Recognition of space weather impact on the ISOPHOT detectors

The effects of changes in the space environment on the ISOPHOT photoconductive detectors over the whole ISO mission were studied using the complete set of responsivity check measurements taken after the curing of the detectors. We found that the responsivity of the Ge-based, low bias voltage far-infrared detectors (P3, C100, and C200) is sensitive to the conditions of the Space Weather. We present evidence that an increased responsivity level (20% - 50%) after curing of the detectors is linked to the onset of geomagnetic storms. The Si-based, high bias voltage detectors P1, P2 and PHT-SS show only small changes in their responsivity. An exception is the PHT-SL array which shows a similar, but less pronounced behaviour as the FIR detectors. While these relations have been demonstrated by our study, a detailed physical understanding is still outstanding. The Space Weather dependent scatter of the responsivity, being the photometric scaling factor (conversion from measured photo current to inband power on the detector), justifies the observing mode design to include frequent monitoring of its actual level.