1. Center for Space Physics,
2. Department of Electrical and Computer Engineering, Boston University, Boston, Massachusetts 02215, USA

Correspondence to: Michael Mendillo^1,2 Correspondence and requests for materials should be addressed to M.M. (Email: mendillo@bu.edu).

On Jupiter's moon Io, volcanic plumes and evaporating lava flows provide hot gases to form an atmosphere that is subsequently ionized. Some of Io's plasma is captured by the planet's strong magnetic field to form a co-rotating torus at Io's distance; the remaining ions and electrons form Io's ionosphere. The torus and ionosphere are also depleted by three time-variable processes that produce a banana-shaped cloud orbiting with Io¹, a giant nebula extending out to about 500 Jupiter radii^{2, 3, 4, 5}, and a jet close to Io^{6, 7, 8, 9}. No spatial constraints exist for the sources of the first two; they have been inferred only from modelling the patterns seen in the trace gas sodium observed far from Io. Here we report observations that reveal a spatially confined stream that ejects sodium only from the wake of the Io–torus interaction, together with a visually distinct, spherically symmetrical outflow region arising from atmospheric sputtering. The spatial extent of the ionospheric wake that feeds the stream is more than twice that observed by the Galileo spacecraft and modelled successfully. This implies considerable variability, and therefore the need for additional modelling of volcanically-driven, episodic states of the great jovian nebula.

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