Auburn University
http://aurora.auburn.edu:80
The DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.2022-10-02T14:16:21ZModeling the loss of inner belt protons by magnetic field line curvature scattering
https://aurora.auburn.edu/handle/11200/50372
Modeling the loss of inner belt protons by magnetic field line curvature scattering
The sudden loss of energetic protons in the inner radiation belt has been observed during geomagnetic storms. It is hypothesized that this sudden loss occurs because of changes in the geomagnetic field configuration which lead to a breakdown of the first adiabatic invariant, mu, in a process called magnetic field line curvature scattering or mu scattering. Comparison of observations to various analytic model predictions for mu scattering induced loss has, however, yielded discrepancies. To better understand how well the analytic models predict the proton loss, test particle simulations are carried out for various magnetic field configurations. Although our simulation results agree well with the analytic models for single-scattering events, the results after cumulative mu scattering can show significant disagreement with the theoretical predictions based on analytic models. In particular, we find the assumption that protons with predicted initial delta mu/mu 0.01 or epsilon > 0.1 are ultimately lost overestimates the proton loss. Based on the test particle simulation results, we develop a new empirical model, called the epsilon-onset model, to predict the minimum value of e at which all protons of a given pitch angle and energy can be assumed to be lost due to mu scattering. By applying our epsilon-onset model as the variable cutoff condition between trapping and detrapping, we obtain very good agreement between theoretical predictions and the simulation results for a range of Kp, suggesting that the epsilon-onset model can potentially serve as an easy-to-use and more reliable predictor of inner belt proton loss due to mu scattering than the previously used fixed-valued cutoff conditions.
Foreshock wave interaction with the magnetopause: Signatures of mode conversion
https://aurora.auburn.edu/handle/11200/50371
Foreshock wave interaction with the magnetopause: Signatures of mode conversion
Our previous hybrid simulation (Shi et al., 2013) under a radial interplanetary magnetic field (IMF) and a supercritical solar wind Mach number has shown that foreshock compressional waves originated from the quasi-parallel (Q-parallel to) shock are mode converted to kinetic Alfven waves (KAWs) at the Alfven resonance surface of the subsolar magnetopause. In this paper, three-dimensional global dayside mode conversion is investigated for cases under various solar wind conditions using the global hybrid model. The global patterns and propagations of KAWs are distinguished and presented. Under a near-critical Mach number (M-A = 3), KAW structures due to mode conversion exhibit a feature of broader excitation regions in the magnetopause boundary layer (MPBL) compared to supercritical Mach number (M-A = 5) shocks. For cases with an oblique IMF with supercritical Mach numbers (M-A = 5), the amplitude of magnetosheath compressional waves is larger at the quasi-parallel shock (Q-parallel to) than at the quasi-perpendicular (Q-perpendicular to) shock. Downstream of the Q-parallel to shock, there is a general trend that the perturbations of density (N) and magnetic field (B) change from predominantly in-phase in the magnetosheath to antiphase near the MPBL. While downstream of the Q-perpendicular to shock, an antiphase relation between N and B is dominant throughout the magnetosheath and magnetopause except near the shock transition. The compressional drivers are found to reach an extended region of the magnetopause due to the combined effects of wave propagation in the plasma frame and flow convection, leading to a broad region of mode conversion at the magnetopause. Subsequently, the resulting KAWs can be carried to the regions downstream of the Q-perpendicular to shock owing to the flow convection at the magnetopause. The KAWs propagate poleward along the geomagnetic field lines and meanwhile are carried tailward by the ambient flows, and they are more intense in the downstream of Q-parallel to shocks than downstream of Q-perpendicular to shocks.
Investigation of storm timemagnetotail and ion injection using three-dimensional global hybrid simulation
https://aurora.auburn.edu/handle/11200/50370
Investigation of storm timemagnetotail and ion injection using three-dimensional global hybrid simulation
Dynamics of the near-Earth magnetotail associated with substorms during a period of extended southward interplanetary magnetic field is studied using a three-dimensional (3-D) global hybrid simulation model that includes both the dayside and nightside magnetosphere, for the first time, with physics from the ion kinetic to the global Alfvenic convection scales. It is found that the dayside reconnection leads to the penetration of the dawn-dusk electric field through the magnetopause and thus a thinning of the plasma sheet, followed by the magnetotail reconnection with 3-D, multiple flux ropes. Ion kinetic physics is found to play important roles in the magnetotail dynamics, which leads to the following results: (1) Hall electric fields in the thin current layer cause a systematic dawnward ion drift motion and thus a dawn-dusk asymmetry of the plasma sheet with a higher (lower) density on the dawnside (duskside). Correspondingly, more reconnection occurs on the duskside. Bidirectional fast ions are generated due to acceleration in reconnection, and more high-speed earthward flow injections are found on the duskside than the dawnside. Such finding of the dawn-dusk asymmetry is consistent with recent satellite observations. (2) The injected ions undergo the magnetic gradient and curvature drift in the dipole-like field, forming a ring current. (3) Ion particle distributions reveal multiple populations/beams at various distances in the tail. (4) Dipolarization of the tail magnetic field takes place due to the pileup of the injected magnetic fluxes and thermal pressure of injected ions, where the fast earthward flow is stopped. Oscillation of the dipolarization front is developed at the fast-flow braking, predominantly on the dawnside. (5) Kinetic compressional wave turbulence is present around the dipolarization front. The cross-tail currents break into small-scale structures with where is the perpendicular wave number. A sharp dip of magnetic field strength is seen just in front of the sharp rise of the magnetic field at the dipolarization front, mainly on the duskside. (6) A shear flow-type instability is found on the duskside flank of the ring current plasma, whereas a kinetic ballooning instability appears on the dawnside. (7) Shear Alfvenic waves and compressional waves are generated from the tail reconnection, and they evolve into kinetic Alfvn waves in the dipole-like field region. Correspondingly, multiple field-aligned current filaments are generated above the auroral ionosphere.
LOW-ALTITUDE OBSERVATIONS AND MODELING OF QUASI-STEADY MAGNETOPAUSE RECONNECTION
https://aurora.auburn.edu/handle/11200/50369
LOW-ALTITUDE OBSERVATIONS AND MODELING OF QUASI-STEADY MAGNETOPAUSE RECONNECTION
Data from two near-conjugate passes of DE 1 and DE 2 through the cusp/cleft region of the Earth's magnetosphere are presented and compared with model calculations of particle transport from the solar wind to spacecraft locations in the magnetosphere. Comparison of the observed and calculated particle spectra shows that the model can successfully match the spectra at both spacecraft using the same model parameters. This demonstrates that the modeling technique is applicable at both high and low altitudes. We are also able to conclude that the particles originate from a fairly narrow spatial region on the magnetopause even though magnetosheath plasma has access to the magnetosphere over the entire magnetopause in the model. The success of the model in reproducing key features of the observed spectra and the fact that the two satellites in near magnetic conjunction but at different altitudes observed similar, distinctive features at times separated by 10 - 20 min demonstrates that there are quasi-stationary, spatial features in the cusp/cleft region of the Earth's magnetosphere.