Radial diffusion coefficient for electrons at low<i>L</i>values

Newkirk, L. L.; Walt, M.

doi:10.1029/ja073i003p01013

Cited by 47 publications

(15 citation statements)

References 19 publications

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“…Such lifetimes were measured in the artificial belt formed after the 1962 Starfish nuclear detonation and found to be in close agreement with theoretical predictions [Walt, 1964]. However, after a few months the decay rate of trapped electron intensity slowed and this was interpreted as evidence for replenishment by inward diffusion [Newkirk and Walt, 1968;Walt and Farley, 1976]. A similar conclusion was reached in the case of the low-altitude (L ≲ 1.3) natural radiation belt, where observed electron decay rates were also much slower than expected on the basis of atmospheric scattering alone [Selesnick, 2012[Selesnick, , 2016.…”

Section: Introductionsupporting

confidence: 82%

Control of the innermost electron radiation belt by large‐scale electric fields

Selesnick

Blake

2016

JGR Space Physics

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Electron measurements from the Magnetic Electron Ion Spectrometer instruments on Van Allen Probes, for kinetic energies ∼100 to 400 keV, show characteristic dynamical features of the innermost ( L≲1.3) radiation belt: rapid injections, slow decay, and structured energy spectra. There are also periods of steady or slowly increasing intensity and of fast decay following injections. Local time asymmetry, with higher intensity near dawn, is interpreted as evidence for drift shell distortion by a convection electric field of magnitude ∼0.4 mV/m during geomagnetically quiet times. Fast fluctuations in the electric field, on the drift time scale, cause inward diffusion. Assuming that they are proportional to changes in Kp, the resulting diffusion coefficient is sufficient to replenish trapped electrons lost by atmospheric scattering. Major electric field increases cause injections by inward electron transport. An injection associated with the June 2015 magnetic storm is consistent with an enhanced field magnitude ∼5 mV/m. Subsequent drift echoes cause spectral structure.

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Section: Introductionsupporting

confidence: 82%

Control of the innermost electron radiation belt by large‐scale electric fields

Selesnick

Blake

2016

JGR Space Physics

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“…The neoclassical radial diffusion coefficient D L*L* is the lower-right component of the diffusion tensor on the left-hand side of equation (10). We have computed this and plotted it in Figure 5 for several color-coded values of K. Also plotted in Figure 5 as a histogram is the radial diffusion coefficient D LL inferred by Newkirk and Walt (1968) as being needed to account for the longer-than-expected apparent lifetimes observed by Imhof et al (1967). Newkirk and Walt (1968) derived the expected lifetimes from the theory of Walt and MacDonald (1964) for 1.6 MeV electrons.…”

Section: Journal Of Geophysical Research: Space Physicsmentioning

confidence: 99%

“…Imhof et al (1967) hypothesized that the longer apparent lifetimes might be due to radial diffusion acting as a source from higher L-shells to offset the loss from pitch angle diffusion. Newkirk and Walt (1968) calculated the radial diffusion coefficient that would be necessary to produce the apparent lifetimes, assuming that the real lifetimes were dominated by Coulomb collisions and that the radial diffusion process conserved the first two adiabatic invariants, consistent with the then-recent theory of Fälthammar (1965). The radial diffusion coefficients inferred by Newkirk and Walt (1968) showed a strongly inverse variation with L. They acknowledged that this trend was contrary to the radial-diffusion mechanisms studied by Fälthammar (1965), who found a strong increase in the radial diffusion coefficient with increasing L-value.…”

