In physical dimensions and energetic charged particle population, Saturns magnetosphere is intermediate between those of Earth and Jupiter. In terms of planetary radii, the scale of Saturns magnetosphere more nearly resembles that of Earth and there is much less inflation by entrapped plasma than in the case at Jupiter. The orbit of Titan lies in the outer fringes of the magnetosphere. Particle angular distributions on the inbound leg of the trajectory (sunward side) have a complex pattern but are everywhere consistent with a dipolar magnetic field approximately perpendicular to the planets equator. On the outbound leg (dawnside) there are marked departures from this situation outside of 7 Saturn radii (Rs), suggesting an equatorial current sheet having both longitudinal and radial components. The particulate rings and inner satellites have a profound effect on the distribution of energetic particles. We find (i) clear absorption signatures of Dione and Mimas; (ii) a broad absorption region encompassing the orbital radii of Tethys and Enceladus but probably attributable, at least in part, to plasma physical effects; (iii) no evidence for Janus (1966 S 1) (S 10) at or near 2.66 Rs; (iv) a satellite of diameter ≳ 170 kilometers at 2.534 Rs (1979 S 2), probably the same object as that detected optically by Pioneer 11 (1979 S 1) and previously by groundbased telescopes (1966 S 2) (S 11); (v) a satellite of comparable diameter at 2.343 Rs (1979 S 5); (vi) confirmation of the F ring between 2.336 and 2.371 Rs; (vii) confirmation of the Pioneer division between 2.292 and 2.336 Rs; (viii) a suspected satellite at 2.82 Rs (1979 S 3); (ix) no clear evidence for the E ring though its influence may be obscured by stronger effects; and (x) the outer radius of the A ring at 2.292 Rs. Inside of 2.292 Rs there is a virtually total absence of magnetospheric particles and a marked reduction in cosmic-ray intensity. All distances are in units of the adopted equatorial radius of Saturn, 60,000 kilometers.

Pioneer 11 observations of energetic particles in the Jovian magnetosphere

Knowledge of the positional distributions, absolute intensities, energy spectra, and angular distributions of energetic electrons and protons in the Jovian magnetosphere has been considerably advanced by the planetary flyby of Pioneer 11 in November-December 1974 along a quite different trajectory from that of Pioneer 10 a year earlier. (i) The previously reported magnetodisc is shown to be blunted and much more extended in latitude on the sunward side than on the dawn side. (ii) Rigid corotation of the population of protons Ep ≈ 1 million electron volts in the magnetodisc is confirmed. (iii) Angular distributions of energetic electrons Ee > 21 million electron volts in the inner magnetosphere are shown to be compatible with the Kennel-Petschek whistler-mode instability. (iv) A diverse body of magnetospheric effects by the Jovian satellites is found. (v) Observations of energetic electrons in to a radial distance of 1.59 Jovian radii provide a fresh basis for the interpretation of decimetric radio noise emission.

Hot Plasma and Energetic Particles in Neptune's Magnetosphere

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 RN (where RN = radius of Neptune), a relative peak in the spectral index of low-energy electrons at Tritons radial distance, and a change of the proton spectrum from a power law with γ ≥ 3.8 outside, to a hot Maxwellian (kT [unknown] 55 keV) inside the satellites 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 spacecrafts traversal of Earths polar regions at low altitudes; simultaneously, several spikes of spectrally soft electrons and protons were seen (power input ≈ 5 x 10-4 ergs cm-2 s-1) suggestive of auroral processes at Neptune. (iii) Composition measurements revealed the presence of H, H2, and He4, 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 β ≈ 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 ∼40 RN] implying an upper limit to the volume-averaged atomic H density at R ≤ 6 RN of ≤ 20 cm-3; and an estimate of the rate of darkening of methane ice at the location of 1989N1 ranging from ∼105 years (1-micrometer depth) to ∼2 x 106 years (10-micrometers depth). Finally, the electron fluxes at the orbit of Triton represent a power input of ∼109 W into its atmosphere, apparently accounting for the observed ultraviolet auroral emission; by contrast, the precipitating electron (>22 keV) input on Neptune is ∼3 x 107 W, surprisingly small when compared to energy input into the atmosphere of Jupiter, Saturn, and Uranus.

