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Published June 2013 | public
Journal Article

Anomalous and Galactic Cosmic Rays at 1 AU During the Cycle 23/24 Solar Minimum


Anomalous cosmic ray (ACR) intensities at 1 AU at solar minimum generally track galactic cosmic ray (GCR) intensities such as those measured by neutron monitors, albeit with differences between solar polarity cycles. The unusual cycle 23/24 solar minimum was long-lasting with very low sunspot numbers and significantly reduced interplanetary magnetic field strength and solar wind dynamic pressure and turbulence, but also featured a heliospheric current sheet tilt that remained high for an extended period. Peak ACR intensities did not recover to the maximum values reached during the last two A>0 solar minima and just barely reached the last A<0 levels. However, GCR intensities in 2009 (neutron monitor rates and also at ∼200 MeV/nucleon) were the highest recorded during the last 50 years, indicating their intensities were not as heavily modulated during their transport from the outer heliosphere. This unexpected difference in the behavior of ACRs and GCRs remains unexplained, but suggests that either the ACR source intensity may have weakened since the last A<0 epoch, or perhaps that ACR intensities at 1 AU in the ecliptic may be more sensitive than GCRs to the higher tilt angle. This seems plausible if the ACR source intensity is greater at low latitudes during A<0 cycles, while the GCR distribution at the heliospheric boundary is more uniform in latitude. Shortly after an abrupt increase in the current sheet tilt angle in late 2009, both ACR and GCR intensities showed dramatic decreases, marking the end of solar minimum modulation conditions for this cycle.

Additional Information

© 2011 Springer Science+Business Media B.V. Received: 29 September 2010; Accepted: 16 March 2011; Published online: 31 March 2011. This work was supported by NASA under grants NNX08AI11G and NNX10AE45G, and benefitted greatly from discussions held at an International Space Science Institute (ISSI) workshop. The Newark neutron monitor of the Bartol Research Institute is supported by the National Science Foundation under grants ANT-0739620 and ANT-0838839. We thank the Wilcox Solar Observatory (http://wso.stanford.edu) for making the HCS data publicly available. We are especially grateful to the personnel at ISSI for their warm hospitality and very generous support not only during the workshop but also throughout our unplanned extended stay necessitated by the closure of European airspace due to the ash cloud from the Eyjafjallajökull volcano.

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