The Ellipticity of High Frequency Transionospheric Radio Waves

The polarization state of transionospheric high frequency (HF) radio waves can be determined using the crossed-dipole antennas of the Radio Receiver Instrument (RRI) onboard Enhanced Polar Outflow Probe (e-POP) on the CAScade, Smallsat, and IOnospheric Polar Explorer/Swarm-E satellite. Coordinated experiments between ground radars and RRI showed that the radio waves have high ellipticity angles (elliptical or circular polarization) for propagation direction 6 degrees-25 degrees from the perpendicular direction to the local geomagnetic field. However, from magnetoionic theory and a typical assumption of roughly equal power in the O- and X-modes, radio waves can have high ellipticity angles only when the propagation direction is within 10 degrees of perpendicularity. This investigation uses coordinated experiments between the Saskatoon SuperDARN radar and RRI. Magnetoionic modeling reveals that the relative strengths of the O- and X-modes for the HF radio waves transmitted from the Saskatoon SuperDARN change significantly poleward of the radar. Differences in the relative power between the two wave modes were found to significantly modify the polarization state of radio waves, and govern the sense of rotation of the wave. Even a relatively modest 2-5 dB power difference in the X-mode over the O-mode was found to result in unexpected observations of circularly polarized radio waves at aspect angles 6 degrees-10 degrees from perpendicular, and unexpected elliptically polarized waves observed at aspect angles more than 10 degrees from perpendicular. Therefore, a combination of RRI observations and modeling, which accounts for relative differences in the O- and X-mode powers, was used here to better understand the unexpected observations of polarization states of transionospheric radio waves. Radio Receiver Instrument on Enhanced Polar Outflow Probe (e-POP) satellite observed polarization states of transionospheric radio waves in coordinated experiments Circularly polarized radio waves were detected far off from the transverse propagation direction to the geomagnetic field Domination of one mode power (X-mode) caused higher ellipticity of radio waves