Polarization observations of broadband VHF signals by the FORTE satellite

Abstract

Coherent very high frequency (VHF) radio observations with the pair of orthogonal log-periodic array antennas of the FORTE satellite allow us to study thoroughly the polarization properties for a received signal. Eighty-one broadband VHF pulses that were generated by the Los Alamos Portable Pulser (LAPP) have been analyzed. The data are analyzed by computing the Stokes parameters in the time-frequency domain. We first examine the LAPP pulses at high time resolution so as to separate the ordinary and extraordinary ionospheric modes. The two modes have been found to be mirror images of each other in terms of polarization, as would be expected. For each mode the polarization degrades from circular toward elliptical as the nadir angle increases. Antenna pattern effects on this observation are discussed. The tilt of the detected polarization ellipse is found to be tightly associated with the azimuthal direction of the pulse source. The same set of data are then examined with much lower time resolution to intentionally mix together the two split modes, so that the ionospheric Faraday rotation can be detected. With the known geomagnetic field the total electron content (TEC) is computed, which shows good agreement with the TEC computed by dechirping the signal. A case study of an impulsive lightning emission shows that it is highly polarized, indicating that the associated breakdown processes are highly coherent and organized. Finally, we discuss the potential use of the polarization observations for locating terrestrial radio signals.