Absence of Martian Radiation Belts and Implications Thereof

A system of sensitive particle detectors on Mariner IV showed the presence of electrons of energy (Ee) less than 40 kiloelectron volts out to a radial distance of 165,000 kilometers in the morning fringe of the earths magnetosphere but failed to detect any such electrons during the close encounter with Mars on 14-15 July 1965, at the time when the minimum areocentric radial distance was 13,200 kilometers. This result can mean that the ratio of the magnetic dipole moment of Mars to that of the earth (MM/ME) is surely less than 0.001 and probably is less than 0.0005. The corresponding upper limits on the equatorial magnetic field at the surface of Mars are 200 and 100 gammas, respectively. It appears possible that the solar wind interacts directly with the Martian atmosphere.

Spatial Distribution and Time Decay of the Intensities of Geomagnetically Trapped Electrons from the High Altitude Nuclear Burst of July 1962

A one-year observational study of the artificial radiation belt produced by the nuclear burst Starfish on 9 July 1962 is reported. It is estimated that 1.3 x 1025 electrons from radioactive fission products, or some 2.6% of the total yield, were present in geomagnetically trapped orbits at 10 hours after the burst. These electrons disappeared in the manner expected from the atmospheric loss theory of Walt for values of the magnetic shell Parameter L<1.25 earth radii. At increasing values of L the rate of disappearance was progressively more rapid than expected by this theory. The maximum observed value of the apparent mean lifetime of ~2MeV electrons in the time range 4000< Δt/10 000 hours was 2 years at L = 1.5. About 15% of the initially injected electrons (or 0.4 of 1% of the total) survived the first 5 ½ months, about 10% the first year.

Observed currents on the Earth's high‐latitude magnetopause

This paper reports a survey of electrical currents on the Earth`s magnetopause, principally at high latitudes, as inferred from magnetic vector data acquired by the University of Iowa/Langley Research Center/NASA satellite Hawkeye 1. The very eccentric orbit of this satellite had an inclination to the equator of approximately 90 deg, an initial apogee at a radial distance of about 21 RE (Earth radii) over the north pole, a relatively low altitude perigee, an essentially constant semimajor axis of 11.0 RE, and a corresponding period of 51.3 hours. A total of 536 candidate crossings of the magnetopause were examined. Many of these were in bundles of multiple crossings and were considered essentially redundant. Others were poorly defined. A reduced data set of 139 selected cases was analyzed in detail though solar wind dynamic pressure data were available for only 117 of these cases.

Venus - An upper limit on intrinsic magnetic dipole moment based on absence of a radiation belt

On the basis of the absence of energetic electrons (Ee 〉 45 kiloelectron volts) and protons (Ep 〉 320 kiloelectron volts) associated with Venus to within a radial distance of 10,150 kilometers from the center of the planet and using a physical similitude argument and the observational and theoretical knowledge of the magnetosphere of Earth, we conclude that the intrinsic magnetic dipole moment of Venus is almost certainly less than 0.01 and probably less than 0.001 of that of Earth. Corresponding upper limits on the magnetic field at the equatorial surface of Venus are about 350 and 35 x 10-5 gauss, respectively.

Iowa catalog of solar X-ray flux (2–12 Å)

The absolute X-ray flux from the whole disc of the sun in the wave length range 2 to 12 Å has been observed for a prolonged period by University of Iowa equipment on the earth-orbiting satellite Explorer 33 and the moon-orbiting satellite Explorer 35, both of the Goddard Space Flight Center of the National Aeronautics and Space Administration. The observations are continuing at the date of writing (July 1969). A comprehensive catalog of the flux F (2–12 Å) is being produced. The observational technique and the scheme of reducing data are described herein. Sample tabulations and plots are given. A catalog of tabular and graphical data with a time resolution of either 81.8 or 163.6 sec has been completed for the following periods: From Explorer 33: 2 July 1966 to 27 July 1967 From Explorer 35: 26 July 1967 to 18 September 1968 These blocks of data have been delivered to the National Space Science Data Center National Aeronautics and Space Administration Goddard Space Flight Center Greenbelt, Maryland 20771, U.S.A. and made available through that agency to interested workers in solar and ionospheric physics. Further blocks of data will be made available as they are completed. An abridged summary of principal flares is published in the monthly Solar-Geophysical Data of the U.S. Department of Commerce, Environmental Science Services Administration.

