James A. Van Allen

Propagation of a large Forbush decrease in cosmic‐ray intensity past the Earth, Pioneer 11 at 34 AU, and Pioneer 10 at 53 AU

During 1991, large and distinctive Forbush decreases in cosmic-ray intensity were observed at Pioneer 11 at 34 AU on 21 August and at Pioneer 10 at 53 AU on 30 September. An apparently related shock passed Voyager 1 at 46 AU on 14 September. These three events are plausibly associated with each other and with a large Forbush decrease at the Earth on 12 June. The apparent radial speed of propagation was about 820 km s−1, independent of radial distance; and the decrease of cosmic-ray intensity was of similar magnitude (≈ 20 percent) at Earth, Pioneer 11, and Pioneer 10, i.e., over a radial range 1 to 53 AU and over a range of heliocentric ecliptic longitude at least as great as 173°. For over a month following passage of the interplanetary shock by Pioneer 11, detectable intensities of locally accelerated protons of Ep ≈ 1 MeV were present. The causative solar flare for the entire family of apparently related events was identified with reasonable certainty as a 3B/X 12.0 one in active region 6659 on 11 June at W17 N31. These new observations together with comparable ones reported previously provide the basis for a new quantitative model of the propagation of Forbush decreases in the outer heliosphere, described by RWF in a paper submitted to the Journal of Geophysical Research.

On the modulation of galactic cosmic ray intensity during solar activity cycles 19, 20, 21, 22 and early 23

Cross-plots of annual mean sunspot number against the annual mean intensity of galactic cosmic rays for the period 1953–1999 show a striking difference between such modulation loops for solar activity cycles 19 and 21 and those for cycles 20 and 22. The loops for cycles 19 and 21 are broad ovals whereas those for cycles 20 and 22 are nearly flat. A complementary feature of the data is that the cosmic ray intensity decreases more rapidly as the sunspot number increases following solar activity minima when the solar polar magnetic parameter A is negative than when A is positive. It is argued that these facts give some support to including gradient and curvature drifts in theories of the transport of cosmic rays in the heliosphere. Further development of the modulation loop for cycle 23 is awaited with special interest.

Findings on rings and inner satellites of Saturn by pioneer 11

Abstract The introductory part of this paper gives a short account of the theory of absorption by planetary rings and satellites of energetic charged particles that are trapped in a planets magnetic field and describes the observable consequences of such absorption processes. The previously published University of Iowa observations of absorption features during Pioneer 11s passage through Saturns inner radiation belt on 1 September 1979 are critically reanalyzed and related to other evidence on rings and satellites inside 2.9 R s , especially that from the imaging system on Voyagers 1 and 2. It is found (a) that satellites 1979 S1, 1979 S2, and 1980 S3 are almost certainly identical; (b) that the evidence for 1979 S4 is weak to nonexistent; (c) that 1979 S5 and two other nearby absorption features and 1979 S6 and one other nearby absorption feature are probably caused by longitudinal and radial structure of Ring F and not by satellites; and (d) that absorption feature 1979 S3 at 169,200 ± 600 km is identified with the opically observed Ring G.

Recovery of interplanetary cosmic ray intensity following the Great Forbush Decrease of mid‐1991

There was a succession of major solar flares in late-May to mid-June 1991. Their effects on cosmic ray intensity were observed by six spacecraft at various points in the heliosphere and by terrestrial neutron monitors. Study of these observations is summarized as follows: (a) An abrupt and extraordinarily large Forbush decrease in intensity occurred at the Earth on DOY 164/1991 (13 June) and ones of similar magnitude (≈ 20 percent) occurred in an orderly sequence of timing thereafter over ranges of heliocentric ecliptic longitude of essentially 360 degrees; of heliographic latitude +32 degrees to −5 degrees; and of radial distance 1.0 to 53 AU. (b) The apparent radial speed of propagation of the presumably causative blast wave was 865 (±75) km s−1 (c) It appears likely that the distinctive solar flare of DOY 162/1991 (11 June) was the dominant cause of the blast wave. (d) Two different representations of the subsequent recoveries of intensity yield substantially different numerical values of “recovery time” but either set of values shows an increase by a factor of about three between 1.0 and 53 AU.

Gravitational assist in celestial mechanics—a tutorial

In planning certain types of trajectories of spacecraft within the solar system, engineers rely on a technique called gravitational assist, or gravity assist. This technique underlies the feasibility of effecting a net change in both the speed and direction of motion of a spacecraft by passage through the gravitational field of a planet or a planetary satellite. The resulting increase, or decrease, in the kinetic energy of the spacecraft appears to contradict the casual expectation that in such an encounter the kinetic energy of the spacecraft after the encounter would be the same as that before the encounter. This paper describes the December 1973 encounter of the Pioneer 10 spacecraft with the planet Jupiter as a real-life example of gravitational assist. It then discusses the physical principles involved in understanding the dynamics of the encounter and concludes with remarks on the important role of gravitational assist in space exploration with artificial spacecraft and in understanding the motion o...

A Durable Reduction of Cosmic Ray Intensity in the Outer Heliosphere

This paper reports Pioneer 10 (P10) and Pioneer 11 (Pll) observations of the intensity J(Ep > 80 MeV) of galactic cosmic rays in the heliosphere near the heliographic equator during the 24-year period 1972-1996 and out to a heliocentric radial distance of 65 AU. It updates previous P10/Pll determinations of the time dependence of the radial gradient of intensity and emphasizes the recent 10-year period, especially the consequences of the great Forbush decrease in 1991. A fresh analysis compares P10 and Pll data with comparable data from IMP 8 at 1.0 AU. For this purpose, we have made a critical study of the data from three different instruments on IMP 8 and have developed a new time-dependent reference level of intensity at 1.0 AU for the period 1974-1996. Using this reference, we find that as of late 1996, recovery of intensity following the 1991 Forbush decrease has been markedly less complete in the outer heliosphere than at 1.0 AU. As a consequence, the mean radial gradient between 4 and 65 AU is now only about +0.3% AU -1. Our findings favor the latitudinal wedge model of the heliosphere (Van Allen and Mihalov, 1990) and suggest that the modulation boundary of the heliosphere is far beyond 65 AU. Generally concordant, but less decisive, evidence of a similar nature has been reported previously by Van Allen (1993), Van Allen (1996), and Webber and Lockwood (1995b).

Alpha-particle emissivity of the moon: an observed upper limit.

Measurements made by the moon-orbiting spacecraft Explorer 35 during 1967-1968 show that it is unlikely that the alpha-particle emissivity of the moon is greater than 0.064 per square centimeter per second per steradian and exceedingly unlikely that it is greater than 0.128, these values being respectively 0.1 and 0.2 of the provisional estimates made by Kraner et al. in 1966. This result implies that the abundance of uranium-238 in the outer crust (approximately a few meters thick) of the moon is much less than that typical of the earths lithosphere, though it is consistent with the abundance of uranium-238 in terrestrial basalt or in chondritic meteorites.

Early rocket observations of auroral Bremsstrahlung and its absorption in the mesosphere

In the summer and autumn of 1957 the author and his colleagues at the University of Iowa conducted 10 successful balloon-launched rocket (rockoon) flights of Geiger-Mueller tubes during two shipboard expeditions, one to the arctic and the other to the antarctic. Summit altitudes ranged from 77 to 130 km. One flight was conducted in the equatorial zone, six in the northern auroral zone, and three in the southern auroral zone. Detailed results of each flight are presented. Auroral bremsstrahlung was detected on eight of the nine high-latitude flights. By virtue of physical shielding, the detectors were insensitive to photons of energy E 8 keV, we find a typical e-folding energy E* of about 10 keV for a differential photon number spectrum of the form dn/dE = A exp(−E/E*) with A in units of photons (cm² s keV)−1. Essentially, the same value of E* is obtained by two complementary methods: (1) The dependence of counting rate on atmospheric depth Q and (2) the inverse ratio of counting rates of an “unshielded” tube and one with an added shield. A detailed discussion of the bases for interpretation of the flight data is given in an extended appendix. For a representative set of data (flight 64) we find an omnidirectional flux of downward moving photons of 2.4×104 (cm² s)−1 at Q = 0, the integral of the above spectrum from E = 0 to E = ∞ with the explicit understanding that this result does not include the presumably much greater flux of photons having E < 8 keV and a much steeper spectrum. The corresponding electron number flux striking the top of the atmosphere is 9.3×107 electrons (cm² s)−1 with an e-folding energy T* = 30 keV. The corresponding energy flux is 4.5 erg (cm² s)−1. Both of the two latter fluxes represent the integral from electron energy T = 0 to T = ∞ of the high-energy tail of the electron spectrum, again with the explicit understanding that the estimates do not include the steeply rising spectrum for T ≲ 15 keV.

Basic principles of celestial navigation

Celestial navigation is a technique for determining one’s geographic position by the observation of identified stars, identified planets, the Sun, and the Moon. This subject has a multitude of refinements which, although valuable to a professional navigator, tend to obscure the basic principles. I describe these principles, give an analytical solution of the classical two-star-sight problem without any dependence on prior knowledge of position, and include several examples. Some approximations and simplifications are made in the interest of clarity.

Energetic Particle Phenomena in the Earth’s Magnetospheric Tail

Solar electrons of kinetic energy E ≳ 50 keV are used as test particles to study the gross magnetic topology of the earth’s distant magnetospheric tail and electric fields therein. The observations have been made with similar systems of detectors on the earth orbiting Explorer 33 and the moon orbiting Explorer 35. Based on study of electron shadowing by the moon, of the simultaneous intensity of solar electrons in interplanetary space and within the magnetotail, and of the very short time delay in access of impulsively emitted solar electrons into the magnetotail, the following conclusions are proposed: (1) The gross magnetic topology of the distant magnetotail is an “open” one (i.e., dynamic interconnection of geomagnetic field lines to those in the interplanetary medium). (2) There are no closed electrical equipotential surfaces in the magnetotail at downstream distances greater than 64 RE. (3) Solar electrons enter the magnetotail at downstream distances between 64 and 900 RE. (4) Support is given to the idea that the motional electromotive force caused by the motion of the interplanetary magnetic field past the earth is responsible for driving magnetospheric plasma convection and electrical currents in the polar ionosphere.