Multi-Instrument Observations of the Evolution of Polar Cap Patches Associated With Flow Shears and Particle Precipitation

Simultaneous observations from Defense Meteorological Satellite Program, Swarm, Resolute Bay all-sky imagers, GPS Total Electron Content and Super Dual Auroral Radar Network, are used to investigate the evolution and key characteristics of the Tongue of Ionization (TOI) being restructured into a polar cap patch. Six satellites crossed the TOI of patch as it moved from the dayside to the nightside. It was initially hot, then a mix of both cold and hot, and finally it became a cold patch. This suggests that cold patch is not only a result of solar extreme ultraviolet radiation, but may also develop when a hot patch cools down. Soft-electron precipitation and flow shears both contribute to the TOI restructuring and the appearance of polar cap patch. The plasma density of patch at similar to 500 km was at least 4 times higher than at similar to 800 km. The plasma density enhancement gradually decreased as the patch evolved due to decreased production and transport of cold nightside low-density plasma. Moreover, the duskward motion of the patch was influenced by changes in the ionospheric convection pattern. The appearance of high-density plasma is a common phenomenon in the polar ionosphere. High-density polar cap patches usually form near the dayside sunlit region, and then move from the dayside to the nightside with the ionospheric flow. In the paper, we use multiple instruments including six satellites and ground-based observations to carefully investigate an event that started out on the dayside. It was initially a hot tongue of ionization (TOI) of high density and high electron temperature, followed by a mix of hot and cold (high vs. low electron temperature), before it finally became a cold patch. Soft-electron precipitation and flow shears both influenced the dayside high-density TOI plasma restructuring and the appearance of polar cap patch. The enhanced plasma density gradually weakened as the plasma migrated toward the nightside due to decreased production and transport of cold nightside low-density plasma. Moreover, the duskward motion of the patch was influenced by changes in the ionospheric convection pattern. Cold dense plasma in the polar cap can appear either via transport of solar extreme ultraviolet plasma or after a hot patch cools downSoft-electron precipitation and flow shears both influence the Tongue of Ionization restructuring and the appearance of polar cap patchesThe enhanced plasma density is reduced due to decreased production and transport of cold nightside low-density plasma