Abstract
A whistler mode interpretation is provided for the narrowband signal (fnof; ∼ 3 – 4.6 kHz, Δfnof; ∼ 200 – 800 Hz) detected by the plasma wave instrument on Voyager 2 during its encounter with Neptune. Our analysis indicates that this signal may have been generated in a limited spatial region and that it propagated to other regions of the Neptunian magnetosphere in the nonducted whistler mode with wave normal vectors lying close to the whistler mode resonance cone. The observed frequency variation of the emission along the Voyager 2 trajectory is consistent with this interpretation. The source location is estimated to be near the magnetic equator at L ∼ 4 and dipole longitude of 111°W (260° W longitude in Neptune coordinate system). The source frequency and bandwidth are estimated to be 3.6 kHz and 300 Hz, respectively. The waves most likely would have been generated by energetic electrons with 2‐ to 20‐keV parallel energy via a gyroresonance mechanism. Our interpretation of the narrowband emissions places the following limits on the Neptunian thermal plasma density and temperature: (1) Ne,min > 0.16 el/cm3 for 1.2 RN < R < 5RN, (2) Ne,max=597.5 cm−3 at R = 1.3RN, (3) Te,max < 500–1000° K at R ∼ 5 RN. It is also possible that the weak UV aurora observed near Neptune could have been caused by the precipitation of energetic particles by the narrowband emission as a result of wave particle interactions.
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