2008A&A...482..359K

The planets Uranus and Neptune with small apparent diameters are primary calibration standards. We investigate their variability at ∼90 GHz using archived data taken with the IRAM 30 m telescope during the 20 year period 1985 to 2005. We calibrate the planetary observations against non-variable secondary standards (NGC7027, NGC7538, W3OH, K3-50A) observed almost simultaneously. 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% (∼2σ) 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 8.6GHz. 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. 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.