Physics
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm..sh22f09h&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #SH22F-09
Physics
2104 Cosmic Rays, 2114 Energetic Particles, Heliospheric (7514), 2134 Interplanetary Magnetic Fields, 7519 Flares, 7537 Solar And Stellar Variability
Scientific paper
Recent observations from the Voyager 1 spacecraft ( ~73 AU, 33° N) during 1998-1999 have uncovered statistically significant, in-phase ~150-day variations in the intensities of ~27-MeV anomalous cosmic ray (ACR) oxygen, helium, and protons1. The Voyager 1 ACRs vary by ~30% with respect to the trend, and similar galactic cosmic ray variations, if they exist, must be less than ~5%, probably much less. No similar, significant periodicities were detected for these same ACR species at Voyager 2 ( ~57 AU, 20° S), which was in the "sector zone" caused by the wavy heliospheric current sheet (HCS) during this period. We interpret the ACR periodicities as being related to the well-known (though not well understood) ~150- to 158-day solar periodicities, first detected in solar gamma ray flare measurements2. The interplanetary magnetic field strength and solar wind, both shown to sometimes exhibit this periodicity3, are most likely related to the ACR variations, probably by periodically impeding the transport of anomalous cosmic rays as compressed regions with elevated magnetic field strength propagate past the spacecraft. The Voyager 1 ACR variations abruptly changed at the beginning of 2000. This was fully six months before a precipitous decrease in ACR intensities -- increased modulation brought on by the new activity of solar cycle 23. We explore the possibility that the periodic behavior continued into 2000, although sharply modified by the superposed global modulation. Also we investigate the relationship that could exist between this abrupt change in the variations of Voyager 1 ACRs in 2000, the increasing latitudinal extent of the sector zone, and the fact that no similar Voyager 2 periodicities were observed in 1998-1999. 1 Hill, M.E., et al., J. Geophys. Res., in press, 2001. 2 Rieger, E., et al., Nature, 312, 623, 1984. 3 Cane, H.V., et al., Geophys. Res. Lett., 25, 4437, 1998.
Hamilton Douglas C.
Hill Matthew E.
Krimigis Stamatios M.
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