Plasma troughs (or sub-auroral troughs) are persistent large-scale F-region electron density depletion structures located at the interface between the mid-latitude ionosphere and the high-latitude auroral region, and typically extend from the post-afternoon sector to the dawn sector. Polar-cap patches are identified as enhanced plasma densities 2-10 times higher than the background density in the polar-cap ionosphere produced by either solar EUV radiation or low energy (or auroral) electron precipitation.



• Plasma trough morphology, especially latitudinal and longitudinal small-scale (< 100 km) structures, and behavior of the small-scale temporal (< 1 sec) and spatial (10 km – 100 km) variations of the trough
• Relationship between plasma trough and plasmapause
• Temporal variations of electron density and temperature in the polar-cap patches, the movement speed and growth-disappearance process of the polar-cap patches, and spatial scale and spatial structure changes of the polar-cap patches