Tracing the boundary of the South Atlantic Anomaly region with SPHEREx transient flagging rates

The NASA SPHEREx satellite was launched in 03/2025 to survey the full sky between 0.75 - 5.0 um. The image processing of SPHEREx H2RG detectors includes real-time flagging of transient events during integration. SPHEREx follows a polar orbit passing over the South Atlantic Anomaly (SAA) zone multiple times daily, where the transient counts reach as high as >80% of pixels, depleting the pixel inventory for further calibration. The science pipeline flags for re-observation all exposures taken while in the SAA, as well as any with >10% transients regardless of the spacecraft location. After six months, ~1.5% of the exposures outside of the pre-defined SAA zone were flagged. Most are near the poles; but also adjacent to the edges of the SAA, potentially expanding its boundary. Given the importance of the SAA to designing and operating space instruments, we present here the SAA boundary traced by SPHEREx transient flags as a reference for future missions.