| Modulated HF heating of the ionosphere provides a feasible means of artificially generating ELF/VLF whistler waves. Since a lot of observation, both ground-based and space-borne, indicates that some of these artificial ELF/VLF waves generated by ionospheric modulation can propagate directly upwards and penetrate through the ionosphere into the overlying magnetosphere, and others propagate within the Earth-ionosphere waveguide and leak up progressively with each reflection off the ionosphere, this paper studies the propagation of these articficial ELF/VLF waves in the space and the resonant interaction with the energetic electron in the magnetosphere from the aspect of numerical simulation and theoretical analysis.The practical significance of this research lies in:first, the generation of ELF/VLF waves by ionospheric modulation can avoid the disadvantaging of the traditional ELF/VLF launch system which is very large and complex. The ELF/VLF waves generated by ionospheric modulation can propagate within the Earth-ionosphere waveguide and have great potential in the applications on the areas of submarine communication, subsurface targets detection and so on. Second, the ELF/VLF waves generated by ionospheric modulation can can leak into the inner magnetosphere and contribute to resonant interactions with high energy electrons. The wave-induced resonant scattering of the energetic electron will lead to the electron precipitation, which is important in the safety protection of the space facilities.The main results of this study are as follows:1. The thesis performs a comprehensive study on the propagation of the ELF/VLF waves artificially generated by ionosphere modulation. Taking into account the difference of background for propagation, we adopt the full-wave method to investigate the propagation of these artificial waves in lower ionosphere and atmosphere, and ray-tracing technique to simulate the propagation path of these artificial waves in the higher ionosphere and magnetosphere.In the study on downward ELF/VLF waves, we first calculate the strength of the ELF/VLF radiation source due to ionosphere modulation, and then the full-wave model is used to analyze attenuation and reflection of the downward ELF/VLF wave. Also, with the HAARP experiment parameter, we calculated the magnetic field of the ELF/VLF signals on the sea as in PT order, which is consistent with the experimental data. The simulation results indicate that the radiation and propagation of these downward ELF/VLF waves is significantly controlled by HF power, modulation frequency and lunch latitude. For the upward ELF/VLF waves, ray-tracing method is used to simulation the trajectory of these artificial waves, as well as the change of wave normal angle in the magnetosphere. The simulation results show that, ELF/VLF waves launched at low latitudes can bounce between the southern and northern hemisphere (due to magentospheric reflection) during their outward propagation. The waves can finally settle down at certain higher L-shells and become highly oblique in the magnetosphere. The wave trajectory mainly depends on the frequency and the latitude when waves are initially launched. When the frequency is lower or the initial latitude is higher, the waves can propagate further. However, the waves launched at mid-and high-latitude are very likely to propagate along the ambient field line (quasi-parallel propagation) and attenuate at the ionosphere without any magnetospheric reflection.These conclusions are significant in the practical applications of the ELF/VLF waves generated by ionosphere modulation. Take the electron precipitation for example, The conclusion about the dependence of ray trajectory on modulation frequency and launch latitude can help select reasonable modulation frequency and latitude in the HF heating experiment according to the target region of electron precipitation, to make the triggered ELF/VLF waves can reach the region, which is the first step in the artificial precipitation of the electron.2. The thesis implement test particle simulations in homogeneous geomagnetic field to investigate the effects of local resonant scattering of energetic electrons due to triggered monotonic (single-frequency) ELF/VLF waves. We select highly oblique waves at L=3according the ray-tracing results. The test particle summation results indicate, while wave-induced changes in pitch angle and kinetic energy of a single electron are stochastic, the change averaged over all test electrons increases monotonically within the resonance timescale, which implies that resonant scattering is an overall characteristic of energetic electrons under the influence of the artificial whistler waves. Computed resonant scattering rates based on the test particle simulations indicate that aritificial ELF/VLF waves with an observable in situ wave amplitude of-10pT can drive efficient local pitch angle scattering of energetic electrons at the magnetic equator, thereby contributing considerably to their precipitation loss and magnetospheric electron dynamics. When the waves become highly oblique during the propagation, besides the fundamental first order resonance, higher order resonances can also drive efficient electron scattering. The results support the feasibility of generating artificially ELF/VLF whistler waves for controlled removal of energetic electrons in the Earth radiation belts. Since it has been practically feasible to run HF heating experiments to trigger artificial ELF/VLF