Day after day variations of arrival angles and polarisation parameters of Q bursts recorded at Antarctic station “Akademik Vernadsky”

We present variations in the source bearings and polarization of the horizontal magnetic field of Q bursts recorded at Antarctic station “Akademik Vernadsky” (65.2458° S, 64.2572° W). We analyzed the parameters of the “polarization ellipse” inscribed in the initial impulse of the Q-burst, which is formed by a direct wave propagating from lightning to the observer along a short path in the Earth-ionosphere cavity. Three days, from March 31 to April 2, 2019, a significant part of the Q burst flux was stably observed in a narrow northerly azimuth sector associated with the direction to lightnings in South America. During this period, regular deviations of the measured azimuths from the northern direction were observed up to 8–10°, to the west when the morning terminator passed and to the east at the evening terminator. Duration of these deviations was about 4 h. The azimuth deviations were accompanied by characteristic oscillations in the “ellipticity” value of the direct wave field, N-shaped during the passage of the evening terminator, and a mirror-symmetric И-shaped oscillation on the morning terminator. We found that, on average, the polarization of the direct wave field is close to linear when the paths from the sources to the observer were on the day side, while at night side its “ellipticity” increases to 0.1. The observed azimuth deviations were qualitatively interpreted using the concept of geometric optics of the reflection of radio waves from the day-night non-uniformity in the Earth-ionosphere cavity. The measurement period was close to the day of the vernal equinox, when the propagation path from the sources and the lines of the solar terminator run almost parallel and radio waves are incident on the day-night interface at “sliding angles”, which enhances their “reflection".