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

TL;DR

This study analyzes 20 years of 90 GHz observations of Uranus and Neptune, revealing a significant decline in Uranus's brightness temperature linked to viewing angle changes, with implications for calibration accuracy.

Contribution

It provides the first long-term analysis of Uranus and Neptune's brightness variability at 90 GHz, highlighting the importance of viewing geometry for calibration.

Findings

Uranus's brightness temperature declined by ~10% over 20 years.

Uranus's south-pole region is significantly brighter than average.

No significant brightness variation detected for Neptune.

Abstract

The planets Uranus and Neptune with small apparent diameters are primary calibration standards. We investigate their variability at ~90 GHz using archived data taken at the IRAM 30m telescope during the 20 years 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% (~2sigma) 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 by Klein & Hofstadter (2006). Both data sets do moreover show a rapid decrease of the Uranus brightness temperature during the year 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 calibration source, and if accuracies better than 10% are required, the Uranus sub-earth point latitude needs to be taken into account.