Coordinated EISCAT and all-sky imager observations of dayside aurora effects on polar cap patch vertical properties

Polar cap patches are islands of high-density and fast-moving structures in the high-latitude F-region ionosphere. When traversing the auroral oval, the vertical properties of the patches are expected to be modified due to auroral heating. However, observational evidence documenting this vertical evolution is rather rare. Using coordinated observations from the European Incoherent Scatter (EISCAT) Radar and an all-sky imager, we observed sequences of patches moving poleward from subauroral latitudes into the polar cap. Each of these patches corresponded one-to-one to a poleward-moving auroral form. Three typical cases of patches corresponding to different auroral intensities were selected for analysis. In these cases, the radar was in field-aligned mode, pointing its beam into or poleward of the auroral oval. This setup provided an excellent chance to closely examine how auroral activity influenced the vertical properties of the patches. We found that: (1) stronger auroral activities can enhance F-region Peak Heights (hmF2), whereas weaker auroral activities have little or no effect on hmF2. (2) During strong auroral activities, ion temperature (Ti) enhancements were observed at altitudes from ~150 to 600 km, i.e., the F1–F2 region. (3) A dense electron layer, which was observed at an altitude of approximately 150 km and caused by auroral particle precipitation, used to be identified as the lower-altitude part of hot patches. This layer quickly disappeared as it moved poleward, away from the auroral oval. These observations suggest that the hmF2 of patches can be enhanced during strong auroral precipitation associated with the Ti enhancements in different ways as the patches move across the auroral oval. The lower-altitude part of the patch is more dynamic, as it experiences stronger collisions with neutral components and more intense recombination processes compared to the higher-altitude part.