No abstract

A full wave technique has been developed to calculate the field intensities both in the upper ionosphere and on the ground when a dipole source immersed in the lower ionosphere radiates ELF/VLF waves. The radiated wave is divided into a large number of elementary plane waves, for each of which the propagation in the horizontally stratified model consisting of the ionosphere, the free space and the ground is calculated by the full wave technique. Then the plane waves are summed up to give a horizontal distribution of the radiated wave intensities at any altitudes. This technique is applied to investigate the propagation characteristics of radiation from a polar electrojet (PEJ) antenna modulated at an ELF/VLF frequency, which was created by the Tromsø heating facility. Comparison between the results of experiments and the calculation is given in the companion paper [Kimura et al., this issue]. We discuss several propagation characteristics, such as the frequency dependence of the radiated wave propagation up to the upper ionosphere and down to the ground. Under the conditions that the frequency is 2.5 kHz and the ionospheric dc electric field is 5 mV/m, the calculated results are roughly consistent with the experimental results and the radiation efficiency of the PEJ antenna is found to be extremely small, 2.5×10−6 upward and 3×10−8 downward.

A method has been developed for numerically calculating, by means of the full-wave method, the electromagnetic field intensity distribution on the ground and at the satellite altitude (~2000 km) using the electromagnetic wave radiated by an infinitesimal dipole placed in a lower ionosphere. The amplitude and polarization are analyzed for the simultaneous observation data on the ground and the Akebono satellite for the VLF wave radiated in the ionosphere heating experiment by a highpower HF wave. The values computed by the full-wave method agreed well with the observed data if the VLF sources by the heating experiment are at an altitude of 66 km along the east-west direction and the current moment is 4.3 X 104 Am.

[1] Electromagnetic wave radiation from an underground current source related to seismic activity is discussed. In order to estimate the ionospheric effects on the electromagnetic waves associated with the seismic activity, ELF waves in the frequency range from 10 Hz to 1 kHz in the ionosphere radiated from a possible seismic current source modeled as an electric dipole located in the lithosphere, are precisely computed by using a full-wave analysis. In this calculation, the ionosphere is assumed to be an inhomogeneous and anisotropic medium, and the Earth's crust is assumed to be a homogeneous and isotropic conductive medium. Especially, the effects of the geomagnetic field on the ionospheric wave propagation are precisely considered. The results of the calculations in the frequency range from 10 Hz to 1 kHz show frequency dependence in spatial distributions of the wave intensities due to the geomagnetic field-aligned whistler propagation in the ionosphere and the Earth-ionosphere waveguide propagation. Wave intensities which could be observed on the ground and in the ionosphere are determined by assuming the magnitude of the current moment of a seismic dipole source. In a possible situation, the current moment is estimated to be about 80 AÁm/Hz 1/2 which generates a detectable wave magnetic field on the ground just above a seismic source. However, if we try to detect it in the ionosphere, the source current moment must be thousands of times more intense.Citation: Ozaki, M., S. Yagitani, I. Nagano, and K. Miyamura (2009), Ionospheric penetration characteristics of ELF waves radiated from a current source in the lithosphere related to seismic activity, Radio Sci., 44, RS1005,

Abstract. A case of highly dispersed (•,860 s 1/2) whistler-like ELF (700-960Hz) wave was detected with the waveform capture (WFC) receiver aboard the Geotail satellite during a dayside magnetopause skimming. Features of the single event distinguish it from the usual falling tone discrete emissions. By ray-tracing and full-wave calculation, the accessibility of the waves from a ground source of a distorted model magnetosphere were investigated. We propose that a lightning discharge at high latitudes is the most plausible source of the event via a special propagation effect revealed by the ray tracing. We demonstrate that the observed large wavenormal angle with respect to the geomagnetic field, the unusually large level of dispersion, and the lack of the expected nose frequency might be attributable to the frequency-dependent nonducted paths to the satellite. The rare occurrence may partly be caused by the Landau damping by the convecting suprathermal electron beams of several hundred eV. It is also shown that the multiple-hop magnetospherically reflected (MR) whistlers from middle latitudes, though being refracted to higher L shells by the plasmapause, are unable to reach the outer magnetosphere. Interhemisphere ducted propagation is possible for ELF waves along a narrow, modestly (15% or more) enhanced flux tube and shows an upper cutoff frequency governed by the high-latitude minimum-magnetic field "horns" regions and the duct enhancement and diameter. A full-wave calculation also implies no lower-frequency cutoff in the upward ionospheric transmission of lightning radiation down to 200 Hz. Because of the complexity of the various effects that influence the propagation from a ground source and the very low occurrence, effective whistler mode waves diagnostics of the highly variable outer magnetosphere are at present beyond our reach.