Cosmic Rays at 1 AU Over the Deep Solar Minimum of Cycle 23/24Cosmic Ray Transport in the Helioshealth: The View from Voyager

AGU Fall MeetingSan Francisco, CA, USADecember 14-18, 2009

F.B. McDonald1, A.C. Cummings2, B.C. Heikkila3, N. Lal3, D.V. Reames1, E.C. Stone2, 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

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

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!

The deep, continuing solar minimum of cycle 23/24 provides an unprecedented opportunity for modulation studies:

• To quantify the effects of reducing the strength of the Interplanetary Magnetic Field and of transit changes in the current sheet tilt angle.

• Better understanding of unusual epochs in the past such as the Sporer and Maunder minima.

Time Scales• 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) 1 AU Solar Min. Energy Spectra (Cycles 19 – 22)

6

IPB is 24% below the previous minima of cycles 19-22.

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

Comparison of ACE, WIND and NM Data (2000 – 2009.7) withIMP-8 Data Transposed from 20 Years Earlier

Cycle 23/24Cycle 21

Cycle 23/24Cycle 22

Kiel NM Rate 0.5% 1.03%

SANAE NM Rate 1.1% ---

270-450 MeV/n GCR Fe --- 19.4 ± 2%

196 MeV/n He 30 ± 2% 17 ± 1.5%

10-18 MeV/n O -20 ± 4% -43%

% of change from solar maximum to minimum

Kiel Neutron Monitor 1.2% 8.0%

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.

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 3.5 %/Year 6 - 14 MeV E 75.0 %/Year

REGRESSION

MODEL HELIOSPHERE

a) 10 MeV Galactic Cosmic Rays are increasing at a rate of 75%/yr.

b) No radial intensity gradient for 150-380 MeV/n GCR Helium (0.1 ± 0.2%/AU) between V1 and V2.

c) Strong correlation between intensity increases of Cosmic Ray 10 MeV Electrons and 265 MeV/n He.

d) There is expected to be a strong magnetic barrier at the front and flanks of the heliopause (The Axford-Cranfield effect).

Two Possible Solutions

i. The heliopause is highly permeable to energetic particles for reasons that are not understood at this time

ii. The GCR Electrons and Medium energy ions (< 350 MeV/n) gain entry into the heliosheath and the heliosphere through the heliotail region. Stay Tuned.

RESPONSE FUNCTION

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.