Plasma Conditions Govern the Reversible Transition Between STEVE and Picket Fence Emissions

On 6 May 2018, a remarkable subauroral optical event exhibiting three phases of evolution (Strong Thermal Emission Velocity Enhancement [STEVE] → Picket Fence → STEVE) and co‐occurrence with a stable auroral red arc was observed over Alberta, Canada. Using ground‐based instruments (Redline Emission Geospace Observatory and TREx) and satellite measurements (Swarm and DMSP), we characterized the associated ionospheric conditions. The results suggest that a visible STEVE may require density depletion, electron heating, and intense ion drifts. The observed STEVE‐Picket Fence transition indicates that these phenomena may share the same subauroral ion drift channel, yet are modulated by local Ne structure. These findings provide new insights into the physical differences between STEVE and Picket Fence emissions, and offer key observational constraints for understanding magnetosphere‐ionosphere coupling in subauroral regions.