S4 streak frequency data from Peter Ford's psci processing logs.
The frequency of S4 streaking seems to be correlated with quiescent S3 amplitude reject rates (events with pha > 15 keV). The quiescent S3 amp_rej rates are known to correlate well with the cosmic ray rate measured by ACE SIS (Grant, Bautz & Virani 2002). The flares in S4 streak frequency do not seem strongly correlated with S3 amp_rej.
Another representation of the correlation of S4 streak frequency with S3 amp_rej rates. The upper left panel includes all data. The remaining panels include only data taken in the given instrument position (ACIS Imaging = ACIS-I or ACIS-S in focal plane; ACIS CTI = ACIS in the HRC-S position used for CTI measurements; ACIS + HETG = ACIS in focal plane with HETG inserted) The flaring behavior is seen in all instrument configurations except in the ACIS CTI position. This may indicate that the particles responsible for large increases in S4 streaks may be protons with insufficient energy to penetrate the SIM shielding but enough to pass through the HETG.