ARE NEPTUNE HIGHLY DISPERSED WHISTLERS REALLY Z-MODE RADIATION

Evidence is presented that the unusual lightning-generated whistler si gnals detected very near Neptune are possibly impulsive emissions prop agating in the z-mode. Unlike terrestrial whistlers in the plasmaspher e, the Neptunian emissions might be propagating in a region where the plasma-to-cyclotron frequency ratio is much less than one. If the nigh tside plasma density is indeed low, the z-mode will extend down to unu sually low frequencies. The observed events may then be interpreted as z-mode instead of the whistler mode. Propagation in the z-mode may ex plain many of the unusual emission features, including the low frequen cy limit of 6 kHz and the very large dispersions. Given an atmospheric emission source, this result suggests that Neptune's ionospheric elec tron density is relatively low. In order for the dispersive events to be observed at Voyager in the z-mode, some locations in the nightside ionosphere are required to have densities as low as about 100 cm(-3). A possible magnetospheric source for the emission is also discussed.