This graph and accompanying audio file show intense electromagnetic wave turbulence associated with a sharp rise, then abrupt drop-off, of electron density in a region over the north pole of Jupiter's moon Io. The section of increased wave activity beginning about one-third of the way from left to right across the graph corresponds to the region of higher electron density.

The graph presents waves in the thin, charged gas around Io through which NASA's Galileo spacecraft passed as it flew near Io's north pole on Aug. 6, 2001. The horizontal axis is time, a total of 25 minutes during the flyby. On the vertical axis are the frequencies of waves detected by the plasma wave instrument on Galileo, ranging from a few hertz (cycles per second) to a few megahertz. Colors denote wave intensity. The electromagnetic waves near Io travel in plasma, a thin gas of charged particles, but researchers at the University of Iowa have converted the data to audible sound waves to make patterns detectable to the ear. The audio signals are restricted to the lowest frequencies on the graph.

The narrow line of emission near the top of the graph is at a frequency determined by the density of electrons in the plasma. It shows a sharp increase as Galileo enters the region immediately over the northern hemisphere of Io and a correspondingly abrupt decrease as Galileo leaves the region a few minutes later.

The section where the noise is loudest is where Galileo passed through a column of electrons moving along magnetic field lines between Jupiter and Io. The abrupt changes in noise level indicate sharp boundaries to that connection path, called a flux tube. The intensity of the roar is a reminder of the great amount of power -- the equivalent of about 1,000 large commercial power plants -- generated by the movement of Io, an electrically conducting body, through the magnetic field of Jupiter.