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Found 7 entries in the Bibliography.
Showing entries from 1 through 7
| 2019 |
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Variability of the Proton Radiation Belt Significant steady but slow variability of radiation belt proton intensity, in the energy range \~19\textendash200 MeV and for L<2.4, has been observed in an empirical model derived from data taken by Van Allen Probes during 2013\textendash2019. It is compared to predictions of a theoretical model based on measured initial and boundary conditions. Two aspects of the variability are considered in detail and require adjustments to model parameters. Observed inward transport of proton intensity maxima near L=1.9 and associated increasing intensity are caused in the model by inward radial diffusion from an external source while conserving the first two adiabatic invariants. The diffusion coefficient is constrained by these observations and is required to have increased near the start of 2015 by a factor \~2. Observed decay of proton intensity at L<1.6 can be caused only in part by energy loss to free and bound electrons in the local plasma and neutral atmosphere. Another, unknown loss mechanism is required to match observed proton decay rates as a function of energy. Accounting for the expected influence of slow radial diffusion at low L, the additional loss should have a mean lifetime near 22 years, independent of L and energy in the range \~19\textendash70 MeV. Several candidate loss mechanisms are considered\textemdashadded plasma or neutral density, elastic Coulomb scattering, plasma wave scattering, field-line curvature scattering, and collision with orbital debris\textemdashbut none are found viable. Published by: Journal of Geophysical Research: Space Physics Published on: 07/2019 YEAR: 2019 DOI: 10.1029/2019JA026754 |
| 2018 |
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Modeling the Proton Radiation Belt With Van Allen Probes Relativistic Electron-Proton Telescope Data An empirical model of the proton radiation belt is constructed from data taken during 2013\textendash2017 by the Relativistic Electron-Proton Telescopes on the Van Allen Probes satellites. The model intensity is a function of time, kinetic energy in the range 18\textendash600 MeV, equatorial pitch angle, and L shell of proton guiding centers. Data are selected, on the basis of energy deposits in each of the nine silicon detectors, to reduce background caused by hard proton energy spectra at low L. Instrument response functions are computed by Monte Carlo integration, using simulated proton paths through a simplified structural model, to account for energy loss in shielding material for protons outside the nominal field of view. Overlap of energy channels, their wide angular response, and changing satellite orientation require the model dependencies on all three independent variables be determined simultaneously. This is done by least squares minimization with a customized steepest descent algorithm. Model uncertainty accounts for statistical data error and systematic error in the simulated instrument response. A proton energy spectrum is also computed from data taken during the 8 January 2014 solar event, to illustrate methods for the simpler case of an isotropic and homogeneous model distribution. Radiation belt and solar proton results are compared to intensities computed with a simplified, on-axis response that can provide a good approximation under limited circumstances. Selesnick, R.; Baker, D.; Kanekal, S.; Hoxie, V.; Li, X.; Published by: Journal of Geophysical Research: Space Physics Published on: 01/2018 YEAR: 2018 DOI: 10.1002/2017JA024661 |
| 2017 |
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Radiation-Induced Single-Event Effects on the Van Allen Probes Spacecraft Electronic devices on the Van Allen Probes mission have experienced more than a thousand single-event effects (SEE) during the 4.5 years of transit through the inner and outer earth trapped radiation belts. The majority of these SEE have been due to trapped protons determined by the orbit timing and the dose rate response of the engineering radiation monitor. Fault tolerant systems engineering and spacecraft operation have enabled a successful mission to date without a safe mode or spacecraft emergency. Maurer, Richard; Fretz, Kristin; Angert, Matthew; Bort, David; Goldsten, John; Ottman, Geffrey; Dolan, Jeff; Needell, Gerald; Bodet, David; Published by: IEEE Transactions on Nuclear Science Published on: 09/2017 YEAR: 2017 DOI: 10.1109/TNS.2017.2754878 Space vehicles; Probes; Belts; Orbits; Monitoring; protons; Observatories; Van Allen Probes |
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Transverse eV ion heating by random electric field fluctuations in the plasmasphere Charged particle acceleration in the Earth inner magnetosphere is believed to be mainly due to the local resonant wave-particle interaction or particle transport processes. However, the Van Allen Probes have recently provided interesting evidence of a relatively slow transverse heating of eV ions at distances about 2\textendash3 Earth radii during quiet times. Waves that are able to resonantly interact with such very cold ions are generally rare in this region of space, called the plasmasphere. Thus, non-resonant wave-particle interactions are expected to play an important role in the observed ion heating. We demonstrate that stochastic heating by random transverse electric field fluctuations of whistler (and possibly electromagnetic ion cyclotron) waves could