F.B. McDonald 1 , W.R. Webber 2 , D.V. Reames 1

1. The Unusual Time History of Galactic and Anomalous Cosmic Rays at 1 AU over the Solar Minimum of Cycle 23 F.B. McDonald1, W.R. Webber2, D.V. Reames1 1 Institute for Physical Science and Technology, Univ. of Maryland, College Park, MD, USA 2 Dept. of Physics and Astronomy, New Mexico State Univ., Las Cruces, New Mexico, USA AGU Fall Meeting San Francisco, CA, USA 15-19 December 2008

2. Galactic Cosmic Ray Modulation Cycle 23 Solar Minimum → Onset of Cycle 24 • Probes the large scale structure and dynamics of our heliosphere • Cosmic Ray Modulation is dominated by the Sun • Level of solar activity – (CME’s) • Tilt angle of the heliospheric neutral current sheet • Velocity (and density) of Solar Wind • Strength of IP magnetic field Over the modern era (1951 – present) GCR intensity is at its lowest level over the past 1000 years • Time Scales • Short term – (days) forbush decreases • 11 year solar activity – dominated by level of solar activity – effects of ICMEs (reasonable correlation with sunspot numbers) • Heliomagnetic (22 years): very different time histories at 1 AU for qA> 0 minima (when particles drift in over the solar poles and out along the current sheet. Flow pattern reversed for qA<0 (odd cycle) • q A<0 at 1 AU enhancement of GCRs > ~ 1 GV • Suppression of GCRs and ACRs < 1 GV

3. Galactic Cosmic Ray Modulation Cycle 23 Solar Minimum → Onset of Cycle 24 • Cycle 23 → 24 • Ulysses/ACE report decreases in magnitude of IP B field • Lower dynamic pressure of solar wind (McComas, et al) • Tilt angle did not reach as small a value as cycle 21 • Solar Activity remains at a very low level Modulation studies will allow the exploration of the role of these different phenomena. V1, V2 in the heliosheath will provide valuable insight into what is happening! Records of the intensity of Galactic Cosmic Rays over the past ~10,000 years are available from 10Be in polar ice cores. A knowledge of the relation between the sun and cosmic rays provides a better understanding of unusual solar epochs in the past such as the Sporer and Maunder minimums.

4. CLIMAX Neutron Monitor (Solar Rotation Avg > 3 GeV Protons and Monthly Average Sunspots: 1951 - 2002

6. Cosmic Ray Drifts in the Heliosphere (qA > 0) from Jokipii & Giacalone, icrc0077

7. The effect of merged interaction regions at Voyager 1 (32-47 AU) over the solar minimum to solar maximum period of cycle 22.

8. 1 AU Solar Minimum Energy Spectra (Cycles 19 – 22)

9. Comparison of IMP-8 134-250 MeV/n GCR He and Normalized ACE 122-235 GCR Oxygen (1997 – 2006)

10. Comparison of IMP-8 134-250 MeV/n GCR He (1997 2006) and Normalized ACE 122-235 GCR Oxygen (extended through 2008.0)

11. Comparison of ACE Data (2000 – 2008.9) withIMP-8 Data Transposed from 20 Years Earlier

14. Voyager 1,2 GCR H and He and ACR He (30-56 MeV/n He): 2004-2008.9 Voyager-1 2006.14 – 2008.92 % Below Webber/Higbie LIS 150 - 380 MeV/n GCR He 7.4 %/Year 21% 145 – 244 MeV/n GCR He 9.6 %/Year 30% 180 - 350 MeV GCR H 15.5 %/Year 44% 30 – 56 MeV/n ACR He 12.6 %/Year

15. Ken McCracken, 2007 ICRC

16. CONCLUSIONS and DISCUSSION • Solar cycle 23 is different from the previous five cycles in the “Modern Era” • Lowest IP magnetic field (since 1963) • Extended plateau followed by increasing intensity (200 MeV/n GCR He, Neutron Monitors) • Voyager-1/Voyager-2 increase (starting in 2006.62) is temporal (not spatial) • In part, produced by reduced value of IP magnetic field • May take several more years to access all regions of the heliosheath (the flanks and tail) • It is vital to make contemporaneous measurements of 10Be in the polar regions to better understand the deep cosmic ray minima of the past • The very low level of solar activity is continuing