Detailed wave characteristics (incident and azimuthal angles, wave polarization, and their frequency dependences) of tweek atmospherics have been elucidated on the basis of the application of our field‐analysis direction finding to the VLF data observed in southern China. It is then found that the wave polarization of the first‐order mode at the frequency above its cutoff frequency (ƒ1c = 1.7 ∼ 1.8 kHz) is always left handed and it becomes exactly left‐handed circular when the wave frequency decreases down to the ƒ1c, together with the fact the incident angle becomes zero (vertical) when the frequency approaches ƒ1c. Whereas the zeroth‐order mode at frequency below ƒ1c is found to be linearly polarized. These wave properties are first interpreted qualitatively, and then we try to explain them in terms of the full wave theory in the Earth‐ionosphere waveguide in which the realistic electron density profiles of the lower ionosphere are assumed. Finally, it is suggested that the measurement of wave polarization, as given in this paper, would be useful in studying the formation mechanism of tweek tails, the coupling into whistler mode waves of lightning discharges, the lower ionospheric density profile, etc.

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The general consensus now is that some observations of Trimpis are not due to energetic electron precipitation from the magnetosphere\ but are a result of scattering from D region inhomogeneities caused directly by the electric _eld or EM radiation _eld of the lightning discharge[ In this connection a recent topic of interest is that of Sprites\ or cloud to ionosphere discharges "Dowden et al [ "0885#^Strangeways "0885#^Schonland and Craib "0816##[ These are believed to consist of narrow columns of enhanced electron density with a value

The purpose of this paper is to investigate the effect of localized perturbations in the lower ionosphere on subionospheric VLF propagation by means of the finite element methos. Owing to the complexity of the method, we have analyzed only a two‐dimensional case where a localized perturbation lies on the great circle path between the transmitter and receiver. The first‐order mode is assumed to be incident into the waveguide, and we have found significant oscillations in the changes of amplitude and phase in the vicinity of the perturbation (distance less than 600 km), which are due to mode conversion (i.e., conversion to higher‐order modes) and subsequent multiple‐mode propagation. In this paper, we present many computational results of the spatial distribution of scattered fields, frequency dependence, effects of enhancement factor, horizontal and vertical scale, and altitude of the perturbation, etc. Some important findings are summarized as follows: (1) The study of scattered fields has suggested that a single‐mode propagation model does not hold and that the geometrical structure of the perturbation, especially in the horizontal direction, plays an important role in the scattering. (2) The magnitude of the perturbation, that is, the enhancement factor, effects mainly the magnitude of the scattered field.