Theoretical Modeling of the Inner Zone Using GEANT4 Simulations as Inputs

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Theoretical Modeling of theInner Zone Using
GEANT4 Simulations as Inputs

• R. S. Selesnick and M. D. Looper
• The Aerospace Corp., Los Angeles, CA USA
• R. A. Mewaldt
• Caltech, Pasadena, CA USA

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• Current standard NASA AE8/AP8 models of Earths
  radiation belts have known limitations, mostly
  due to limited range of (1960s) observations
  incorporated
• Some energies and spatial regions were not
  sampled, so extrapolation needed
• Only simple (MIN/MAX) account taken of dependence
  on 11-year solar cycle
• Models incorporating newer data also limited
  (e.g., Heynderickx et al. 1999)
• Missions to gather more comprehensive
  measurements in inner zone planned
• Advances in theoretical and computational tools
  allow construction of full physics,
  time-dependent model of inner zone, for
  pure-science or applied use
• GEANT4 simulations of CRAND (Cosmic Ray Albedo
  Neutron Decay) and other processes an important
  part of this NSF-funded project (ATM-0518190)

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• Source Cosmic Ray Albedo Neutron Decay (CRAND)
• Cosmic ray protons modulated by solar activity
  (F10.7), e.g., Usoskin et al. 2002
• Geomagnetic access to atmosphere modeled using
  vertical rigidity cutoff plus transmission
  function to account for off-vertical variation

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• Monoenergetic protons incident isotropically at
  200 km altitude
• Atmosphere modeled with average of NRLMSISE-00 N,
  O, and Ar densities over latitude, longitude,
  local time, and season, with typical geomagnetic
  conditions F10.7 150, Ap 4 (Picone et al.
  2002)
• GEANT4 physics list LHEP_PRECO_HP seemed
  reasonable
• Neutrons exiting top of simulation tabulated
• Mac OS X, dual 2.5GHz G5

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• Neutron production convolved with incident
  modulated/cut-off proton spectrum
• Agreement with observations pretty good,
  especially at crucial higher energies

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• Tangent to a given proton orbit traced back to
  top of atmosphere
• Neutron flux integrated along line of sight to
  give source term (drift averaged)
• Assumes decay proton acquires neutron flight
  direction and energy
• Losses include drift-averaged energy loss and
  scattering, parametrized by the real time history
  of solar activity in realistic atmospheric/ionosph
  eric models
• Adiabatic energization/de-energization due to
  secular changes in magnetic field (with real time
  history) also included important for long-lived
  particles (centuries)
• Loss and adiabatic terms integrated back in time
  to give proton energy history at given K, and L
  (adiabatic invariants for bounce and drift
  motion)
• Energy histories convolved with source function
  and integrated along path to give temporal and
  spatial dependence of fluxes

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• Model results are solid curves, with AP8 shown as
  dashed curves
• Note AP8 stops at 400 MeV, but actual data that
  were used to build it only went to 150 MeV
• These results were from a one-year preliminary
  study we are now in a multi-year project to
  improve the model
• Long residence times at high altitudes mean that
  slower processes (nuclear scattering, radial
  diffusion) will need to be added to the model

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• Other improvements include addition of neutrons
  from cosmic-ray helium to CRAND source (about 10
  of proton incident flux, but higher yield)
• Light-ion secondaries (deuterium, tritium, alphas
  and helium-3, e.g., Looper et al. 1996) have been
  observed and modeled (with AP8 inputs), Selesnick
  Mewaldt 1996
• Both of these, as well as nuclear scattering of
  protons off atmospheric and exospheric nuclei,
  require GEANT4

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Suggestions would be appreciated

• I am a relatively new (1-2 years) GEANT4 user
• The addition of pre-packaged physics lists has
  been a huge help to me!
• Suggestions as to the best ones, for CRAND
  process and others we hope to add, would help
• We are looking for secondaries (neutrons, light
  ions) in the energy range of a few MeV to a few
  GeV, from proton (and later, alpha and other
  light ion) primaries of tens of MeV to tens of
  GeV incident on atmospheric nuclei
• We are also interested in the losses of fragile
  secondaries like deuterium at energies of tens to
  hundreds of MeV due to breakup on atmospheric
  nuclei

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References

• Heynderickx, D., M. Kruglanski, V. Pierrard, J.
  Lemaire, M. D. Looper, and J. B. Blake, A low
  altitude trapped proton model for solar minimum
  conditions based on SAMPEX/PET data, IEEE Trans.
  Nucl. Sci., 46, 14751480, 1999
• Kanbach, G. C. Reppin, and V. Schonfelder,
  Support for Crand theory from measurements of
  Earth albedo neutrons between 70 and 250 MeV, J.
  Geophys. Res., 79, 5159, 1974
• Looper, M. D, J. B. Blake, J. R. Cummings, and R.
  A. Mewaldt, SAMPEX observations of energetic
  hydrogen isotopes in the inner zone, Radiation
  Meas., 26, 967978, 1996
• Picone, J. M., A. E. Hedin, D. P. Drob,
  NRLMSISE-00 empirical model of the atmosphere
  Statistical comparisons and scientific issues, J.
  Geophys. Res., 107, 1468, doi10.1029/2002JA009430
  , 2002
• Preszler, A. M., S. Moon, and R. S. White,
  Atmospheric neutrons, J. Geophys. Res., 81, 4715,
  1976
• Selesnick, R. S. and R. A. Mewaldt, Atmospheric
  production of radiation belt light isotopes, J.
  Geophys. Res., 101, 19,74519,757, 1996
• Usoskin, I. G., K. Alanko, K. Mursula, and G. A.
  Kovaltsov, Heliospheric modulation strength
  during the neutron monitor era, Sol. Phys., 207,
  389399, 2002