Recognition of space weather impact on the ISOPHOT detectors

The effects of changes in the space environment on the ISOPHOT photoconduct ive detectors over the whole ISO mission were studied using the complete se t of responsivity check measurements taken after the curing of the detector s. We found that the responsivity of the Ge-based, low bias voltage far-inf rared 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 geomagneti c storms. The Si-based, high bias voltage detectors Pi, 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 physi cal 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 obs erving mode design to include frequent monitoring of its actual level.