PMOD TSI: SOHO keyhole effect, and possible degradation over time
Leif Svalgaard [Leif@Leif.org], Stanford University, Palo Alto, California
Comparing the latest PMOD composite with SORCE/TIM I find that the ratio PMOD/SORCE
has been declining steadily since 2004.0 [a total of 80 ppm]. Extrapolating the decline back to
1996.75 [previous solar minimum] the decline amounts to 172 ppm. In addition, the ratio seems to
have 'spikes' of the order of 50-100 ppm [thus not trivial]. These spikes coincide with times when
SOHO was in a 'keyhole' condition [KH], and are thus artifacts. During a KH there is a slight data
loss and calculating a daily average has to be done VERY carefully [especially the correction for
solar distance]. Another possibility is the effect of some differences in the thermal environment
during KHs. I conclude that if we assume a correct SORCE/TIM calibration, the difference between
the latest solar minimum and the previous minimum may not be real and that the Sun even this
minimum may have just returned to the same constant conditions prevailing when there is no solar
activity.