Browsing by Author "Ali, Ashar F."

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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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    Magnetic field power spectra and magnetic radial diffusion coefficients using CRRES magnetometer data
    (Wiley, 2015) Ali, Ashar F.; Elkington, Scot R.; Tu, Weichao; Ozeke, Louis G.; Chan, Anthony A.; Friedel, Reiner H.W.
    We used the fluxgate magnetometer data from Combined Release and Radiation Effects Satellite (CRRES) to estimate the power spectral density (PSD) of the compressional component of the geomagnetic field in the ∼1 mHz to ∼8 mHz range. We conclude that magnetic wave power is generally higher in the noon sector for quiet times with no significant difference between the dawn, dusk, and the midnight sectors. However, during high Kp activity, the noon sector is not necessarily dominant anymore. The magnetic PSDs have a very distinct dependence on Kp. In addition, the PSDs appear to have a weak dependence on McIlwain parameter L with power slightly increasing as L increases. The magnetic wave PSDs are used along with the Fei et al. (2006) formulation to compute DBLL[CRRES] as a function of L and Kp. The L dependence of DBLL[CRRES] is systematically studied and is shown to depend on Kp. More significantly, we conclude that DELL is the dominant term driving radial diffusion, typically exceeding DBLL by 1–2 orders of magnitude.