The polarisation state of transionospheric radio waves detected by RRI on Swarm-E

The Radio Receiver Instrument (RRI) on-board the Swarm-E satellite is used to determine the full polarisation state of transionospheric radio waves. Co-ordinated experiments between ground transmitters and RRI have shown that the detected radio waves can have ellipticity that varies from linearly to circularly polarised when observed poleward of the transmitter at low elevations. Circular polarisation states are expected for waves propagating traverse to the local geometric field; however, these circular states were observed over a much wider range of aspect angles than expected. The present study compares the results of radio transmissions from the Saskatoon SuperDARN radar with a transionospheric radio wave model to estimate the relative strengths of received O- and X-mode waves along the satellite track. At selected low elevations, the X-mode of the radio wave dominates over the O-mode, resulting in a circular ellipticity, although the propagation is not transverse to the magnetic field. This work presents new ways to utilize ellipticity polarisation observations of radio waves to better understand the structure of the ionosphere, complementing Faraday rotation observations which have been extensively used.