| Powerful high-frequency radio waves modulated at ELF/VLF can efficiently modify the ionosphere, which can act as an equivalent virtual dipole source for the generation of ELF/VLF wave, and these signals can propagate downward into the earth-ionosphere waveguide. In this paper, based on the theory of ionospheric heating, the heating model in different regions of the ionosphere is established; the mechanism of the excited ELF/VLF wave and its propagation in both ionosphere and earth-ionosphere waveguide is analyzed; at the same time the final field on the sea surface is calculated completely. The main works are as follows:1?The relevant parameters of the ionosphere are briefly introduced, and the ionospheric international references model IRI-2012 and atmosphere model MSIS90 are also be given. By using this two models, the changes of electron density, ion density, neutral particle density and electrical conductivity of the ionosphere at different latitudes and time are be analyzed.2?Based on Ohmic heating theory, the model of lower ionosphere heating is been introduced. In this paper the heating locations are Troms? and Arecibo respectively, the former is in the high latitudes while the latter corresponds to low latitudes. The results show that, when the continuous wave is injected into the ionosphere, the saturation time to reach a steady state by electron density is far longer than that for electron temperature. Meanwhile, the two different values at final steady state correspond different initial electron temperature and density.3?In the lower ionosphere for the default value of the natural electric field is 25 m V/m in high latitude while 5m V/m in low latitude. Hall conductivity, Pedersen conductivity and current density by square wave modulated heating can be calculated. The results show that the oscillation period of Hall conductivity and Pedersen conductivity equals to the modulation period, and the phase of the two is opposite. As for the current density and the height of the equivalent dipole, Troms? is greater than Arecibo.4?In the lower ionosphere the final current density generated by two different modulation modes: the square wave modulation and beat-wave modulation are compared, the results show that though beat-wave modulation is less efficient but generates the best quality signal.5 ? Compare with the lower ionospheric heating, the transport process in the high ionosphere cannot be neglected that makes high ionospheric heating more complex. Based on the discrete of the momentum equation, energy equation and continuity equation the variation of electron temperature and density with time can be got by using chase method. Moreover, the theoretical derivation of the beat-wave heating is introduced in detail, and the current density generated by pondermotive force and thermal pressure are numerically simulated. The results show that the pondermotive current density is larger than thermal pressure current density for two orders of magnitude.6?By analyzing the T matrix of the stratified ionosphere, we can conclude that the attenuation rate of ELF/VLF wave propagation is higher when the geomagnetic latitude is low, so in high latitude areas the ELF/VLF wave is more able to propagate downward into the earth-ionosphere waveguide. At the same time, the title downward propagating wave is difficult to spread down for the total reflection occurs. Therefore, the numerical expression of the field on the sea surface generated by the dipole source in the stratified ionosphere is obtained through the quasi-longitudinal approximation.7?Take both the lower and high ionospheric heating into consideration, and put the amplitude and height of the equivalent dipole source generated from the two situations into the numerical expression of the magnetic field on the sea surface, we can easily get the results that the filed from beat-wave heating in high ionosphere is smaller than lower ionosphere heating by about one power of ten. |
PDF Full Text Request