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Are Neptune's highly dispersed whistlers really Z‐mode radiation?
Author(s) -
Farrell W. M.
Publication year - 1996
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/95gl03090
Subject(s) - whistler , ionosphere , plasmasphere , physics , neptune , magnetosphere , hiss , electron precipitation , mode (computer interface) , very low frequency , lightning (connector) , plasma , van allen radiation belt , geophysics , atmospheric sciences , electron , computational physics , astrophysics , astronomy , planet , power (physics) , quantum mechanics , computer science , operating system
Evidence is presented that the unusual lightning‐generated whistler signals detected very near Neptune are possibly impulsive emissions propagating in the z‐mode. Unlike terrestrial whistlers in the plasmasphere, the Neptunian emissions might be propagating in a region where the plasma‐to‐cyclotron frequency ratio is much less than one. If the nightside plasma density is indeed low, the z‐mode will extend down to unusually low frequencies. The observed events may then be interpreted as z‐mode instead of the whistler mode. Propagation in the z‐mode may explain many of the unusual emission features, including the low frequency limit of 6 kHz and the very large dispersions. Given an atmospheric emission source, this result suggests that Neptune's ionospheric electron 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.