The Unusual Time History of Galactic and Anomalous Cosmic Rays in the Heliosphere 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
SOHO 23Northeast Harbor, Maine, USA
September 21-25, 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 sheetVelocity (and density) of Solar WindStrength of IP magnetic field
Over the modern era (1951 – present) GCR intensity is at its lowest level over the past 1000 years
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.
Galactic Cosmic Ray ModulationCycle 23 Solar Minimum → Onset of Cycle 24
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
Cosmic Ray Drifts in the Heliosphere (qA > 0)
from Jokipii & Giacalone, icrc0077
1 AU Solar Minimum Energy Spectra (Cycles 19 – 22)
Comparison of ACE Data (2000 – 2009.7) withIMP-8 Data Transposed from 20 Years Earlier
COSMIC RAY MODULATION
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 3.5 %/Year6 - 14 MeV E 75.0 %/Year
REGRESSION
MODEL HELIOSPHERE
Ken McCracken, 2007 ICRC
DISCUSSION and CONCLUSIONS
The ongoing period of very low solar activity has a multi-faceted effect on the modulation process.
• The tilt angle changes of the current sheet play a major role, especially for ACR O and perhaps for NMs but less for 135-250 MeV/n GCR He. This is probably because the GCR He is close to the cross-over rigidity that defines the different behavior for low rigidity and high rigidity particles in different phases of the heliomagnetic cycle.
• The lower IPB field will have a significant effect through-out the heliosphere. At the Voyagers in the heliosheath there is an on-going increase of 265 MeV/n He (7.4%/year) and H (15.5%/year) that is temporal and not spatial in nature. Since drift effects are not expected to be important in the heliosheath, this increase is probably related to the lower B field.
• The lower solar wind velocity and pressure will have an effect on the dimensions of the heliosphere and probably on the particle diffusion coefficients.
• The pronounced difference between the 8-18 MeV/n ACR 0 and GCR C is probably due to the fact that the Compton-Getting factor for C is expected to be zero in this energy interval based on ACE measurements over the previous solar minimum.
DISCUSSION and CONCLUSIONS
• Over the last 1000 years there have been previous epochs of low solar activity that have resulted in significant increases in the GCR intensity. As measured by archival data from 10Be in polar ice cores and 10C in tree rings.
• Caballero-Lopez et. al. modeled the cosmic ray intensity variations from 850-2000 AD by varying the strength of the heliospheric magnetic field. At the Maunder Minimum a value as low as 2nT was required for certain periods. Reductions in the solar wind speed and density could lead to larger values of IPB for those periods.
• The current Quiet Sun period should provide insight into the changes that occurred on the Sun and in the IP medium over those very unusual earlier periods. It is especially important to get contemporaneous measurements of 10Be.
