Solar control of F region radar backscatter: Further insights from observations in the southern polar cap

Abstract The role of solar wind and illumination in production of small‐scale F region plasma irregularities is investigated using a 4 year data set collected by the Super Dual Auroral Radar Network (SuperDARN) facility at the McMurdo station, Antarctica (MCM). Statistical analysis of ionospheric echoes detected by MCM shows that radar backscatter from the polar F region occurs in wide and persistent bands in range that exhibit systematic changes with local time, season, and solar cycle. It is demonstrated that all variations considered together form a distinct pattern. A comparison with the F region model densities and ray tracing simulations shows that this pattern is largely controlled by the F region solar‐produced ionization during the day. During the night, however, MCM observations reveal a significant additional source of plasma density in the polar cap as compared with the model. An example of conjugate radar observations is presented that supports the idea of polar patches being an additional source of ionization on the nightside. Echo occurrence exhibits a clear peak near the solar terminator, which suggests that small‐scale irregularities form in turbulent cascade from large scales. Further, echo occurrence is enhanced for particular interplanetary magnetic field (IMF) orientations during the night. Observations indicate that solar illumination control of irregularity production is strong and not restricted to the nightside. Indirect solar wind control is also exerted by the IMF‐dependent convection pattern, since the gradient‐drift instability favors certain orientations between the plasma density gradients and convection velocity.