Las Cruces CRS April 21-22, 2011

F.B. McDonald1, A.C. Cummings2, E.C. Stone2 , B.C. Heikkila3, N. Lal3, W.R. Webber4

1 Institute for Physical Science and Technology, Univ. of Maryland, College Park, MD, USA2 California Institute of Technology, Pasadena, CA, USA3 NASA/Goddard Space Flight Center, Greenbelt, MD, USA4 Dept. of Physics and Astronomy, New Mexico State Univ., Las Cruces, New Mexico, USA

A New Look at the Heliosphere and Solar Modulation

IPB is 28% below the previous minima of cycles 19-22.

Heliospheric neutral current sheet approaches minimum value at a slower rate than previously observed.

Wang, Robbrecht and Sheely (2009) have pointed out that these effects appear to be related – the weaker polar fields result in the apparent refusal of the heliospheric current sheet to flatten closer to the equator.

Comparison of ACE, WIND and NM Data (2000 – 2010.3) withIMP-8 Data Transposed from 20 Years Earlier

Table 1

Comparison of Cycle 21 and 22 with 23/24

Cycle 23/24 Cycle 21

Cycle 23/24 Cycle 22

Hermanus N.M (Rc = 4.9 GV) 0.7% 2.8% SANAE NM Rate (Rc = 0.8 GV) 0.96% 3.6% 270 – 450 MeV/n GCR Fe 19.4± 2% 196 MeV/n GCR He 30 ±2 % 17±1.5 % 10 – 18 MeV/n ACR 0 -20±4 % - 40 ± 3% 10 – 18 MeV/n GCR C - 11 ± 1% Ulysses KET 2.5 GV electrons --- ~ 30% Ulysses KET 2.5 GV protons --- ~ 0 Russian Murmansk Balloon Flights (Rc = 0.6 GV)

15% 13%

10-21 MeV/n ACR He 13% 15%

The 23/24 Solar minimum intensities were from the end of 2009 except Ulysses KET (end of 2008), the Murmansk Balloon Flights (April 2009) and ACE Iron (2009.3)

% of change from solar maximum to minimum

SANAE 4NMD 0.45% 3.1%

200 MeV/n GCR He 3.5% 0.7%

8-18 MeV ACR O 40% 0.2%

Cosmic Ray response to 5° transient increase in the tilt angle of heliospheric neutral current sheet.

Ken McCracken, 2007 ICRC

RESPONSE FUNCTION

GCR He and H in the Heliosheath

• Radial Gradients < 0.2 %/AU over 2005-2009.2

• The V2 decrease starting in 2009.2 is not understood

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 %/Year 6 - 14 MeV GCR E 75.0 %/Year

Regression

Cosmic Ray Modulation

1 AU Comparison

Low Energy Galactic Cosmic Ray Electrons (2 – 160 MeV)(Raw Data)

• Data from 1997.0 – 2002.0 indicates the background level of the electron telescope.

• There is no radial intensity gradient despite the large radial separation.

• The background is produced by higher energy protons (close to minimum ionizing)

Low Energy Galactic Cosmic Ray Electrons (2 – 160 MeV) versus HEP

• Background correction procedure for selected electron channels. HEP rates correspond to H > 200 MeV

• The electron intensity closely tracks the integral rate of GCR H > 200 MeV from 1986 to 2002. In addition, the electron “radial intensity gradient” was negative in 1987 between V1 and V2, clearly a background effect. The detector is background dominated until it reaches the vicinity of the heliosheath.

V1,V2 Background Subtracted Low Energy Galactic Cosmic Ray Electrons

The large electron increases that occur just prior to the V1 TSX are associated with the passage of strong IP transients. The V2 TSX occurred closer to solar minimum and the large increases at energies less than 30 MeV immediately after crossing are interpreted as local acceleration at the TS.

Voyager Electron Energy Spectra

• The V2 electron spectra is from shortly after the termination shock crossing.

• The V2 spectra is steeper than that of V1 which is interpreted as due to reacceleration at the termination shock.

• V1 spectral slope = 1.6V2 spectral slope = 2.9

DISCUSSION and CONCLUSIONS

The ongoing period of very low solar activity has a multi-faceted effect on the modulation process.

• The increase in 135 - 250 MeV/n GCR He appears to be strongly related to the decrease in the heliospheric magnetic field.

• The tilt angle changes of the heliospheric current sheet plays a major role for ACR O and for Neutron Monitors.

• 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.

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.