Relationship between lightning discharges and rapid changes in cross polarization discrimination of the Ka-band satellite signal

Rapid changes in cross-polarization discrimination (XPD) of the Ka-band satellite radio wave signal (19.45 GHz) are observed at 1 or 0.1 sec intervals in 77 thunderstorm events from 1990 to 2006 at Osaka Electro-Communication University. About one third of the rapid changes are found to coincide with the cloud-to-ground lightning strokes on the south side of our earth station at the distance up to 20 km. Also, more than two thirds of them show that cross-polar phases move toward −90 deg, which means that canting angles of ice crystals move toward the horizontal direction. The XPD values and the canting angles of ice crystals associated with cross-polar phases are, for the first time, estimated just around the point of lightning discharges, using the observed XPD and cross-polar phases before and after the rapid changes. The XPD value at the lightning point is considered to become about 10 dB lower than other place before the lightning due to the almost vertical alignment of ice crystals in the thundercloud. In addition, the XPD values and cross-polar phases are directly traced during the rapid changes at the discharging point using those measured at 0.1 sec intervals. It is found that the XPD values due to the alignment of ice crystals almost disappear during about 0.5 sec, yielding the increase in XPD at the lightning point, while the mean canting angle of ice crystals at the lightning point moves from nearly vertical to horizontal direction during this short period. In the case of larger thunderclouds, however, the vertical alignment of ice crystals at the lightning point and their nearly horizontal alignment in other places seem to cause the increase in XPD due to depolarization cancellation before the lightning, and give rise to the decrease of XPD after the lightning.