Temporal variation of particle-induced background rate of ACIS-S3, FAINT mode. The transparent crosses are based on the rate of S3 events rejected onboard for exceeding the upper-level PHA threshold; they are scaled by ≈2% to be compared to the particle-induced background rate. The red and green ellipses enclose the observations at especially high and low particle-induced background rates. MJD stands for modified Julian date.