Browsing by Author "Malaspina, David M."

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    Electric and magnetic radial diffusion coefficients using the Van Allen probes data
    (Wiley, 2016) Ali, Ashar F.; Malaspina, David M.; Elkington, Scot R.; Jaynes, Allison N.; Chan, Anthony A.; Wygant, John; Kletzing, Craig A.
    ULF waves are a common occurrence in the inner magnetosphere and they contribute to particle motion, significantly, at times. We used the magnetic and the electric field data from the Electric and Magnetic Field Instrument Suite and Integrated Sciences (EMFISIS) and the Electric Field and Waves instruments (EFW) on board the Van Allen Probes to estimate the ULF wave power in the compressional component of the magnetic field and the azimuthal component of the electric field, respectively. Using L∗, Kp, and magnetic local time (MLT) as parameters, we conclude that the noon sector contains higher ULF Pc-5 wave power compared with the other MLT sectors. The dawn, dusk, and midnight sectors have no statistically significant difference between them. The drift-averaged power spectral densities are used to derive the magnetic and the electric component of the radial diffusion coefficient. Both components exhibit little to no energy dependence, resulting in simple analytic models for both components. More importantly, the electric component is larger than the magnetic component by one to two orders of magnitude for almost all L∗ and Kp; thus, the electric field perturbations are more effective in driving radial diffusion of charged particles in the inner magnetosphere. We also present a comparison of the Van Allen Probes radial diffusion coefficients, including the error estimates, with some of the previous published results. This allows us to gauge the large amount of uncertainty present in such estimates.
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    Simulation of radiation belt wave-particle interactions in an MHD-particle framework
    (Frontiers Media S.A., 2023) Chan, Anthony A.; Elkington, Scot R.; Longley, William J.; Aldhurais, Suhail A.; Alam, Shah S.; Albert, Jay M.; Jaynes, Allison N.; Malaspina, David M.; Ma, Qianli; Li, Wen
    In this paper we describe K2, a comprehensive simulation model of Earth’s radiation belts that includes a wide range of relevant physical processes. Global MHD simulations are combined with guiding-center test-particle methods to model interactions with ultra low-frequency (ULF) waves, substorm injections, convective transport, drift-shell splitting, drift-orbit bifurcations, and magnetopause shadowing, all in self-consistent MHD fields. Simulation of local acceleration and pitch-angle scattering due to cyclotron-scale interactions is incorporated by including stochastic differential equation (SDE) methods in the MHD-particle framework. The SDEs are driven by event-specific bounce-averaged energy and pitch-angle diffusion coefficients. We present simulations of electron phase-space densities during a simplified particle acceleration event based on the 17 March 2013 event observed by the Van Allen Probes, with a focus on demonstrating the capabilities of the K2 model. The relative wave-particle effects of global scale ULF waves and very-low frequency (VLF) whistler-mode chorus waves are compared, and we show that the primary acceleration appears to be from the latter. We also show that the enhancement with both ULF and VLF processes included exceeds that of VLF waves alone, indicating a synergistic combination of energization and transport processes may be important.
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    Testing the Organization of Lower-Band Whistler-Mode Chorus Wave Properties by Plasmapause Location
    (Wiley, 2021) Malaspina, David M.; Jaynes, Allison N.; Elkington, Scot; Chan, Anthony; Hospodarsky, George; Wygant, John
    Lower-band whistler-mode chorus waves are important to the dynamics of Earth's radiation belts, playing a key role in accelerating seed population electrons (hundreds of keV) to relativistic (>1 MeV) energies, and in scattering electrons such that they precipitate into the atmosphere. When constructing and using statistical models of lower-band whistler-mode chorus wave power, it is commonly assumed that wave power is spatially distributed with respect to magnetic L-shell. At the same time, these waves are known to drop in power at the plasmapause, a cold plasma boundary which is dynamic in time and space relative to L-shell. This study organizes wave power and propagation direction data with respect to distance from the plasmapause location to evaluate what role the location of the plasmapause may play in defining the spatial distribution of lower-band whistler-mode chorus wave power. It is found that characteristics of the statistical spatial distribution of equatorial lower-band whistler-mode chorus are determined by L-shell and are largely independent of plasmapause location. The primary physical importance of the plasmapause is to act as an Earthward boundary to lower-band whistler-mode chorus wave activity. This behavior is consistent with an equatorial lower-band whistler-mode chorus wave power spatial distribution that follows the L-shell organization of the particles driving wave growth.