Abstract
Abstract. In the present work, a test particle simulation is performed in a model of analytic Ultra Low Frequency, ULF, perturbations in the electric and magnetic fields of the Earth's magnetosphere. The goal of this work is to examine if the radial transport of energetic particles in quiet-time ULF magnetospheric perturbations of various azimuthal mode numbers can be described as a diffusive process and be approximated by theoretically derived radial diffusion coefficients. In the model realistic compressional electromagnetic field perturbations are constructed by a superposition of a large number of propagating electric and consistent magnetic pulses. The diffusion rates of the electrons under the effect of the fluctuating fields are calculated numerically through the test-particle simulation as a function of the radial coordinate L in a dipolar magnetosphere; these calculations are then compared to the symmetric, electromagnetic radial diffusion coefficients for compressional, poloidal perturbations in the Earth's magnetosphere. In the model the amplitude of the perturbation fields can be adjusted to represent realistic states of magnetospheric activity. Similarly, the azimuthal modulation of the fields can be adjusted to represent different azimuthal modes of fluctuations and the contribution to radial diffusion from each mode can be quantified. Two simulations of quiet-time magnetospheric variability are performed: in the first simulation, diffusion due to poloidal perturbations of mode number m=1 is calculated; in the second, the diffusion rates from multiple-mode (m=0 to m=8) perturbations are calculated. The numerical calculations of the diffusion coefficients derived from the particle orbits are found to agree with the corresponding theoretical estimates of the diffusion coefficient within a factor of two.
Highlights
Determining the source and acceleration mechanism of energetic (MeV) particles is one of the main current subjects of research in radiation belt physics
Fei et al (2006) used power spectral densities calculated from the MHD waves, produced by a global MHD simulation of a magnetic storm; test particles were traced in the global MHD fields, and their study showed that the radial diffusion coefficients describe the electron transport quite well, with the asymmetric terms making significant contributions at larger L-shells
We show the results from testparticle simulation in a background dipole magnetic field with superimposed field fluctuations, and we calculate numerically the diffusion coefficient DLL
Summary
Determining the source and acceleration mechanism of energetic (MeV) particles is one of the main current subjects of research in radiation belt physics. Fei et al (2006) used power spectral densities calculated from the MHD waves, produced by a global MHD simulation of a magnetic storm; test particles were traced in the global MHD fields, and their study showed that the radial diffusion coefficients describe the electron transport quite well, with the asymmetric terms making significant contributions at larger L-shells. It has been demonstrated by theoretical calculations and computer simulations that poloidal waves can be mode-converted to toroidal waves which are resonantly excited on closed magnetic field lines where the frequency of the poloidal waves matches the local Alfven frequency (Kivelson and Southwood, 1985; Wright and Rickard, 1995) In the process they transfer their perturbation energy and are dampened. The diffusion rates obtained through the simulation are compared to existing theoretical calculations, which associate the diffusion rate of the electrons with the Power Spectral Density, PSD, of the fluctuations
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