A NEW LOOK AT THE HELIOSPHRE AND SOLAR MODULATION The V1/V2 Saga and the Deep Minimum of Solar Cycle 23/24 Voyager CRS Science Team: E.C. Stone (PI) 1, A.C. Cummings 1, B.C. Heikkila 2, N. Lal 2, F.B. McDonald and W.R. Webber 3 1 California Institute of Technology, Pasadena, CA, USA 2 NASA/Goddard Space Flight Center, Greenbelt, MD, USA 3 Dept. of Physics and Astronomy, New Mexico State Univ., Las Cruces, New Mexico, USA F.B. M c Donald Institute for Physical Science and Technology, Univ. of Maryland, College Park, MD, USA Modeling Studies: H. Moraal 4 and R. Caballero-Lopez 5 4 Northwest University, Potchefstroom, South Africa 5 National Autonomous University, Mexico City, Mexico

OUTLINE

GALACTIC COSMIC RAYS

Galactic Cosmic Ray Modulation 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 ~ 1 GV Suppression of GCRs and ACRs < 1 GV Cosmic Ray Drifts in the Heliosphere (qA > 0)1 AU Solar Min. Energy Spectra (Cycles 19 – 22)

IPB is 24% below the previous minima of cycles Heliospheric neutral current sheet approaches minimum value at a slower rate than previously observed.

Comparison of ACE, WIND and NM Data (2000 – ) with IMP-8 Data Transposed from 20 Years Earlier Cycle 23/24 Cycle 21 Cycle 23/24 Cycle 22 HERMANUS NM RateXX% SANAE NM Rate1.1% MeV/n GCR Fe ± 2% 196 MeV/n He30 ± 2%17 ± 1.5% MeV/n O-20 ± 4%-43%

VOYAGER Spacecraft Mass:722 kg Power: 3 RTG (Plutonium Oxide) Antenna: 3.7 m Requires 34 m / 70 m dish Bit Rate (Dlink): X-Band 160 bits/s January 15, 2008: Voyager AU 34.17° N Voyager AU27.79° S

Science Team E.C. Stone (PI), A.C. Cummings (Caltech) N. Lal, B.C. Heikkila (GSFC) F.B. McDonald (Univ. of Maryland) W.R. Webber (New Mexico State Univ.) Energetic Particle Coverage H: MeV He: MeV/n Z = 1-28 (Resolves Isotopes) E: 2.5 – 140 MeV CRS EXPERIMENT

July 9, 2007:Voyager AU 34.14° N Voyager AU27.48° S TRAJECTORY

The Heliosheath at the nose is estimated to be AU wide. At the TS, radial velocity of the solar wind will decrease by a factor of 2.4 – 4 depending on the strength of the TS, and will continue to decrease as 1/r 2 where r is the heliocentric distance. The intensity of the transverse component of the interplanetary magnetic field jumps by the same factor and continues to increase proportional to r across the heliosheath. HELIOSHEATH

Time-dependent structure of the heliopause and heliosheath: Rayleigh-Taylor- like and Kelvin-Helmholtz-like instabilities driven by interstellar neutrals Zank, 2007

An Overview of GCR He Modulation in the Heliosphere Over the Past 39 Years Concentrate on Solar Minimum/Maximum Periods

COSMIC RAY MODULATION

Cycle 23 Voyager Data 01/01-02/22 / (52 Day Periods) 07/02-08/23 / (52 Day Periods)

COSMIC RAY MODULATION Cycle 23 Voyager Data 01/01-02/22 / (52 Day Periods) 07/02-08/23 / (52 Day Periods)

GCR He and H in the Heliosheath Radial Gradients < 0.2 %/AU over The V2 decrease starting in is not understood

Voyager – % Below Webber/Higbie LIS MeV/n GCR He 7.4 %/Year21% 145 – 244 MeV/n GCR He 9.6 %/Year30% MeV GCR H15.5 %/Year44% 30 – 56 MeV/n ACR He 3.5 %/Year MeV GCR E 75.0 %/Year

REGRESSION

COSMIC RAY MODULATION

Webber, 2010

MODEL HELIOSPHERE a)10 MeV Galactic Cosmic Ray Electrons are increasing at a rate of 75%/yr. b)No radial intensity gradient for 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.

DISCUSSION and CONCLUSIONS The ongoing period of very low solar activity has a multi-faceted effect on the modulation process. The increase in MeV/n GCR He at 1 AU 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 at 1 AU. 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. 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 10 Be in polar ice cores and 10 C in tree rings. 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 10 Be.