08/4/2009NAS - SHINE-Suprathermal Radial Evolution (1-11 AU) of Pickup Ions and Suprathermal Ions in the Heliosphere N. A. Schwadron Boston University, Department of Astronomy D. J. McComas and H. Elliott Southwest Research Institute M. E. Hill Johns Hopkins Univ. Applied Physics Lab D.C. Hamilton and R. D. DiFabio Univ. of Maryland R. K. Squier Georgetown University

06/23/2008NAS - SHINE-Suprathermal Distribution Morphology Knee from locally produced pickup ions Tail from stochastic and/or shock acceleration Injection for stochastic acceleration? Acceleration mechanism in the tail?

06/23/2008NAS - SHINE-Suprathermal Cassini’s Unique Trajectory

4 EPREM The Energetic Particle Radiation Environment Model (EPREM) is a physical 3D kinetic model for the transport of energetic particles. - Capable of simulating the transport of protons, electrons, and heavier ions. - Currently run on an event-by-event basis (boundary conditions from ACE & GOES)

06/23/2008NAS - SHINE-Suprathermal EPREM Solver Solves Extended focused transport equations Also included perpendicular diffusion, gradient and curvature drift

EPREM Validation (Stochastic Acceleration) 06/23/2008NAS - SHINE-Suprathermal

06/23/2008NAS - SHINE-Suprathermal The He + /He ++ Ratio with R He + a pickup ion –Density and distribution at the knee fall off as 1/R He ++ a solar wind ion –Density and suprathermal distribution fall off as 1/R 2 He + /He ++ ratio in Energetic Particles should rise with R if locally accelerated

06/23/2008NAS - SHINE-Suprathermal Magnitude of He + /He ++ He + /He ++ increases with higher speeds of injection into stochastic acceleration 12 Ion speed/Solar Wind Speed Distribution Function

06/23/2008NAS - SHINE-Suprathermal Comparison to Cassini (MIMI/CHEMS) Radial dependence confirms increase with R He+/He++ in fed into stochastic acceleration at ~half the solar wind speed (in SW frame)

06/23/2008NAS - SHINE-Suprathermal He + /He ++ invariance with Energy He + /He ++ almost invariant with energy/nuc Requires acceleration rate that depends on particle speed

06/23/2008NAS - SHINE-Suprathermal Puzzling Radial Evolution! He ++ He + “simple” distance varition of stochastic models of stochastic acceleration do not work

06/23/2008NAS - SHINE-Suprathermal Tails have familiar E -1.5 power-law

06/23/2008NAS - SHINE-Suprathermal A Tail Rising with Distance 12 Ion speed/Solar Wind Speed Distribution Function Tail effectively energized with distance r

06/23/2008NAS - SHINE-Suprathermal Conclusions from Cassini/MIMI/CHEMS Ions fed into stochastic acceleration at around v/u = 0.5 in solar wind frame Stochastic acceleration does not have a simple profile with distance (e.g., constant or 1/R) He +/ He ++ ratio similar from 13 to 41 keV/nuc –The acceleration rate (13-41 keV/nuc) best organized by particle speed (e.g., He + and He ++ tails grow at the same rate) –The acceleration rate must be ~day -1 to compensate adiabatic cooling Need to have continuous stochastic accelerationœ through the interplanetary medium

Pickup Hydrogen Distributions in the Solar Wind at ~11 AU D.J. McComas, H.A. Elliott, and N.A. Schwadron Pickup Hydrogen Distributions in the Solar Wind at ~11 AU D.J. McComas, H.A. Elliott, and N.A. Schwadron

SWAP Observation 10/15/08 at ~11.3 AU

8 Hours of data – 32 s Cadence

8-Hour Averaged Ion Distribution Function Analytic: Vasyliunas and Siscoe [1976] Simulations: EPREM [Schwadron et al., 2007] 4:1 inward/outward ratio remains!

New Horizons/SWAP Initial Results Significant suprathermal tail above 2 Vsw “cutoff” Pickup ion anisotropy of inward:outward ions ~4:1  very similar to that observed on Ulysses at ~5 AU –Anisotropy might be some combination of an even longer than previously realized mean free path and significantly more radial magnetic field than expected –We may be missing something Evidence for continued stochastic acceleration of the suprathermal tails

06/23/2008NAS - SHINE-Suprathermal Sources of Stochastic Acceleration –Random Scattering Centers –Transit time damping

06/23/2008NAS - SHINE-Suprathermal Stochastic vs Shock Acceleration Evidence for Stochastic Acceleration: –Local acceleration observed well away from shocks [e.g., Schwadron et al., 1996] –Local acceleration observed where shocks have not yet formed [e.g., Chotoo et al., 2000] At higher energies we see strong evidence for diffusive shock acceleration A two-step process –Where is the injection into stochastic acceleration? –Where is the transition into a diffusive- acceleration dominated regime?

06/23/2008NAS - SHINE-Suprathermal Can the tails be caused by shocks? Very unlikely The observations reported include quiet time data only With a fixed scattering mean free path, the ability for ions from shocks to diffuse across a growing spread in longitudes diminishes with distance