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Van Allen, James A.

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

7 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

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 followed: (1) An abrupt and extraordinarily large Forbush decrease in intensity occurred at the Earth on day of year (DOY) 164/1993 (13 June) and ones of similar magnitude (approximately equal 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. (2) The apparent radial speed of propagation of the presumably causative blast wave was 865 (+/- 75)km/s. (3) It appears likely that the distinctive solar flare of DOY 162/1991 (11 June) was the dominant cause of the blast wave. 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.

Van Allen, James A.↗
NASA NTRS1992

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

Observations of a large Forbush decrease in cosmic-ray intensity at the earth, Pioneer II and Pioneer 10 are reported. The Pioneer 10 data show a large impulsive decrease in cosmic-ray intensity at the greatest distance from the sun observed to date. The apparent radial speed of propagation of the Forbush decrease was about 820 km/s, independent of the radial distance. The observations reported here and similar previous observations provide the basis for a new quantitative model of the propagation of Forbush decreases in the outer heliosphere.

Van Allen, James A.↗
NASA NTRS1988

Solar cycle dependence of the radial gradient of cosmic ray intensity

Observation of the interplanetary intensity of cosmic rays (Ep greater than 80 MeV) by Pioneers 10 and 11 now spans a 16-year time period 1972-1988 and heliocentric radial distances, out to 43.7 AU for Pioneer 10 and 25.8 AU for Pioneer 11. Solar modulation continues to be present at the current distances of both spacecraft. The radial gradient of intensity is measured continuously over the slowly varying, outward moving radial segment r10-r11. The 50-day mean values of the gradient G vary systematically and cyclically in phase with solar activity as measured by sunspot number, with a maximum value of about 2.1 percent/AU at sunspot maximum and a minimum value of about 1.2 percent/AU at sunspot minimum.

Van Allen, James A.↗
NASA NTRS1987

An upper limit on the sizes of shepherding satellites at Saturn's ring G

An accurate analytical theory of the absorption of energetic magnetospheric particles by an inert satellite is developed for the case in which the radius b of the satellite is much less than the equatorial gyroradius r(g) of the particles and in which r(g) is in turn much less than the radius r of the satellite's orbit. In previous interpretations of Pioneer 11 observations, an estimate of the lifetime against absorption of energetic protons at Saturn's ring G has been made. This lifetime is used in the framework of the absorption theory to establish an upper limit on the sizes of shepherding satellites associated with the ring. The resulting upper limit, ignoring the absorption of the optically observed particulate matter, is given for an assemblage of N satellites of various radii. It is noted that ring G lies outside the Roche limit. No shepherding satellites at ring G were detected optically by Voyagers 1 and 2, and the searches were not comprehensive in longitudinal coverage.

Van Allen, James A.↗
NASA NTRS1987

Interplanetary proton (Ep = 0.61-3.41 MeV) events observed with Pioneer 11, 1973-1986 and out to 22.4 AU

A survey of interplanetary proton (Ep = 0.61-3.41 MeV) events is summarized in graphical and tabular form for the period April 1973 - December 1986. The observations were obtained by an effectively continuous data stream from the University of Iowa instrument on the NASA Ames spacecraft Pioneer 11 as it moved outward in the solar system from 1.0 to 22.4 AU. A total of 265 distinct events are identified. The spectra and intensities of the protons, presumed to be originally of solar origin, are influenced dramatically by propagative and accelerative processes in the interplanetary medium.

Van Allen, James A.↗
NASA NTRS1960

Distribution of Trapped Radiation in the Geomagnetic Field

The altitude dependence (360 to 2090 km) of the intensity of geomagnetically trapped radiation as measured with Explorer I (satellite 1958a) is given for a number of geographic locations. It is found that all intensity data in the vicinity of the magnetic dip equator and over the full range of longitude and of altitude can be represented satisfactorily by a single function of the scalar magnetic field intensity B. The value of B at the lower boundary of the inner zone of trapped radiation is a monotonically increasing function of magnetic dip latitude; such data from all available geographic locations are well represented by a single curve.

Yoshida, Sekiko↗