1. Department of Astronomy and Center for Space Physics, Boston University, Boston, Massachusetts 02215, USA

Abstract

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, R_Io) and detection of high-speed sodium jets³ suggested that sodium is present only sporadically at 30/R_Io (ref. 4). Sodium emission has been reported at greater distances⁵, even as far as 60R_Io (ref. 6) but these observations have been controversial in view of suggestions⁷ that the detection of sodium beyond 10R_Io was implausible on theoretical grounds and probably indistinguishable from terrestrial sodium airglow. Here we report on the detection of sodium to distances beyond 400 R_I, 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.