Relationship between Lightning Discharges and Rapid Changes in Cross Polarization Discrimination of the Ka-band Satellite Radio Signal

Abstract

Rapid changes in cross-polarization discrimination (XPD) of the satellite radio wave signal (19.45GHz) are observed at 1sec interval in the thunderstorm events. About one third of the rapid changes are found to coincide with the cloud-to-ground lightning strokes which occurred on the south side of our earth station at the distance up to 20km. The distribution of the lightning location indicates that the center of thunder clouds primarily exists to the westward of the radio wave propagation path. At the moment of the rapid changes, more than half the observed data indicate the decrease in XPD. Also, more than two thirds of them show the changing direction of cross-polar phases toward i90deg, which means the decrease in canting angles of ice crystals possibly causing the depolarization changes. These features seem to be enhanced in the middle stage of each lightning event The decrease in XPD may be related to cancellation efiects of depolarization due to the difierence in canting angles between the ice crystals near the lightning and those in other places far from the lightning. Thus, the measurements of XPD and cross-polar phase are shown to be important to reveal the electriflcation process of thunder clouds and to predict lightning discharges. In satellite communication links, degradation of cross-polarization discrimination (XPD) of the radio wave is caused by raindrops and ice crystals along the propagation paths. In thunderstorm events, moreover, rapid changes of XPD and cross-polar phase relative to co-polar phase are fre- quently observed at time intervals of less than 1sec (1,2). These peculiar phenomena are considered to be related to the change of electric fleld due to lightning discharges as well as the aerodynamic forces of convective air ∞ows (3). Up to now, however, detailed mechanisms of the rapid changes of XPD and cross-polar phase are not understood very well. In this study, statistical distributions of the changes of XPD and cross-polar phase are examined using the Ka-band beacon signal observations of Japan's domestic communication satellites (4) which have been conducted for the past 17 years at 1sec (partly 0.1sec) interval in Osaka Electro- Communication University. The thunderstorm events are observed more than 70 times from 1990 to 2006, and more than 2500 examples of rapid XPD changes are obtained during these years in our university. The occurrence time and the amplitude of these changes are then compared with the lightning location and time provided by the Kansai Electric Power Company in Osaka area. Also, the decrease and increase in XPD and cross-polar phase are discussed in relation to the alignment of ice crystals due to electrostatic force and convective air ∞ow in thunder clouds.