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 radii2,3,4,5, and a jet close to Io6,7,8,9. 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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We thank P. Kervin and staff of the AEOS installation for their expertise and collegiality in the use of our HDI system for these measurements. The Boston University HDI image slicer was built under a Defense University Research Instrumentation Program (DURIP) grant via sponsorship by the Office of Naval Research (ONR). Observations were made possible by a grant from the joint NSF/AFOSR programme for AEOS, and data analysis was funded by a grant from the NSF Planetary Astronomy Program.
Author Contributions The observations were made by J.B., J.W. and M.M., who also guided the overall study. S.L. developed the HDI techniques and conducted the data analysis with methodological expertise provided by J.K, W.C.K., J.B. and J.W. M.M. wrote most of the paper, with contributions and interpretation of the data from J.W. and J.B.
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
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Mendillo, M., Laurent, S., Wilson, J. et al. The sources of sodium escaping from Io revealed by spectral high definition imaging. Nature 448, 330–332 (2007). https://doi.org/10.1038/nature06000
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