Some General Aspects of Geomagnetically Trapped Radiation

This paper provides a sketch of the physical principles of radiation belts and an abridged summary of the present State of knowledge of the geomagnetically trapped radiation. As such, it is intended as an introduction to detailed papers on specific topics which constitute the remainder of the Bergen conference.

The cosmic ray intensity above the atmosphere near the geomagnetic pole

SummaryWith the help of new high altitude intensity measurements of Meredith, Van Allen and Gottlieb with single Geiger counters near the north geomagnetic pole, an assessment of existing knowledge of the low rigidity end of the primary cosmic ray spectrum is presented. The new Iowa rocket experiments fully confirm and substantially extend previous evidence for the marked flattening of the integral primary cosmic ray spectrum below a magnetic rigidity of about 1.5–109 volts. In particular, they indicate a complete or nearly complete absence of the major components (H, He) of the primary radiation in the following spectral regions: (a) For hydrogen, the magnetic rigidity range 1.2–109 volts to 0.18-109 volts (kinetic energy range 590 MeV to 18 MeV). (b) For helium, the magnetic rigidity range 1.2 · 109 volts to 0.37·109 volts (kinetic energy range 700 MeV to 72 MeV). The low end of these ranges is far below that attainable by any means except rocket-borne apparatus. This observed absence of low rigidity primaries is not inconsistent with a solar dipole moment as large as 0.6–1034 gauss-cm3. But there may be an entirely different physical cause. Due to the low relative abundance of primary nuclei ofZ > 2, the present data are not of sufficient accuracy to conclusively exclude their presence in the low rigidity region of the spectrum. A crucial test of the solar cut-off hypothesis is available in this connection. Preliminary roeket flights of pulse ionization chambers have been made with the intention of investigating the intensities of low rigidity heavy nuclei.RiassuntoServendosi dei risultati delle nuove misure d’intensità a grandi altezze eseguite da Meredith, Van Allen e Gottlieb con singoli contatori Geiger, vicino al polo geomagnetico Nord, si presenta un quadro d’insieme delle conoscenze attuali sull’estremità di bassa rigidità dello spettro primario dei raggi cosmici. I nuovi esperimenti con razzi eseguiti nello Iowa pienamente confermano e sostanzialmente ampliano le prove preesistenti di un appiattimento marcato dello spettro primario integrale dei raggi cosmici al disotto di una rigidità magnetica di circa 1,5·109 V. In particolare se ne può dedurre l’assenza completa o quasi completa dei componenti principali della radiazione primaria (H, He) nelle seguenti regioni spettrali: (a) Per l’idrogeno, l’intervallo di rigidità magnetica da 1,2·109 V a 0,18·109 V (intervallo d’energia cinetica: 590 MeV-18 MeV). (b) Per l’elio, l’intervallo di rigidità magnetica da 1,2 ·109 V a 0,37·109 V (intervallo d’energia cinetica: 700 MeV-72 MeV). L’estremità inferiore di questi intervalli è molto al disotto di quella raggiungibile con qualsi mezzo, eccetto le apparecchiature portate da razzi. L’assenza di primaria di bassa rigidità che è stata constatata non è incompatibile con un momento di dipolo solare fino a 0,6 · 1034 gauss ·cm3; può però esser dovuta ad una causa fisica del tutto differente. A causa dell’abbondanza relativamente scarsa di nuclei primari conZ > 2, i dati attualmente disponibili non sono sufficientemente precisi per escludere la loro presenza nella regione dello spettro di bassa rigidità magnetica. A tal riguardo è possibile una verifica cruciale dell’ipotesi del cut-off solare. Sono stati eseguiti con razzi lanci preliminari di camere di ionizzazione a impulso allo scopo di studiare le intensità dei nuclei pesanti di bassa rigidità.