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SHINE 2009 Old Orchard Inn, Nova Scotia, Canada 3-7 August 2009

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Voyager Observations of Anomalous Cosmic Rays A. C. Cummings and E. C. Stone, Caltech F. B. McDonald, University of Maryland B. Heikkila and N. Lal, Goddard Space Flight Center W. R. Webber, University of New Mexico. SHINE 2009 Old Orchard Inn, Nova Scotia, Canada 3-7 August 2009.

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Voyager Observations of Anomalous Cosmic Rays A. C. Cummings and E. C. Stone, CaltechF. B. McDonald, University of MarylandB. Heikkila and N. Lal, Goddard Space Flight CenterW. R. Webber, University of New Mexico

SHINE 2009Old Orchard Inn, Nova Scotia, Canada

3-7 August 2009

  • What do the observations (gradients, energy spectra) in the heliosheath tell us about the ACR transport and are these observations consistent with the models?
    • We think evolution of ACR spectra disfavor conventional stochastic acceleration models
      • acceleration is not happening where V1 and V2 are
    • Observations also disfavor shock motion models
      • V1 and V2 both saw similar, modulated ACR spectra at time of shock crossings
  • Can criteria be established by which some of the theories and/or models could be eliminated by observations in the next few years in the heliosheath?
    • Harder question
      • Surviving models involve modulation of ACRs from remote source
      • Fisk/Gloeckler unconventional stochastic acceleration model can fit ACR spectra at least at one location
      • Kota/McComas & Schwadron flanks/tail model shows radial evolution similar to observations
      • Lazarian & Opher/Drake et al magnetic-reconnection-near-heliopause model presumably will be able to do something similar
    • ACR source intensity greater in A<0 vs A>0 will need to be explained
v1 spectral evolution in heliosheath


V1 spectral evolution in heliosheath








Low-energy TSP part of spectrum not changing – being convected to S/C from TS; ACRs from remote source are unrolling – source effectively beyond V1.

Thanks to LECP team for low-E Ions, He, and O data

acr spectral evolution in heliosheath for v1 and prior to and in heliosheath for v2

ACR spectral evolution in heliosheath for V1 and prior to and in heliosheath for V2

Result of subtracting TSP and GCR

“backgrounds” -> ACRs are steadily

unrolling towards source spectrum

Note: at high energies, no intensity change and no gradient between V1 and V2


ACR spectrum is also unrolling at V1 in heliosheath.

Can separate intensity change due to spacecraft motion thru radial gradient from that due to temporal change in modulation beyond V1 by comparing V1/V2 gradient with gradients from these lines.

acr models
ACR Models
  • Acceleration at site on termination shock remote from nose of heliosphere
    • Along flanks/tail (McComas & Schwadron 2006, Kota 2007, Schwadron et al. 2007)
    • Does yield radial dependence at different energies similar to observations (see Kotal & Jokipii 2006)
  • Conventional stochastic acceleration in heliosheath (e.g., Zhang 2006, Langner & Potgieter 2006, Langner et al. 2006, Ferreira et al. 2007, Moraal et al. 2007)
    • Not enough energy in turbulence – Fisk & Gloeckler 2009
    • No evidence of local acceleration inside 110 AU – this presentation
  • Unconventional stochastic acceleration, mostly near heliopause (Fisk & Gloeckler 2009)
    • Redistribution of energy from core to tail
    • Have matched V1 energy spectra for several species
    • Would predict positive radial gradient, qualitatively in agreement with observations
  • Magnetic reconnection in vicinity of heliopause (Lazarian & Opher 2009, Drake et al. 2009)
    • Magnetic reconnection leads to contracting magnetic islands leads to Fermi acceleration
    • Would predict positive radial gradient too
  • Source of “classic” ACRs is not where V1 and V2 penetrated the termination shock (TS).
  • Observed energy spectra of energetic particles >0.05 MeV/nuc consists of a low-energy component accelerated locally at the TS, the “classic” ACR component accelerated at a remote location, and the galactic cosmic (GCR) component.
  • Intensities of ACRs at mid-energies are increasing even now in the heliosheath. Some of the increase is due to modulation changes beyond V1 and some is due to motion of the spacecraft through a radial gradient.
  • There is almost no gradient of higher-energy ACRs, implying that we are seeing the source intensity of these particles and that the mean free path is very large (V1 and V2 are separated by ~120 AU).
  • There is a strong A>0 vs A<0 source intensity effect, with the intensity being higher by a factor of 2-3 in the A<0 portion of the solar cycle.
  • Need ever more sophisticated models to calculate energy spectra of more than one ACR species at V1 and V2 along their trajectories.
    • Include turning of solar wind velocity
    • Address A<0 vs A>0 ACR source intensity change