waves at a number of frequencies and produce a broadband frequency spectrum in certain spatial region, we also evaluate the effects of energetic electrons resonant scattering driven by the discrete, multi-frequency artificially generated ELF/VLF waves. The results indicate a stochastic behavior of electrons and a linear profile of pitch angle and kinetic energy variations averaged over all test electrons, which are similar to that associated with single-frequency wave. We also investigate the dependence of diffusion coefficients on the frequency interval (Af) of the discrete, multi-frequency waves. We find that there is a threshold value of Af for which the net diffusion coefficient by multi-frequency whistlers is inversely proportional to Af (proportional to the frequency components Nw) when Af is below the threshold value but it remains unchanged with increasing Af when Δf is larger than the threshold value. It is interpreted due to that the resonant scattering effect of broadband waves is the sum of the effect of each frequency in the "effective frequency band". Our results suggest that modulation frequencies of HF heating the ionosphere can be appropriately selected with reasonable frequency intervals so that better performance of controlled precipitation of high energy electrons in the plasmasphere by artificial ELF/VLF whistler waves can be potentially achieved.Moreover, the computed local diffusion coefficients based on test particle simulations are also quantitatively compared with the quasi-linear diffusion rates obtained using the Full Diffusion Code (FDC) of University of Canifornia, Los Angeles (UCLA), and our test particle simulation results show good agreement with quasi-linear coefficients, confirming the applicability of both methods to evaluate the local resonant diffusion effect of artificial generated ELF/VLF whistlers.3. The thesis extends the localized test particle simulation to bounce-averaged test particle simulation in dipole magnetic field. Based on this more real model, we investigate the effects of energetic electrons resonant scattering driven by parallel-propagating artifical ELF/VLF waves and by highly oblique waves in the outer radiation belt, respectively. Since the parallel-propagating ELF/VLF waves generated at high latitude has more potential in the applications on electron precipitation in outer radiation, its resonant scattering on energetic electron is our focus. We evaluate the effect of resonant scattering of monistic wave, broadband waves with different frequency interval, and non-linear interaction of large amplitude waves, respectively. The simulation results associated with the parallel-propagating ELF/VLF waves show that the bounce-averaged pitch angle scattering driven by the parallel-propagating artificial ELF/VLF waves with an observable in situ amplitude (or mean-root squared amplitude for mult-frequency waves) of～10pT, can be intense near the loss cone with a rate of～10-5s-1, which supports the feasibility of artificial triggering of ELF/VLF whistler waves at middle and high latitude for removal of high energy electrons in days. In addition, the effect of resonant scattering of mult-frequency waves with the same means squared amplitude but different frequency interval are nearly the same. Our investigation also indicates that wave amplitude strongly affects the resonant effect. When wave amplitude is below several hundred of pT, bounce-averaged pitch angle diffusion coefficients are proportional to the square of amplitude. However, when amplitude is large enough, non-linear effect dominates. The electrons are then phase bunched by the wave field and show advective changes of their energy and pitch angle. Some particles could also be phase trapped and stay resonant with the wave for an extended period of time.The feature of resonant interaction between energetic electron and the highly oblique ELF/VLF waves is much more complex than that associated with the parallel-propagating waves, as the Landau resonance and several cyclotron resonances may occur in one period as the electron bounce along the magnetic field. As a result of the spatial limitation of the oblique artificial ELF/VLF waves, only several cyclotron resonances happen for electrons with small equatorial pitch angle, in which lower order resonance generally driven more intense pitch angle scattering and energy diffusion; for electrons with large equatorial pitch angle, only Landau resonance happens; for electrons with medium equatorial pitch angle, both cyclotron resonances and Landau resonance happen in which the Landau resonance generally dominates. The bounce-averaged pitch angle diffusion coefficients driven by artificial highly oblique ELF/VLF waves with an observable in situ amplitude of～10pT, can be～10-5s-1, which supports the feasibility of artificial triggering of highly oblique ELF/VLF whistler waves at low latitude for removal of high energy electrons in days.Moreover, take the cases of parallel-propagating ELF/VLF waves for example, the bounce-averaged diffusion coefficients based on test particle simulations are also quantitatively compared with the quasi-linear bounce-averaged diffusion rates calculated using the FDC, and our test particle simulation results show good agreement with quasi-linear coefficients, confirming the applicability of both methods to assess the bounce-averaged resonant diffusion effect of artificial generated ELF/VLF whistlers. |
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