Abstract: STEVE (Strong Thermal Emission Velocity Enhancement) is a white/mauve optical arc in the subauroral region ionosphere that reflects the coupling among the ionosphere, magnetosphere, and thermosphere. We analyze a STEVE event on 16 April 2021 using multi-instrument observations to investigate its morphological evolution and energy sources under relatively quiet solar wind and weakly disturbed geomagnetic conditions. By integrating optical observations from THEMIS all-sky imagers with ionospheric and magnetospheric measurements from GNSS TEC, the Swarm satellites, AMPERE field-aligned currents, NOAA POSE, and the Arase (ERG) spacecraft, we find that this STEVE event lasted for approximately 1.5 hours and evolved into Picket Fence structures. The STEVE region exhibited typical localized signatures of electron heating, electron density depletion, fast westward ion flow, and downward field-aligned currents. The elevated westward plasma velocity (~4 km/s), together with the unusually high electron temperatures (~8,000 K during STEVE and >20,000 K during the picket fence), suggests that frictional heating associated with subauroral ion drifts (SAIDs) alone cannot account for the observed thermal enhancement. Magnetospheric observations reveal broadband electron energy flux enhancements (dominated by <10 keV electrons), indicating that low-energy electron precipitation likely supplied additional energy to the STEVE region. This study demonstrates that, even during weakly disturbed periods, the combined effects of SAID and low-energy electron precipitation can substantially elevate electron temperatures in the subauroral ionosphere, thereby facilitating the formation of STEVE and picket-fence structures.