POLARIZATION OF VLF ATMOSPHERICS NEAR THE RESONANCE FREQUENCY OF EARTH - IONOSPHERE WAVEGUIDE BY OBSERVATIONS IN THE AURORAL REGION

Ground-based recording of ELF-VLF waves with right-handed (R) and left-handed (L) circular polarization has been performed at the frequency range of 0-10 kHz in Northern Finland. Monitoring showed a difference in the behavior of VLF waves with R- and L-polarization. The waves with a perfect circular L- polarization were observed at and just above the Earth-ionosphere waveguide cutoff frequency, which is the critical frequency of the first transverse resonance in Earth-ionosphere waveguide (around 1.6-2.3 kHz). To study the features of VLF wave propagation near the mode cutoff frequencies, we derived the attenuation of VLF waves with right (R)- and left (L)-polarization from the full wave equation for different models of the ionosphere. Our calculations show that a maximum in the spectra of VLF waves near the critical frequencies of 1.6 - 2.3 kHz due to small absorption of L waves and excitation of resonance waves in Earth - ionosphere waveguide. Introduction The atmospheric waveforms are radiated from lightning and able to propagate up to several thousands of kilometers very often. The atmospherics offer an easy way to analyze the effects of Earth-ionosphere wave-guide on propagating ELF-VLF waves. The atmospheric contain a great deal of information about the state of the ionosphere along the propagation path. Important peculiarities of Earth-ionosphere waveguides have been obtained from the analysis of a special type atmospheric, so-called the tweek ELF/VLF atmospherics. The tweeks have a sharp maximum in amplitude and strong dispersion near the critical frequency of the waveguide, which is ~ 1.6-2.3 kHz (Mikhaylova, 1988). The wave polarization of tweek at the frequency above the cutoff frequency is always left-handed (Yedemsky et al., 1992; Hayakawa et al., 1995). The propagation mechanism of tweek atmospherics near the mode cutoff frequencies in the presence of an anisotropic homogeneous ionosphere was investigated by Yamashita (1977). Numerical calculations showed very low attenuation in a narrow frequency range just above the mode cutoff frequency. These results are consistent with the observations of a sharp maximum in the tweek spectra. Explanation of the left-handed (L) polarization at the frequency above the cutoff frequency has been given by Ryabov (1992) for a simple model that was earlier used by Yamashita (1977). In the model it is assumed that the Earth-ionosphere waveguide has a plane sharp boundary, and the Earth's magnetic field is vertical. In this paper we analyze properties of spectrum, and polarization of atmospherics in auroral latitudes and explain the observed features based on the solutions of full wave equation. Experimental data