The possible influence of Jupiter on cosmic radiation at the Earth

Abstract

Analysis of cosmic ray data in sidereal time from underground mu-meson telescopeat different depths in New Mexico and in Bolivia indicates the presence of a long-term modulation of the cosmic ray anisotropy in sidereal time. The period of this rfiodulation is close to 11 or 12 years. These new data, along with data from the underground telescope at Hobart, are examined in relation to suggestions that such a long-term modulation in the cosmic ray anisotropy in sidereal time can be produced by a possible shielding of incoming galactic cosmic rays by J. upiter's magnetosphere. This model is reviewed in the light of the new cosmic ray data presented here and the data on Jupiter's magnetosphere obtained in the flyby by Pioneer 10. These data suggest that it is unlikely that Jupiter's magnetosphere is capable of causing the observed modulation. A correlation between the phase of the modulation and the cycle of solar activity is noted. Recently there have been a number of suggestions that Jupiter may affect the intensity of cosmic rays observed at the earth. Pizzella and Venditti (1973), in an investigation using 4 years (1961-1965) of neutron monitor data for stations with cutoff energies above 6.3 GeV, found evidence that the cosmic ray flux is larger for a preferred direction (130 o from the earth- Jupite? line) that remained invariant in a reference system cor- otating with Jupiter. They pointed out that Jupiter has a strong magnetic field and a Van Allen belt, suggesting the possible acceleration of cosmic ray protons up to about 10 : eV by Jupiter's magnetosphere. The peak in cosmic ray intensity, which appears to 'follow' Jupiter, has an angular velocity (30.35 o per year) corresponding to the period of Jupiter (11.8 years) based on the 4 years of data. Although Pizze!la and Venditti admit that this period could be associated with some other 11- or 12-year cycle, such as the solar cycle, they prefer to conclude that Jupiter contributes to the cosmic ray flux at the earth. Cini-Castagnoli et al. (1973) have also investigated a possible modulation of the cosmic ray intensity by Jupiter, using data from undergrourid mu-meson telescopes. They consider Jupiter not as a source of cosmic rays but as a magnetic shield that could impede the progress of galactic cosmic rays ap- proaching the earth. At sufficiently high energies they note that there should be minimal deflection of cosmic rays between Jupiter and earth; since the declination of Jupiter varies by about +22 o over its 11.8-year period, direct line of sight of Jupiter by a terrestrial cosmic ray telescope occurs only at cer- tain times for telescopes at different latitudes on earth. They note that the Hobart (42oS) vertical underground telescope 'sees' Jupiter within its range only during the period 1959-1961; an analysis of a limited amount of data from the Hobart telescope showed some evidence for a small effect. In a more comprehensive paper Jacklyn and Cini-Castagnoli (1974) again note that because of Jupiter's greatest southerly declination during 1959-1961, its effect should be seen by the Hobart telescope, whereas during 1965-1967, when Jupiter is at its most northerly declination, there should not be any observable effect at Hobart. The position of the earth, and of Jupiter, in June of each year since 1958 is shown in Figure 1. In addition to its variation in right ascension, the declination of Copyright ¸ 1974 by the American Geophysical Union. Jupiter also varies with a period of 11.8 years; the greatest southern declination occurred during 1959-1960 and again during 1971-1972, whereas the greatest northern declination occurred during 1965-1966. The range in declination is about +22 o . The latitudes of cosmic ray telescopes in New Mexico, Bolivia, and Hobart are also shown in Figure I for com- parison. Jacklyn and Cini-Castagnoli (1974) proposed a model in which Jupiter's magnetosphere acts as a magnetic shield to cosmic rays that have sufficiently high energy not to be deflected by the interplanetary magnetic field between Jupiter and earth. This model predicts that when Jupiter falls within the field of view of a telescope, there should be a deficiency in the cosmic ray intensity from the direction of Jupiter. During 1959-1960 and 1971-1972 Jupiter has a right ascension in the region of 18 hours, whereas during 1965-1966 the right ascen- sion is close to 6 hours. The Jupiter model would then predict the results shown in Table 1. These results would, of course, be superimposed on any other sidereal anisotropy that also ex- isted. It is to be noted that a minimum in intensity at 6 hours is