WAVE CHARACTERISTICS OF TWEEK ATMOSPHERICS DEDUCED FROM THE DIRECTION-FINDING MEASUREMENT AND THEORETICAL INTERPRETATION

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 (f1c. = 1.7 approximately 1.8 kHz) is always left handed and it becomes exactly left-handed circular when the wave frequency decreases down to the f1c, together with the fact the incident angle becomes zero (vertical) when the frequency approaches f1c. Whereas the zeroth-order mode at frequency below f1c 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.