Polarization observations of lightning‐produced VHF emissions by the FORTE satellite

Abstract

Following an earlier polarization study for a well‐defined man‐made very high frequency (VHF) signal by using the two orthogonally oriented, linear polarization antennas aboard the FORTE satellite, we report in this paper similar polarization observations for lightning‐produced radiation. A selected group of 313 transionospheric pulse pairs (TIPPs) that were geolocated by the National Lightning Detection Network (NLDN) has been analyzed. The TIPPs have been examined with high time resolution so that the magnetoionic modes can be resolved. Most of the TIPPs have been found highly polarized, with 40% of them far above the background polarization level. The polarization ellipticity and the orientation of the ellipse of the split modes have been examined as a function of the nadir and the azimuthal angles as referenced to satellite coordinates, and they are found in agreement with the predictions based on the antenna beam pattern. The original and the reflected pulses in a TIPP show nearly the same properties of polarization, except the latter appears less polarized. However, no recognizable polarization has been observed for the VHF signals accompanying more common discharge processes of initial ground strokes, dart leaders, and K streamers that usually produce continuous VHF radiation. Observations of a sequence of impulsive radiation bursts that is apparently associated with a normal negative cloud‐to‐ground flash indicate they are somewhat polarized, though not as much as the TIPPs. On the basis of the polarization observations, the possible breakdown mechanisms that are responsible for the VHF radiation have been discussed. For the highly polarized TIPP events, if they follow a cone‐shaped discharge geometry, the half‐angle of the cone is estimated to be less than 22°.