Effects of Increasing Solar Modulation on Anomalous Cosmic Ray Intensities

Abstract

Anomalous Cosmic Rays (ACRs) are accelerated far out in the heliosphere at the termination shock. As they diffuse back through the solar system to 1 AU, they are modulated in intensity. The intensity of ACRs at 1 AU observed by the Solar Isotope Spectrometer has been decreasing since September 1997 as solar activity has been increasing. Oxygen at 7.1-10 MeV/nuc has decreased by a factor of at least 80. Nitrogen and neon at similar energies have also had similar large decreases. We examine the changing fluxes of various ACR species, and the changing elemental and isotopic composition of ACRs. ACR oxygen, in particular, may no longer be observable in 2001 at 1 AU above the background of solar, interplanetary, and galactic cosmic ray particles. The 22 Ne/20 Ne ratio at 15 MeV/nuc has also been increasing since September 1997 from a value of about 0.1 to a value more consistent with the GCR ratio of 0.5. This suggests that GCRs now dominate the quiet time flux of Ne at energies of 10 to 30 MeV/nucleon.

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