Simulation of SAR Arc Formation: Separating SAPS and IMHF Mechanisms

Abstract Stable auroral red (SAR) arcs are known to be associated with both subauroral polarization streams (SAPS) and inner magnetospheric heat fluxes (IMHF), though their relative roles remain unclear. Using Thermosphere–Ionosphere–Electrodynamics General Circulation Model (TIEGCM) simulations with IMHF incorporated within the SAPS channel, we distinguish their contributions to redline emissions during the 17 March 2015 geomagnetic storm. Nightside SAPS‐driven arcs with ∼50 Rayleighs (R) exhibit local time dependence, contrasting with the brighter (∼100 R), uniform IMHF‐driven arcs. Theoretical analysis shows that they operate via independent mechanisms: SAPS enhances O 2 + at lower altitudes (∼200–300 km), increasing recombination to produce redline emissions near 250 km. In contrast, IMHF heats electrons at higher altitudes (∼400 km), exciting oxygen through thermal excitation. These results demonstrate that while IMHF remains an important driver of SAR arcs, SAPS can also generate subauroral red emissions through a distinct lower‐altitude mechanism, thereby challenging the traditional IMHF‐centric paradigm.