Predictions for Uranus from a radiometric Bode's law

Abstract

The magnetospheres of three planets, Earth, Jupiter and Saturn, are sources of intense, nonthermal radio bursts. Recent studies^1–3 have shown that the emissions from these sources undergo pronounced long-term intensity fluctuations that are caused by the solar wind interaction with the magnetosphere of each planet. Determinations by spacecraft of the low-frequency radio spectra and radiation beam geometry now permit a reliable assessment of the overall efficiency of the solar wind in stimulating these emissions. We find that earlier estimates⁴ of how magnetospheric radio output scales with the solar wind energy input must be greatly revised, with the result that, while the efficiency is much lower than previously thought, it is remarkably uniform from planet to planet. This result has prompted our formulation a ‘radiometric Bode's law’ from which a planet's magnetic moment can be estimated from its radio emission output. (This terminology is by analogy with the ‘magnetic Bode's law’, sometimes called Blackett's law⁵.) Applying the radiometric scaling law to Uranus, we estimate the low-frequency radio power likely to be measured by the Voyager 2 spacecraft as it approaches this planet. We also show how measurements of the Uranus radio flux by Voyager 2, which will probably be made in early to mid-1985, can be used to estimate the planetary magnetic moment and solar wind stand-off distance before the in situ measurements.