Space Research & Planetary Sciences (WP)


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Plasma-driven Sodium Emission at an Io as an "exo-Io" proxy: Fig. 3, Oza et al. 2019, ApJ

This 10-hour sequence was obtained on a single night of ground based narrowband imaging by Nick Schneider and John Trauger. The observations and analysis appear in Schneider+ 1991 and Schneider+ 1995. The images demonstrate two physical phenomena:

Top panel
Na D1 and D2 doublet sequence. Io starts in front of Jupiter and orbits to the right, then reverses to give a sense of continuous motion. Io’s image in reflected sunlight saturates the detector, but the Io sodium cloud is clearly visible as two features extending to Io’s right. Europa saturated disk starts  at the right edge of the image, too bright to allow imaging of Europa's own endogenic source of Na (Brown+ 96; Leblanc+ 2002), and another moon’s disk appear on the left side of the image. The Na clouds and jets at Io are ejected directly from Io’s atmosphere due to plasma-driven atmospheric sputtering. The additional Na streams at upper left are independent of Io's location and are attributed to dissociative recombination of the ion NaCl+ in the Io-plasma torus (see Wilson+. 2002 for a full review).  The three Na sources altogether contribute to a vast cloud about Jupiter extending to 1000 Jovian radii.

Bottom panel
S+ sequence. Singly ionized sulfur emission from Io demonstrates the gyration of the Io plasma torus with Jupiter's magnetic field.

The two panels are described in Figure 3 of Oza et al. 2019, ApJ in the context of active exomoon (exo-Io) alkaline signatures. The plasma-driven Na loss visibly occurring in this video can be estimated at any irradiated rocky body by employing Eqn. 7 of Oza et al. 2019, ApJ.

Schneider, N.M., et al., “Molecular Origin of Io’s Fast Sodium”, Science 253, 1394-1397, 1991, doi: 10.1126/science.253.5026.1394
Schneider, N.M., J.T. Trauger, “The Structure of the Io Torus”, Astrophys. J. 450, 450-462, 1995, doi: 10.1086/176155