Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.u34a..06l&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #U34A-06
Physics
[2104] Interplanetary Physics / Cosmic Rays, [2114] Interplanetary Physics / Energetic Particles, [2162] Interplanetary Physics / Solar Cycle Variations
Scientific paper
Since the early 1970's, anomalous cosmic ray (ACR) intensities at 1 AU at solar minimum have generally tracked the galactic cosmic ray (GCR) intensities as measured by neutron monitors. Throughout the current A<0 cycle, however, the ACR intensities are a factor of 3-4 lower than expected from scaling neutron monitor rates. Also, although there have been no major solar particle events for almost 3 years and sunspot numbers have been at minimum levels for more than a year, ACR intensities at 1 AU are still almost a factor of 2 below their maximum values during each of the last two A>0 solar minima and have just barely reached the last A<0 levels, suggesting that heliospheric conditions are not yet at minimum modulation levels. Simultaneously, however, GCR intensities are at their highest levels ever recorded during the Space Age. The low ACR intensities are probably associated with the fact that ACRs drift inward along the heliospheric current sheet (HCS) during A<0 cycles, and the tilt of the current sheet is still relatively high for solar minimum, dipping only to ~20 degrees so far. However, since the last polarity reversal in 2000, both GCR and ACR rates no longer scale with HCS tilt angle in the same way they did for the previous three solar minima. Compared with the last A<0 cycle, both ACR and GCR intensities are actually much higher now for a given HCS tilt angle than they were in the mid-1980's. We present updated measurements of the intensity variations of ACRs at 1 AU throughout the solar cycle, compare them with GCR measurements and HCS tilt angle as described above, and discuss possible explanations for the different behavior between the present A<0 epoch and the previous one.
Cummings Alan C.
Leske Richard A.
Mewaldt Richard A.
Stone Edward C.
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