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, USA2 Dept. of Physics and Astronomy, New Mexico State Univ., Las Cruces, New Mexico, USA

NRL13 February, 2009

Galactic Cosmic Ray ModulationCycle 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 GVSuppression of GCRs and ACRs < 1 GV

Galactic Cosmic Ray ModulationCycle 23 Solar Minimum → Onset of Cycle 24

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!

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

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.

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

Cosmic Ray Drifts in the Heliosphere (qA > 0)

from Jokipii & Giacalone, icrc0077

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

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

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

Comparison of IMP-8 134-250 MeV/n GCR He (1997-2006) and Normalized ACE 122-235 GCR Oxygen (extended through 2009.0)

Comparison of ACE Data (2000 – 2009.0) withIMP-8 Data Transposed from 20 Years Earlier

The red points are the N-S average tilt from Hoeksema around the 1987 minimum, shifted by 21 years, while the black points are the tilt from 2004-2009 (Webber, 2009).

The black points are the N-S average tilt from Hoeksema from 2004-2009, while the red points are the ACE 135-250 MeV/n O normalized to IMP-8 He (Webber, 2009).

• Reduction of 150-380 MeV/n GCR He Solar Min to Solar Max in Cycle 23.

IMP8 (1AU)

Factor of 4.4

V2 (63.5 AU) 33%

V1 (81 AU) 22%

• Essentially all of the modulation associated with the 11 year solar activity cycle occurs in the region of the supersonic solar wind.

The Current State of the Heliosphere as Defined by Galactic Cosmic Rays

COSMIC RAY MODULATION

COSMIC RAY MODULATION

Voyager 1,2 GCR H and He and ACR He (30-56 MeV/n He): 2004-2009.0

Voyager-1 2006.14 – 2008.92 % Below Webber/Higbie LIS

150 - 380 MeV/n GCR He 7.4 %/Year21%

145 – 244 MeV/n GCR He 9.6 %/Year30%

180 - 350 MeV GCR H 15.5 %/Year44%

30 – 56 MeV/n ACR He 12.6 %/Year

Ken McCracken, 2007 ICRC

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