Study Of ELF Waves Excited By Ionospheric Artificial Modulation Caused By The Incidence Of High-power High-frequency Radio Waves At Low Latitude

Powerful high-frequency radio waves can efficiently heat the lower ionosphere. If using HF heater modulated at ELF, the electron temperature will oscillate with the modulation frequency and lead to periodic changes of the electric conductivity, resulting in periodica-lly variations of the current present in the heated region which can act as an equivalent ELF virtual antenna for the generation of electromagnetic waves at the modulation frequenccy.The significance of researches on ELF radiation by artificial ionospheric modulation lies as the following:(1) It helps to better understand the physical processes in the ionosphere and discover new ionospheric phenomena, offering the better theory support for modifying actively and control intentionally the ionosphere parameters;(2) The traditional ELF antenna has drawback on covering a huge area and establishing difficultly which can be overcomed by radition of ELF waves by the artificial ionospheric modulation.(3) The ELF waves radiated by artificial ionospheric modulation can propagate into the earth-ionosphere waveguide for the submarine communication and detection of the underground objects.(4) The ELF waves radiated by artificial ionospheric modulation can propagate into the magnetosphere and have wave-particle interaction, leading to the precipitation of the energetic electron so as to reduce its number for protecting the space instrument and prolonging their lifetime.Early in the 1950’s theoretical researches on the possibilities of artificial ionospheric turbulences by powerful HF waves began to be developed. First and exploratory experiments of controlled local heating of the ionosphere by HF radio waves from powerful ground-based transmitters were conducted in the 1970’s, along with some very interesting heating phenomena in which ELF waves radiation by artificial ionospheric modulation having important significance on the science research and military applications become increasing focus so that a large number of experimental campaigns have been implemented and received more and more attractive and valuable observations. Because of the presence of large auroral electrojet currents, these experiments are primarily at the high latitudes and polar region. The observations of many experiments show that there is an electric field eastward driving the strong current called the equatorial current that can offer the precondition for the ionospheric modulation, which also need analysis and should be settled. However, For the costly finan- cial support for HF transmitters and heating experiments, numerical methods basing on the heating physical model and initial ionospheric background model begin to play important roles in deepening our knowledge about ionospheric modulation using the high powerful HF waves. Due to that existed heaters and heating experiments are almost at the high and middle magnetic latitudes, during modeling the background ionospheres take corresponding ionosphere models at high or middle magnetic latitudes for the sake of comparison. Because there exists no HF transmitters in our country, it is of great need to argue for scientifically the possibilities and feasibility of conducting ionospheric modulation experiments in China. Based on the above considerations and current research developments on ionospheric modulation, in this paper both theoretical study on ELF radiation and propagation resulting from artificial ionospheric modulation and numerical simulation are focused on. The conclusions in this paper can be mainly outlined as the following:1. With a summary review of existed ionospheric modulation experimental observations, this paper expounds the mechanisms of ionospheric modulation and builds up the physical model of ionospheric modulation in one dimension basing on the ionospheric HF heating model. Beside the Perdesen conductivity and Hall conductivity, the Cowling conductivity is also taken into primary consideration. The effects of different parameters are estimated in the numerical simulation and compare from the modulation results in high latitude in the same condition.The results of modeling show that the artificial modulation occurs below 90 km in the lower ionosphere at low latitudes.The ionospheric conductivity can oscillate with the pulse cycle. The Pedersen conductivity is opposite to the Hall conductivity near 75 km and the Pedersen conductivity phase will reverse at higher altitudes.The modification of the Hall conductivity dominates in the lower ionosphere and at higher altitudes the variation of the Pedersen conductivity is larger than the Hall conductivity.The effect of the modulated heating in the lower ionosphere in the X-mode is better than that in the O-mode.The radiated power may have a critical value. Keeping increasing the radiated power will not have a larger influence on the changes of the conductivities. For the different modulation frequencies, the effect will be less when the modulated frequency increases because the heating time in every pulse of different frequencies is different. It can estimate the electron characteristic time according to the ionospheric current oscillation at some altitude. The effects of ionosph- eric modulation at low latitude is also obvious which is better than the effects at high latitude on the conductivity modification, However, the electric field at high latitude is one order of magnitude larger than that at law latitude so that the total effects at high latitude is still better. Within the error bounds, the results of modeling correspond to the experiment observations favorably and satisfactorily.2. The full-wave infinite method is used to establish the propagation model of ELF waves at lower ionosphere and introduce the arithmetic processes carefully. In this model, the lower ionosphere is considered as a stratified medium. The area we discussed is separa-ted into two parts:the area above the source and the area below the source including the ionosphere and atmosphere. For the first area, the signal radiated from the source will be reflected by the upper boundaries in each layer. It is assumed that the boundary of the top layer is free, so the reflection coefficient is equal to zero and we use recursive method to calculate Rui by Ru(i+1). For the second area, the signal radiated from the source will be reflected by the lower boundary. We assume that the sea is conduction so the electric field is zero, then we calculate the reflection coefficient using Maxwell equation in free space and use Rd(i+1) to obtain Rdi recursively. In this way, we can get all reflection coefficients in each layer. The exitation coefficient can be obtained by the source producing by the artificial ionospheric modulation so that we can get the amplit- ude of the upward propagating and downward propagating mode waves. For the area above the source, we use Ui to calculate Ui-1 recursively and get the Di of downward propa- gating mode waves using the reflection coefficient. For the area below the source, we use Di+1 to calculate Di recursively and get the Ui of upward propagating mode waves using the reflection coefficient. In this way, we can get all the Fourier composition of the field in each layer, then, we use the inverse transform to calculate the total field.The results of the model show that the full-wave model is correspond to the experiment observations favorably and satisfactorily. The field amplitude of the radiated signals on the sea is～pT. The upward propagating mode wave is in a narrow beam corresponding to the size of the radiating current region and the downward propagating mode waves can propagating into the Earth-ionosphere guide. With the decreasing latitude, the attenuation of the waves become larger, so the field received on the ground is smaller. On the other hand, the attenuation of the radiated waves from the same position become larger with the decreasing modulation frequency and the critical angle is too small at low latitude so that even if the radiating source can be formed at low latitude due to the natural equatorial electrojet, the waves radiated from the source will be hard to propa- gate into the Earth-ionosphere waveguide because the larger attenuation resulting of energy losing greatly.3. The ray tracing theory is illustrated carefully in this paper. The 2D ray tracing eqations about the LF wave propagation in the magnetosphere and magnetosphere background model is given. In addition, the phase refractive index as well as its partial derivatives needed in numerical stimulation. The theory of the cold plasma dispersion relation is studied. We discuss the reason of the magnetosphere reflection which shows that the refractive index surface including ions is very different from that including electrons only. The surface only including electrons is not close so that the surface has the resonance cone and the phase refractive index in this area will become infinite which make the magnetosphere reflection can not happen. On the other hand, the surface only including ions is close for the lower wave frequency so that the magnetospheric reflection happes during the ray change its direction.The results of the numerical stimulation according to the ray tracing theory in the magnetosphere show that we can get the ray path. For different signals starting from the same latitude, the L-shell they can propagate become smaller with the increasing frequency, while the lower frequencies lead to the increasing L-shell they can propagate. The signals can reflect between northern and southern hemispheres and propagate to the far area and during the process, the variation of the L-shell become smaller which is close to the constant in the end. On the other hand, the wave normal angle becomes smaller. It means that the ELF waves tend to propagate along the ambient magnetic field vector. For the signals starting from the higher latitude, it is easier to form the PL mode whistlers.