The polarization ellipticity of transionospheric HF radio waves as observed by RRI on e-POP/Swarm-E

The Radio Receiver Instrument (RRI) on e-POP/Swarm-E detects incoming radio waves. For slew-to-transmitter passes towards the Ottawa HF transmitter (45.4°N, 75.6°W), a series of ten experiments were conducted at 10.4 MHz in April 2016 and 2017. These passes covered approximately a 15° span in both latitude and longitude around the transmitter location. Large ellipticity magnitudes were observed beyond the aspect angles suggested by magnetoionic theory. A distinct switch from left-hand to right-hand polarization was detected for passes that changed from west to east of the transmitter. During periods of unsettled ionospheric conditions, large oscillations in ellipticity magnitude were observed. Modeling efforts indicate that, along with the magnetic field geometry, the relative power of the O- and X-modes govern both the magnitude of ellipticity and the sense of rotation of the HF radio waves.