THE EXTENDED SODIUM NEBULA OF JUPITER

THE detection of a cloud of neutral sodium near Jupiter's moon Io1 has led to the use of sodium as a tracer of processes in the jovian environment. Although relatively rare in the Io-Jupiter system, sodium atoms are easily detected because of their high efficiency for scattering sunlight at wavelengths of ∼5,890 Å. Direct imaging of the sodium cloud2 has suggested that sodium atoms are a common feature close to Io (at distances of about six Io radii, RIo) and detection of high-speed sodium jets3 suggested that sodium is present only sporadically at ∼30/RIo (ref. 4). Sodium emission has been reported at greater distances5, even as far as 60RIo (ref. 6) but these observations have been controversial in view of suggestions7 that the detection of sodium beyond ∼10RIo was implausible on theoretical grounds and probably indistinguishable from terrestrial sodium airglow. Here we report on the detection of sodium to distances beyond ∼400 RI, an observation that requires the ejection rate of sodium atoms to be increased. By relating the shape of this great nebula to conditions in the plasma torus surrounding Jupiter, we show that ground-based imaging techniques can provide information about distant planetary magnetospheres. © 1990 Nature Publishing Group.