The BCS detectors are effected by a background caused by particles trapped in the Earth's magnetic field. Many of these are rejected by the setting of the lower level SCA, but occasionally some are seen in channels 1 and 2 - they appear as a flare-like spike, but are not seen at the corresponding time in channels 3 and 4.
Over the South Atlantic Anomaly, the background caused by trapped particles is particularly intense and the High Voltage supplies to the BCS detectors are switched off (using time-tagged commands).