Long-term observations of Uranus and Neptune at 90 GHz with the IRAM 30 m telescope - (1985-2005)

Context. The planets Uranus and Neptune with small apparent diameters are primary calibration standards. Aims. We investigate their variability at similar to 90 GHz using archived data taken with the IRAM 30 m telescope during the 20 year period 1985 to 2005. Methods. We calibrate the planetary observations against non-variable secondary standards (NGC7027, NGC7538, W3OH, K3-50A) observed almost simultaneously. Results. Between 1985 and 2005, the viewing angle of Uranus changed from south-pole to equatorial. We find that the disk brightness temperature declines by almost 10% (similar to 2 sigma) over this time span indicating that the south-pole region is significantly brighter than average. Our finding is consistent with recent long-term radio observations at 8.6 GHz. Both data sets show a rapid decrease of the Uranus brightness temperature during 1993, indicating a temporal, planetary scale change. We do not find indications for a variation of Neptune's brightness temperature at the 8% level. Conclusions. If Uranus is to be used as a calibration source, and if accuracies better than 10% are required, the Uranus sub-earth point latitude needs to be taken into account.