Section: Introductionmentioning

confidence: 99%

“…Newkirk and Walt (1968) calculated the radial diffusion coefficient that would be necessary to produce the apparent lifetimes, assuming that the real lifetimes were dominated by Coulomb collisions and that the radial diffusion process conserved the first two adiabatic invariants, consistent with the then-recent theory of Fälthammar (1965). The radial diffusion coefficients inferred by Newkirk and Walt (1968) showed a strongly inverse variation with L. They acknowledged that this trend was contrary to the radial-diffusion mechanisms studied by Fälthammar (1965), who found a strong increase in the radial diffusion coefficient with increasing L-value. Roederer et al (1973) suggested that the radial diffusion needed to explain the behavior found by Imhof et al (1967) might be caused by pitch angle scattering from Coulomb collisions in the presence of the drift-shell splitting associated with nondipolar azimuthal asymmetries intrinsic to Earth's main magnetic field, as modeled (for example) by the International Geomagnetic Reference Field (IGRF).…”

Section: Introductionmentioning

confidence: 99%

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Neoclassical Diffusion of Radiation‐Belt Electrons Across Very Low L‐Shells

et al. 2018

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In the presence of drift‐shell splitting intrinsic to the International Geomagnetic Reference Field magnetic field model, pitch angle scattering from Coulomb collisions experienced by radiation‐belt electrons in the upper atmosphere and ionosphere produces extra radial diffusion, a form of neoclassical diffusion. The strength of the neoclassical radial diffusion at L < 1.2 exceeds that expected there from radial‐diffusion mechanisms traditionally considered and decreases with increasing L‐shell. In this work we construct a numerical model for this coupled (radial and pitch angle) collisional diffusion process and apply it to simulate raw count‐rate data observed aboard the Gemini spacecraft for several years after the 1962 Starfish nuclear detonation. The data show apparent lifetimes 10–100 times as long as would have been expected from collisional pitch angle diffusion and Coulomb drag alone. Our model reproduces apparent lifetimes for >0.5‐MeV electrons in the region 1.14 < L < 1.26 to within a factor of 2 (comparable to the uncertainty quoted for the observations). We conclude that neoclassical radial diffusion (resulting from drift‐shell splitting intrinsic to International Geomagnetic Reference Field's azimuthal asymmetries) mitigates the decay expected from collisional pitch angle diffusion and inelastic energy loss alone and thus contributes importantly to the long apparent lifetimes observed at these low L‐shells.

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“…The intensity decayed over the next several years, and decay rates were compared to predictions based on theory of electron scattering by the upper atmosphere [Walt, 1964]. They were found to be in substantial agreement for a few months after the Starfish detonation, but subsequent slower decay at lower altitudes (or lower L values) was attributed to replenishment of atmospheric losses by inward radial diffusion [Newkirk and Walt, 1968] (see also the review by Walt and Farley [1976, and references therein]).…”

Section: Introductionmentioning

confidence: 99%

Stochastic simulation of inner radiation belt electron decay by atmospheric scattering

Selesnick

2016

JGR Space Physics

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Decay of inner radiation belt electron intensity, resulting from elastic and inelastic collisions with neutral atoms, ions, and free electrons of the upper atmosphere, ionosphere, and plasmasphere, is described by stochastic Monte Carlo simulation. Modified collision cross sections allow detailed simulation of large‐angle scattering and large‐energy‐loss collisions while preserving mean effective scattering and slowing‐down rates resulting from all collisions. Scattering from bound electrons and δ‐ray production are also included. Results show that traditional methods describing diffusion of the mirror point magnetic field, equivalent to diffusion in equatorial pitch angle, and energy loss by continuous slowing down are generally good approximations. Updated formulae for these approximations are provided. The drift‐averaging approximation is also shown to provide a generally accurate description of trapped electron decay. The approximate methods overestimate decay rates by small factors, and the detailed stochastic simulation should be used when greater accuracy is required.

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Radial diffusion coefficient for electrons at lowLvalues

Cited by 47 publications

References 19 publications

Control of the innermost electron radiation belt by large‐scale electric fields

Control of the innermost electron radiation belt by large‐scale electric fields

Neoclassical Diffusion of Radiation‐Belt Electrons Across Very Low L‐Shells

Stochastic simulation of inner radiation belt electron decay by atmospheric scattering

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