Io's Sodium Clouds

There are several processes which eject atoms and molecules from Io's atmosphere. The sodium clouds, which are the most easily oberved consequences of atmospheric escape, are formed primarily by two types of ejection processes: (1) atmospheric sputtering forms the banana cloud, and (2) pickup ion neutralization forms the stream and jet. Some of the sputtered sodium and nearly all of the stream and jet sodium is ejected entirely from the Jupiter system, forming the huge sodium nebula around Jupiter.

  • Banana cloud. Atmospheric sputtering ejects sodium atoms from Io in all directions and at relatively low speeds. These atoms end up in orbits similar to Io's, and gradually spread out along Io's orbit both ahead of and behind Io. Those atoms trailing Io also orbit at a slightly larger distance from Jupiter, placing them in the least hospitable portion of Jupiter's plasma torus. As a result, they are quickly ionized and incorporated into the torus, which cannot be seen in these images of sodium emission. The atoms orbiting ahead of Io, however, orbit closer to Jupiter, where there is relatively little plasma to ionize them. As a result, they spread out to relatively large distances ahead of Io in its orbit. The resulting banana cloud thus extends primarily ahead of Io and slightly inside of Io's orbital distance.

  • Stream. The stream is formed by a two-step process: (1) molecular ions containing sodium are ripped out of Io's atmosphere by the electric and magnetic fields of Jupiter's magnetosphere and picked up into the plasma torus, then (2) over the course of several hours, these molecular ions dissociatively recombine with electrons, releasing sodium atoms which are visible in these images of sodium emission. The molecular ions which spawn the sodium atoms are invisible in these images, so it looks as if the sodium atoms are magically appearing in the torus. The magnetic and electric fields have no influence on neutral particles, so the new sodium atoms find themselves free to zoom away from Jupiter at high speed, and into the nebula which surrounds Jupiter. Visually, the stream looks like a long fuzzy "rope" of sodium atoms leading Io that flaps up and down. This flapping motion is the result of the recently-escaped pickup ions oscillating north and south along Jupiter's magnetic field lines. The sodium atoms being created by the ions reflect the locations and velocities of their parent ions.

  • Jet. The jet is very similar to the stream, the main difference being how long it takes the ions to recombine and relase sodium atoms. The jet is formed by ions which recombine only seconds after leaving the densest regions of Io's atmosphere, compared with hours for the stream. These "fresh" pickup ions move in tight circles perpedicular to the magnetic field, so the sodium atoms that are produced by their recombination fly out into a flat, disk-shaped region, the orientation of which depends on Io's changing magnetic longitude relative to Jupiter. As the hours go by, the magnetic field orientation at Io changes, and so does the orientation of the Jet. The ions in Io's ionosphere escape from the side of Io facing away from Jupiter, so the jet points away from Jupiter as well. There is no jet pointing towards Jupiter.