explain this weak and slow transverse heating of H+ and O+ ions in the inner magnetosphere. The essential element of the proposed model of ion heating is the presence of trains of random whistler (hiss) wave packets, with significant amplitude modulations produced by strong wave damping, rapid wave growth, or a superposition of wave packets of different frequencies, phases, and amplitudes. Such characteristics correspond to measured characteristics of hiss waves in this region. Using test particle simulations with typical wave and plasma parameters, we demonstrate that the corresponding stochastic transverse ion heating reaches 0.07\textendash0.2 eV/h for protons and 0.007\textendash0.015 eV/h for O+ ions. This global temperature increase of the Maxwellian ion population from an initial Ti\~0.3Ti\~0.3 eV could potentially explain the observations. Artemyev, A.; Mourenas, D.; Agapitov, O.; Blum, L.; Published by: Physics of Plasmas Published on: 02/2017 YEAR: 2017 DOI: 10.1063/1.4976713 electric fields; Electrostatic Waves; protons; Van Allen Probes; Wave power; Whistler waves |
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Spectra of keV protons related to ion-cyclotron wave packets We use the Fokker-Planck-Kolmogorov equation to study the statistical aspects of stochastic dynamics of the radiation belt (RB) protons driven by nonlinear electromagnetic ion-cyclotron (EMIC) wave packets. We obtain the spectra of keV protons scattered by these waves that show steeping near the gyroresonance, the signature of resonant wave-particle interaction that cannot be described by a simple power law. The most likely mechanism for proton precipitation events in RBs is shown to be nonlinear wave-particle interaction, namely, the scattering of RB protons into the loss cone by EMIC waves. Khazanov, K.; Sibeck, D.; Tel\textquoterightnikhin, A.; Kronberg, T.; Published by: Physics of Plasmas Published on: 01/2017 YEAR: 2017 DOI: http://dx.doi.org/10.1063/1.4973323 Diffusion; Particle precipitation; protons; Van Allen Probes; wave particle interactions; Wave power |
| 2016 |
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Inward diffusion and loss of radiation belt protons Radiation belt protons in the kinetic energy range 24 to 76 MeV are being measured by the Relativistic Electron Proton Telescope on each of the two Van Allen Probes. Data have been processed for the purpose of studying variability in the trapped proton intensity during October 2013 to August 2015. For the lower energies (≲32 MeV), equatorial proton intensity near L = 2 showed a steady increase that is consistent with inward diffusion of trapped solar protons, as shown by positive radial gradients in phase space density at fixed values of the first two adiabatic invariants. It is postulated that these protons were trapped with enhanced efficiency during the 7 March 2012 solar proton event. A model that includes radial diffusion, along with known trapped proton source and loss processes, shows that the observed average rate of increase near L = 2 is predicted by the same model diffusion coefficient that is required to form the entire proton radiation belt, down to low L, over an extended (\~103 year) interval. A slower intensity decrease for lower energies near L = 1.5 may also be caused by inward diffusion, though it is faster than predicted by the model. Higher-energy (≳40 MeV) protons near the L = 1.5 intensity maximum are from cosmic ray albedo neutron decay. Their observed intensity is lower than expected by a factor \~2, but the discrepancy is resolved by adding an unspecified loss process to the model with a mean lifetime \~120 years. Selesnick, R.; Baker, D.; Jaynes, A.; Li, X.; Kanekal, S.; Hudson, M.; Kress, B.; Published by: Journal of Geophysical Research: Space Physics Published on: 03/2016 YEAR: 2016 DOI: 10.1002/2015JA022154 |
| 2015 |
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The Van Allen Probes mission provides an unprecedented opportunity to make detailed measurements of electrons and protons in the inner magnetosphere during the weak solar maximum period of cycle 24. The MagEIS suite of sensors measures energy spectra and fluxes of charged particles in the space environment. The calculations show that these fluxes result in electron deposition rates high enough to cause internal charging. We use omnidirectional fluxes of electrons and protons to calculate the dose under varying materials and thicknesses of shielding. We show examples of charge deposition rates during the times of nominal and high levels of penetrating fluxes in the inner magnetosphere covering the period from the beginning of 2013 through mid-2014. These charge deposition rates are related to charging levels quite possibly encountered by shielded dielectrics with different resistivities. Using a simple model, we find temporal profiles for different materials showing the long-term charge deposition rate and estimated charge density levels reaching high levels. These levels are an indicator of internal charging rates that satellites might possibly experience in the inner magnetosphere. The results are compared with charge densities that can induce internal electrostatic discharge. Skov, Tamitha; Fennell, Joseph; Roeder, James; Blake, Bernard; Claudepierre, Seth; Published by: IEEE Transactions on Plasma Science Published on: 09/2015 YEAR: 2015 DOI: 10.1109/TPS.2015.2468214 artificial satellites; dielectric materials; electrons; Energy measurement; MAGEis; Magnetosphere; particle detectors; protons; Van Allen Probes |
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