Neptune's microwave spectrum from 1 mm to 20 cm

Abstract

We present VLA observations of Neptune at 1.3, 2, 6, and 20 cm. The total flux densities and disk-averaged brightness temperatures are compiled in Table II. Averaged values for the brightness temperatures are: 119 ± 13°K, 152 ± 11°K, and 318 ± 16°K at 1.3, 2, 6, and 20, respectively. High resolution images of the planet show an error in the ephemeris of −0.6″ in RA. Further, we have recalibrated previous observations by other researches to accurately determine the shape of Neptune's spectrum. In contrast to Uranus' spectrum, the brightness temperature of the planet increases with increasing wavelength. A comparison of Neptune's spectrum with realistic model atmosphere calculations implies an excess emission at 20 cm of 0.4 ± 0.13 mJy. If all emission is thermal radiation from Neptune's atmosphere, the ammonia abundance must be a factor of ∼50 below the solar nitrogen value throughout Neptune's atmosphere (at least down to the 120- to 150-bar level). An alternative explanation could be that Neptune emits ∼0.3–0.5mJy synchrotron radiation at 20 cm. This amount of synchrotron radiation is expected from the planet if it has a magnetic field with a strength of ∼0.5 G at the surface, and a particle distribution similar to that in Jupiter's radiation belts.