Spatial Variation of Polarization Ellipticity for Transionospheric HF Radio Waves Observed by RRI on e‐POP/Swarm‐E

Abstract The Radio Receiver Instrument (RRI) on Enhanced Polar Outflow Probe (e‐POP) onboard CASSIOPE/Swarm‐E can detect transionospheric radio waves from ground‐based transmitters. The RRI crossed‐dipoles were slewed toward the Natural Resources Canada (NRCan) Ottawa transmitter during 10 experiments in April 2016 and 2017 to determine the polarization characteristics of the transionospheric HF radio waves. Six experiments were during geomagnetically quiet intervals and four during unsettled ionospheric conditions. The detected radio waves exhibit elliptical to circular polarized states over a broad region, extending beyond the theoretical range for aspect angle northward of the transmitter. In accordance with magnetoionic theory, the radio waves are mostly linear when the radio wave propagation direction and the geomagnetic field are antiparallel, which occurs to the south of the transmitter. During unsettled ionospheric conditions, the ellipticity magnitude can take on any value between and whereas during quiet conditions, the ellipticity magnitude has a variation within . Dominant regions of O‐ and X‐mode waves, derived from a power distribution model, hint at a plausible explanation for the observed variations in ellipticity magnitude and rotation sense of radio waves.