Io plasma torus and the source of jovian narrow-band kilometric radiation

Narrow-band jovian kilometric radiation (nKOM) is distinguishable from broad-band jovian kilometric radiation (bKOM), both discovered by the Voyager spacecraft1–3, by its generally lower intensity, its more restricted range of centre frequency, its narrower bandwidth, and its temporally less sporadic nature4. When observed at northern and southern magnetic latitudes nKOM polarization was predominantly left-handed and right-handed respectively, opposite to bKOM4,5. At nearly the same longitude on consecutive jovian rotations nKOM sometimes lagged the magnetic field rotation4 by 3–5%; no such lag was detected for bKOM6,7. It was proposed that bKOM is produced within ˜2° of the magnetic equator in the outer lo plasma torus by conversion of electrostatic upper hybrid (ESUH) waves to ordinary (O) mode electromagnetic radiation via a radio window8,9. Here we show that nKOM seems to be produced by the same conversion process, also operating in the lo torus, but at latitudes ≥8°. The difference in latitude for nKOM and bKOM sources stems from the condition required for mode conversion that the plasma density gradient should be normal to the magnetic field vector. The difference in these locations allows an explanation of most of the similarities and differences between the two radiations.