Bibliography
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Found 1109 entries in the Bibliography.
Showing entries from 1101 through 1109
| 1969 |
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Radial Diffusion of Starfish Electrons A study of the change in electron intensities in the Starfish electron belt from January 1, 1963, to November 3, 1965, indicates that radial diffusion, both inward and outward from L of 1.40, was a significant loss mechanism for these electrons during this period. For L values of 1.20 and below, the indicated steepening of the pitch-angle distributions during this period has been interpreted as the result of a radial diffusion source for each L shell concentrated near the geomagnetic equator. Since pitch-angle diffusion lifetimes are not well known for 1.20 < L < 1.65, a definitive radial diffusion coefficient cannot be computed from these data. A maximum reasonable diffusion coefficient (mean square displacement per unit time) computed for this range of L for this period has a minimum at L of 1.31, and a value of 4.4 \texttimes 10-5 RE\texttwosuperior/day at that point. This maximum coefficient, representing an average over a 3-year period, is more than an order of magnitude too small to account for the apparent radial diffusion of natural electrons into this region that took place in September 1966. The results are, however, consistent with population of the inner zone by radial electron diffusion occurring during relatively short periods during which the diffusion coefficient is enhanced by two or three orders of magnitude. Published by: Journal of Geophysical Research Published on: 07/1969 YEAR: 1969 DOI: 10.1029/JA074i014p03591 |
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Convection Electric Fields and the Diffusion of Trapped Magnetospheric Radiation We explore here the possible importance of time-dependent convection electric fields as an agent for diffusing trapped magnetospheric radiation inward toward the earth. By using a formalism (Birmingham, Northrop, and Fälthammar, 1967) based on first principles, and by adopting a simple model for the magnetosphere and its electric field, we succeed in deriving a one-dimensional diffusion equation to describe statistically the loss-free motion of mirroring particles with arbitrary but conserved values of the first two adiabatic invariants M and J. Solution of this equation bears out the fact that reasonable electric field strengths, correlated in time for no longer than the azimuthal drift period of an average particle, move particles toward the earth at a rate at least an order of magnitude faster than electric fields whose source is a fluctuating current on the magnetopause. Published by: Journal of Geophysical Research Published on: 05/1969 YEAR: 1969 DOI: 10.1029/JA074i009p02169 |
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Diffusion of Equatorial Particles in the Outer Radiation Zone Expansions and contractions of the permanently compressed magnetosphere lead to the diffusion of equatorially trapped particles across drift shells. A general technique for obtaining the electric fields induced by these expansions and contractions is described and applied to the Mead geomagnetic field model. The resulting electric drifts are calculated and are superimposed upon the gradient drift executed by a particle that conserves its first (μ) and second (J = 0) adiabatic invariants. The noon-midnight asymmetry of the unperturbed drift trajectory (resulting from gradient drift alone) is approximated by means of a simple model. In this model the angular drift frequency is found to be the geometric mean of a particle\textquoterights angular drift velocities at noon and midnight. The radial diffusion coefficient D = (\textonehalf) (ΔL)\texttwosuperior/time is calculated as a function of the McIlwain parameter L and in terms of the spectral density of fluctuations in the stand-off distance of the magnetosphere boundary. Because the unperturbed drift trajectories are asymmetric, drift-resonant diffusion of particles is produced by spectral components at all harmonics of the drift frequency, although the first (fundamental) harmonic is the major contributor. Schulz, Michael; Eviatar, Aharon; Published by: Journal of Geophysical Research Published on: 05/1969 YEAR: 1969 DOI: 10.1029/JA074i009p02182 |
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Particle fluxes in the outer geomagnetic field The outer geomagnetic field comprises the outer radiation belt, consisting of electrons with energies of 104\textendash107 ev, and the unstable radiation zone. The outer radiation belt is bounded on its inner side by a gap, which is at various times located at a distance of 2.2\textendash3.5 RE and in which a considerable precipitation of electrons from radiation belts occurs, possibly owing to a high intensity of electromagnetic waves. The boundary separating the outer radiation belt from the unstable radiation zone is at λ \~ 71\textdegree and \~9 RE in the equatorial plane on the sunlit side, and at 7\textendash8 RE in the equatorial plane on the nightside. Beyond this, the unstable radiation zone extends out to the magnetosphere boundary and up to λ \~ 77\textdegree on the sunlit side, and out to 14\textendash15 RE on the nightside. The relatively rapid electron intensity variations with periods of 1\textendash7 days are essentially absent at distances less than that of the outer belt but are distinctly seen in the outer belt. In the unstable radiation zone the intensity of electrons with energies of the order of 105 ev changes by several times, and good correlation is observed with the increase in Kp. Analysis of the outer belt data shows that this belt is formed partly by electron diffusion into the magnetosphere (like the belt of protons with energies of 105\textendash107 ev) and partly by the simultaneous acceleration of electrons at various distances from the earth. A comparison of electron intensity changes with the solar activity cycle shows little or no correlation for electrons with Ee > 40 kev. The intensity of electrons with Ee > 500 kev has changed significantly; in 1964 it was 30 times lower than in 1959. The absence of significant dependence of the diffusion coefficients for electrons with E \~ 104\textendash105 ev on the phase of the solar activity cycle shows that the relatively weak magnetic disturbances that do not change with the phase of the cycle are of major importance in diffusion. This suggests that these magnetic disturbances appear at great distances from the sun because of the instabilities of plasma itself and, therefore, that they depend little on solar activity. Vernov, S.; Gorchakov, E.; Kuznetsov, S.; Logachev, Yu.; Sosnovets, E.; Stolpovsky, V.; Published by: Reviews of Geophysics Published on: 02/1969 YEAR: 1969 DOI: 10.1029/RG007i001p00257 |
| 1968 |
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Radial Diffusion Coefficient for Electrons at 1.76 < L < 5 Radial diffusion by nonconservation of the third adiabatic invariant of particle motion is assumed in analyzing experiments in which electrons appeared to move across field lines. Time-dependent solutions of the Fokker-Planck diffusion equation are obtained numerically and fitted to the experimental results by adjusting the diffusion coefficient. Values deduced for the diffusion coefficient vary from 1.3 \texttimes 10-5 RE\texttwosuperior/day at L = 1.76 to 0.10 RE\texttwosuperior/day at L = 5. In the interval 2.6 < L < 5, the coefficient varies as L10\textpm1. Assuming a constant electron source of arbitrary magnitude at L = 6 and the above diffusion coefficients, the equatorial equilibrium distribution is calculated for electrons with energies above 1.6 Mev. The calculation yields an outer belt of electrons whose radial distribution is in good agreement with the observed belt. The calculated distribution also exhibits an inner belt at L ≈ 1.5. However, the calculated intensity of the inner belt relative to the outer belt is several orders of magnitude smaller than the experimental ratio. Published by: Journal of Geophysical Research Published on: 12/1968 YEAR: 1968 DOI: 10.1029/JA073i023p07231 |
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Radial Diffusion Coefficient for Electrons at Low L Values An empirical evaluation of the diffusion coefficient for trapped electrons diffusing across low L shells is obtained by adjusting the coefficient to account for the observed radial profile and the long-term decay rate of the trapped electron flux. The diffusion mechanism is not identified, but it is assumed that the adiabatic invariants \textmu and J are conserved. The average value of the coefficient for electrons > 1.6 Mev energy is found to decrease monotonically from \~4 \texttimes 10-6 RE\texttwosuperior/day at L = 1.16 to \~2 \texttimes 10-7 RE\texttwosuperior/day at L = 1.20. Published by: Journal of Geophysical Research Published on: 02/1968 YEAR: 1968 DOI: 10.1029/JA073i003p01013 |
| 1966 |
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Velocity Space Diffusion from Weak Plasma Turbulence in a Magnetic Field The quasi-linear velocity space diffusion is considered for waves of any oscillation branch propagating at an arbitrary angle to a uniform magnetic field in a spatially uniform plasma. The space-averaged distribution function is assumed to change slowly compared to a gyroperiod and characteristic times of the wave motion. Nonlinear mode coupling is neglected. An H-like theorem shows that both resonant and nonresonant quasi-linear diffusion force the particle distributions towards marginal stablity. Creation of the marginally stable state in the presence of a sufficiently broad wave spectrum in general involves diffusing particles to infinite energies, and so the marginally stable plateau is not accessible physically, except in special cases. Resonant particles with velocities much larger than typical phase velocities in the excited spectrum are scattered primarily in pitch angle about the magnetic field. Only particles with velocities the order of the wave phase velocities or less are scattered in energy at a rate comparable with their pitch angle scattering rate. Published by: Physics of Fluids Published on: 12/1966 YEAR: 1966 DOI: 10.1063/1.1761629 |
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Limit on Stably Trapped Particle Fluxes Whistler mode noise leads to electron pitch angle diffusion. Similarly, ion cyclotron noise couples to ions. This diffusion results in particle precipitation into the ionosphere and creates a pitch angle distributon of trapped particles that is unstable to further wave growth. Since excessive wave growth leads to rapid diffusion and particle loss, the requirement that the growth rate be limited to the rate at which wave energy is depleted by wave propagation permits an estimate of an upper limit to the trapped equatorial particle flux. Electron fluxes >40 kev and proton fluxes >120 kev observed on Explorers 14 and 12, respectively, obey this limit with occasional exceptions. Beyond L = 4, the fluxes are just below their limit, indicating that an unspecified acceleration source, sufficient to keep the trapped particles near their precipitation limit, exists. Limiting proton and electron fluxes are roughly equal, suggesting a partial explanation for the existence of larger densities of high-energy protons than of electrons. Observed electron pitch angle profiles correspond to a diffusion coefficient in agreement with observed lifetimes. The required equatorial whistler mode wide band noise intensity, 10-2γ, is not obviously inconsistent with observations and is consistent with the lifetime and with limiting trapped particle intensity. Published by: Journal Geophysical Research Published on: 01/1966 YEAR: 1966 DOI: 10.1029/JZ071i001p00001 |
| 1965 |
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Effects of time-dependent electric fields on geomagnetically trapped radiation. Large-scale electric potential fields in the magnetosphere are generally invoked in theories of the aurora. It is shown in the present article that irregular fluctuations of such fields cause a random radial motion of trapped energetic particles by violating the third adiabatic invariant. When the first and second invariants are conserved, any radial motion of the particles is associated with a corresponding energy change. Some particles move outward and others inward; but, if there is a source in the outer magnetosphere and a sink farther in, there will be a net inward transport and an associated net energy gain. This mechanism supplements that of particle transport by magnetic disturbances, which has already been discussed in the literature. The transport and acceleration of energetic particles by fluctuating electric potential fields have a formal similarity to the so-called stochastic mode of acceleration in synchrocyclotrons. In the magnetosphere, the rate of displacement of trapped particles is found to depend on the spectral power density of the fluctuating electric fields at the azimuthal drift frequency and its harmonics. Which of these frequencies is most important depends on the spatial structure of the fluctuations. The observational data needed for numerical evaluation of the rate of transport are still lacking, but the formulas derived serve the purpose of indicating what properties of the fields are important and ought to be measured experimentally. The effects of magnetic time variations, which have been discussed in the literature under special assumptions, are considered in a more general way. A first-order result is given, which applies not only to initial phases of magnetic storms but also to other types of magnetic time variations. Published by: Journal of Geophysical Research Published on: 06/1965 YEAR: 1965 DOI: 10.1029/JZ070i011p02503 |