Galactic and Anomalous Cosmic Rays at 1 AU During the Recent Unusual Solar Minimum (Invited)

Details Galactic and Anomalous Cosmic Rays at 1 AU During the Recent Unusual Solar Minimum (Invited) Galactic and Anomalous Cosmic Rays at 1 AU During the Recent Unusual Solar Minimum (Invited)

Dec 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufmsh53d..01l&link_type=abstract

American Geophysical Union, Fall Meeting 2010, abstract #SH53D-01

Physics

[2104] Interplanetary Physics / Cosmic Rays, [2114] Interplanetary Physics / Energetic Particles, [2162] Interplanetary Physics / Solar Cycle Variations

Scientific paper

Anomalous cosmic ray (ACR) intensities at 1 AU at solar minimum have generally tracked galactic cosmic ray (GCR) intensities such as those measured by neutron monitors, albeit with differences between solar polarity cycles. Throughout the present A<0 cycle ACR intensities have been consistently a factor of 3-4 lower than expected from scaling neutron monitor rates during the last A>0 cycle, similar to what was observed during the last A<0 period in the mid-1980's. Although the present solar minimum was deep and long-lasting with very low sunspot numbers and no major solar particle events, peak ACR intensities just barely reached their last A<0 levels. Meanwhile, GCR intensities (neutron monitor rates) were at the highest levels recorded during the last 50 years, indicating that these particles had easier access to the inner heliosphere than ACRs. During A<0 cycles particles preferentially drift inward along the heliospheric current sheet (HCS), and the HCS tilt angle dropped much more slowly this cycle than it did during the previous two cycles. Using neutron monitor data along with ACR and GCR measurements from the Advanced Composition Explorer spacecraft, we find that for a given HCS tilt angle both ACR and GCR intensities were actually much higher this cycle than they were during the last A<0 cycle. Evidently there was less overall modulation, but ACRs were more sensitive to the larger tilt of the HCS than GCRs, perhaps because of the latitudinal distribution of ACRs at their source. The HCS tilt angle increased abruptly in late 2009, and by early 2010 both ACR and GCR intensities showed dramatic decreases, apparently marking the end of solar minimum modulation conditions for this cycle. We present measurements of ACR and GCR intensities at 1 AU throughout the solar cycle and discuss possible explanations for the different behavior between the present A<0 epoch and the previous one. This work was supported by NASA grants NNX08AI11G and NNX10AE45G.

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