GCRs & ACRs Intensities during the last Solar Minimum: Similarities and Differences J. Kόta & J.R. Jokipii University of Arizona, LPL 32 nd ICRC Beijing, August 15, 2011

Last Unusual Solar Minimum Sunspot number low Magnetic field weakest ever recorded Solar Wind pressure low Tilt Angle remained moderate until 2008 did not flatten as in other minima GCRs at record high level ! Neutron Monitors, ACE Iron ACRs track GCRs but did not surpass previous levels Implication: ACR source intensity at TS ? Spectrum may have steepened (Moraal, Leske, Jokipii)

GCRs at record high level in 2009 (Mewaldt et al) MeV/n Iron at ACE Black: prediction from earlier cycle GCR increase is the result of : decreasing B decreasing tilt ?

Last Solar Minimum

ACRs in the last 3 solar cycles (from Ace News) ACRs did not surpass their 1987 level

Modeling GCRs 2.5 D code Changing tilt and field strength are propagated at the SW speed

Modulation of GCRs Changing B and tilt : Qualitative similarity with observations Changing tilt only ( ): GCRs insensitive to high tilt, Peak when HCS flattens

How about Anomalous CRs?  ACRs are high, but not record-setting (Mewaldt et al, ACE news 136). Why are not ACRs at record level?

Steepening ACR Source Spectrum at TS? Plane shock: Steepens if Shock weakens Spherical: (cooling): Escape: Solar Max: Harder Solar Min: Softer

Simulated Time-evolution of ACRs (1-D) ACRs would peak before solar minimum ?? Not observed (though different recovery has been observed (McDonald et al, 2000) Caveat: 1-D spherical model. 2-D with particle drifts may be different Min Max Min

Summary Last unusual solar Minimum offers unique possibilities to gain new insight into the modulation mechanisms GCRs are in qualitative agreement with expectations and simulations ACR source intensity at TS may be lower at weaker Solar Minima. Tempting but.. Simulations tend to predict maximum ACR fluxes before Solar Min. 2-D models may resolve this problem. Results are preliminary, work in progress.

2.5 D Solution: Different Phases of HCS

Global structure of Heliosphere GCR ACRACR SEP

Time Evolution of GCR & ACR (1D) GCR peaks w delay after Solar Min (at Sun) ACR peaks before Solar Min? Not observed Min B

● Are Anomalous Cosmic Rays (ACRs) indeed accelerated at the SW termination shock (TS) ? Likely yes but Bluntness of TS counts ● Weaker B, weaker scattering ● more cooling in expanding SW ● Steeper spectrum ● Lower flux at Earth Try to model… How are ACRs accelerated? Current paradigm

Modeling GCR 2.5 D – including HCS Solar Min Solar max Record high level: combination of weak B and low tilt of HCS ● HCS counts at low tilt only

Summary oLast solar minimum offers unique insight into the physics of GCR modulation, and explore the role of different mechanisms (B, Tilt, etc) oResults are preliminary, work in progress oLong way to go (next: need realistic heliosheath)

Concept/Approach ● “Make everything as simple as possible, but not simpler “

Modeling oOld Model (1988): fully 3-D, + includes TS, CIRs, steady-state in corotating frame - Heliosheath crude, difficult to improve oNew Model: 2.5 D Cheating: omitting radial B, take circles instead of spiral. Time variation are easier to include Can be improved to have realistic Heliosheath (still in progress)

Ariadne’s Thread When Theseus was to go into the labyrinth to fight the Minotauros, Ariadne gave him a ball of thread so that he can find his way out of labyrinth For charged particles, magnetic field lines act as Ariadne’s thread. Field lines are wound into Parker spiral Competing processes: fast field-aligned pro- pagation (long path) vs. cross-field transport.

GeV Cosmic Rays vs Tilt of HCS Sol Max peak flat

Re-acceleration of GCR at the TS Re-acceleration

Constraints oAge: few years - Sampex measuring charge state multiply charged component yields limit for age (Adams, Mewaldt, Jokipii) oInjection IBEX - ENAs Energetic Neutral Atoms created by charge exchange between energetic ions & slow neutrals Surprising results just to come out ACR Mostly singly charged

Model heliosphere, the way to go: include realistic heliosheath Spherical TS: self- inconsistent offset sphere little better

Simulating GCRS

Parker Spiral Field 26-day rotation + radial solar wind = tightly wound spiral field TS

Parker’s Diffusive Equation Diffusive transport equation of energetic charged particles: - assumes near isotropic distribution Diffusion (anisotropic) Drift Convection Cooling/ Acceleration Source Related to regular gyro- motion Polarity/charge dependent – 22 year wave

CR modulation in nutshell Flattish Current Sheet – Particle drifts important Transient Barriers (GMIRs)

ACR: Time-evolution in 2.5D model Sol Max: Sol Min: