DASP Blog: e-POP and HF wave polarisation

 In this DASP blog, Ceren Eyiguler (Athabasca University) discusses their recent paper on the spatial variability of HF wave polarisation, as observed by e-POP. If you have any questions, reach out to Ceren at eeyiguler@athabascau.ca.

When high-frequency (HF) radio waves travel through the ionosphere, their polarisation is modified by interactions with charged plasma and the Earth’s magnetic field. In our recent study, we investigated how these transionospheric signals vary as they propagate, building on a series of earlier studies (Gillies, 2006; Gillies et al., 200l; Gillies et al., 2010; Gillies et al., 2012) that applied magnetoionic theory and ray tracing to establish the theoretical and observational basis for using satellite measurements to study transionospheric HF propagation.

 

We used the Radio Receiver Instrument (RRI) on the e-POP scientific suite aboard the Canadian CASSIOPE (Swarm-E) satellite to track HF signals transmitted from a transmitter in Ottawa, Canada (Danskin et al., 2018; Kalafatoglu Eyiguler et al., 2023) under both geomagnetically quiet and unsettled conditions and compared the direct observations with theoretical expectations.

 

As radio waves traverse the ionosphere, they split into two modes due to the birefringent nature of the medium: the ordinary (O-) mode and the extraordinary (X-) mode. For linearly polarised transmissions with equal power in both modes, magnetoionic theory predicts strong variability in polarisation only when the propagation direction is nearly perpendicular to the geomagnetic field (within about ±10°) (Pandey et al., 2024). Outside this range, the waves are expected to remain linearly polarised. However, RRI observations revealed elliptically and even circularly polarised signals across a much wider range, extending to about 40° from perpendicular, particularly during disturbed conditions.

 

Figure 1: Ellipticity variation with respect to the azimuth and elevation angles superimposed on the relative power of the wave modes (X/O) for the observations during (a) quiet-time and (b) unsettled conditions. The Ottawa transmitter is at the centre of the polar plot. The azimuth angles are on the outer circle of the polar plot. The elevation angle decreases radially outwards from the centre. The inner colour bar shows the colour scale for the ellipticity angle χ in degrees, and the outer colour bar shows the colour scale for the X/O power ratio in dB (Kalafatoglu Eyiguler et al., 2025).

 

To explain the wider range of large ellipticity magnitudes, we modelled the distribution of power between the O- and X-modes as a function of radar boresight, magnetic field orientation, and radio wave propagation path. 

 

Figure 1 shows the modelled mode power distribution as shaded contours, with blue indicating O-mode dominance and red indicating X-mode dominance. The circles represent RRI ellipticity angle observations, where blue and red markers denote left-hand and right-hand polarisation (negative and positive ellipticity, respectively). The elevation angle decreases radially outward from the Ottawa transmitter at the centre of the figure. Around 10° elevation and lower, the wave path is nearly perpendicular to the geomagnetic field. The results reveal a clear correspondence between regions of O- or X-mode dominance and the observed polarisation sense. Right-hand polarised signals occur where the X-mode carries more power, while left-hand polarised signals occur where the O-mode dominates.

 

The key factor is the relative power balance of the two modes. When one mode carries more power, the combined signal departs from the predictions of equal-power models, often taking on elliptical or circular polarisation. In some passes, the dominant sense of rotation even switched from left-hand (O-mode) to right-hand (X-mode) as the satellite crossed regions of changing mode dominance. Unsettled ionospheric conditions enhanced these effects, producing stronger departures from the quiet-time expectations.

 

Our results demonstrate that the power distribution between O and X modes is a critical factor in determining HF polarisation. This insight refines our understanding of how space weather influences radio propagation and highlights the need to extend traditional models to account for unequal contributions of power from each mode. By characterising how radio wave polarisation varies along the propagation path, RRI observations provide an important diagnostic of ionosphere-radio wave-magnetic field interactions. Improved modelling of polarisation variation has practical implications for long-distance HF communication, space-based receivers, and over-the-horizon radar systems that rely on stable and predictable signal characteristics.

 

This work was carried out in collaboration between the University of Saskatchewan and the University of Calgary, with funding support from the Canadian Space Agency (CSA) and the European Space Agency (ESA).

 

References

 

Danskin, D., Hussey, G., Gillies, R., James, H. G., Fairbairn, D. T., and Yau, A. W. (2018). Polarization characteristics inferred from the Radio Receiver Instrument on the Enhanced Polar Outflow Probe. Journal of Geophysical Research: Space Physics, 123(2):1648–1662.

 

Gillies, R. (2006). Modelling of transionospheric HF radio wave propagation for the ISIS II and e-POP satellites. Available at https://harvest.usask.ca/items/44bfcca1-c31e-46ca-a67d-cf115e93a2ed.

 

Gillies, R., Hussey, G., James, H., Sofko, G., and André, D. (2007). Modelling and observation of transionospheric propagation results from ISIS II in preparation for e-POP. Annales Geophysicae, 25:87–97.

Gillies, R., Hussey, G., Sofko, G., and James, H. (2010). Relative O-and X-mode transmitted power from superdarn as it relates to the RRI instrument on epop. Annales Geophysicae, 28(3):861–871.

 

Gillies, R., Hussey, G., Sofko, G., and James, H. (2012). Modeling measurements of ionospheric density structures using the polarization of high-frequency waves detected by the Radio Receiver Instrument on the Enhanced Polar Outflow Probe. Journal of Geophysical Research: Space Physics, 117(A4).

 

Kalafatoglu Eyiguler, E., Pandey, K., Howarth, A., Holley, W., Danskin, D., Hussey, G., Gillies, R., and Yau, A. (2023). Effect of spacecraft attitude on radio wave polarization measurements for the Radio Receiver Instrument on Swarm-E. Advances in Space Research, 72:4836–4855.

 

Kalafatoglu Eyiguler, E., Pandey, K., Hussey, G., Danskin, D., Gillies, R., and Yau, A. (2025). Spatial variation of polarization ellipticity for transionospheric HF radio waves observed by RRI on e-POP/Swarm-E. Journal of Geophysical Research: Space Physics, 130(2):e2024JA032822.

Interested in writing a blog post for DASP about a recent paper? Email DASP at dasp.dpae@gmail.com, or Dan Billett at daniel.billett@usask.ca. 

 

Pandey, K., Kalafatoglu Eyiguler, E. C., Hussey, G. C., Danskin, D. W., Gillies, R. G., and Yau, A. W. (2024). The ellipticity of high frequency transionospheric radio waves. Journal of Geophysical Research: Space Physics, 129(4):e2023JA032022