Changes in Polarization State of Transionospheric Radio Waves Driven by Difference in O- and X-mode Power

The Radio Receiver Instrument (RRI) of the Swarm-E/e-POP satellite can determine the polarization state of transionospheric radio waves. Coordinated experiments between ground transmitters and RRI show that the radio wave ellipticity oscillates from linear to circular when observed poleward of the transmitters. The RRI observations showed circularly polarized waves farther away from transverse propagation alignment. For radio waves with similar power in the O and X-modes, the magnetoionic theory shows that the radio waves are circularly polarized for transverse propagation only (aspect angle ~90°). However, the X-mode power was found to dominate over the O-mode at low elevations to the north of transmitter. The radio wave polarization states are modeled after accounting for difference in power between the O- and X-modes. The results show that the dominance of X-mode at select elevation angles resulted in significant change in the radio wave ellipticity.