PIDT Report: Revolution 722

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On November 3, 1997, a Coronal Mass Ejection on the Sun was observed by the SOHO satellite. A large increase in the proton flux was observed in revolution 722 that had a negative impact on the operation of the four instruments. PIDT performed an analysis of the ISOPHOT observations during this revolution, and this report summarizes our findings.

• For the FIRST time in the mission, the six detectors showed signals out of range (QPAS message from RTA/QLA). For the P3 detector, which is most sensivity to the space environment, the signal increase was nearly 50 percent off from the average value, with a signal of 19.49 V/s. Note that there were four orbits during the mission where the signal of P3 was over 19 V/s, but this is the first time that we have observed the event associated to a large proton flux.
• From the archived data, we extracted the FCS measurements associated to ALL the measurements. The data was loaded into PIA to derive the glitch rate. Visual inspection showed that for all the measurement the ramps were of very poor quality, not acceptable for science use. The glitch rate seems to be constant during the whole orbit, we estimate the typical value to be around 0.5 glitch/sec/pixel.
• The reponsivity check of the PHT-S detector at deactivation (P84) was also examined, and also shows the same problems compared with the measurement in activation.
• The responsivity checks during revolution 721, both in activation and deactivation, did not show any peculiar behaviour. The quality of the observations during that observation should be OK. The responsivity checks after revolution 722 returned immediately to the nominal values. The glitch analysis shows no peculiar behaviour, with only a slight glitch rate enhancement at the beginning of revolution 723, and we conclude that the science results are not affected by this event.
• The examination of the ISOPHOT RAM dump at the end of the revolution shows no RAM corruption during the orbits 720 to 725.

We recomend the replanning of the revolution 722, as the high glitch rate during the orbit makes the data not scientifically valid. We can not estimate the probability to observe again a solar event similar to the experienced in revolution 722. If the levels of proton flux in the future reach a value similar to those observed in that orbit, it is clear that not useful science could be produced during these conditions.

Figures:

• Figure 1 : The responsivity check (AOT P83) vs. orbit since revolution 200. Notice the big increase in the responsivity at revolution 722.
• Figure 2 : These are three figures, each show the ramps for the pixel 60 of the ISOPHOT-S subinstrument, for the revolutions 720, 722 and 723 (721 is not available, as it was performed a manual activation, but it should be noted that the quality of the ramps at deactivation for ISOPHOT-S was nominal). Notice the high rate of glitches in revolution 722, and how they are back to nominal in revolution 723.
• Figure 3 : These are three figures, each show the ramps for detectors P1, P2, and C100 (pixel 5) for FCS measurements in revolution 722. Notice the high rate of glitches for ALL the detectors.

Additional Information:

• Timetable : Timetable of space events related to the coronal mass ejection from the Sun, compared to the schedule of ISO operations.
• Affected Observations : List of ISOPHOT observations during revolution 722.

Héctor Castañeda
ISOPHOT Instrument Dedicated Team

November 26, 1997