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Voyager 1 evidence for ion‐cyclotron instability in the vicinity of the Io plasma torus
Author(s) -
Thorne R. M.,
Scarf F. L.
Publication year - 1984
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/gl011i003p00263
Subject(s) - physics , jovian , atomic physics , cyclotron , ion , plasma , instability , scattering , torus , astrophysics , optics , nuclear physics , planet , quantum mechanics , saturn , mechanics , geometry , mathematics
A pronounced enhancement in low frequency wave activity was observed as Voyager 1 traversed the Io plasma torus. Wave intensification was particularly strong in the frequency range below the proton gyrofrequency. The data are interpreted as evidence for the excitation of ion‐cyclotron waves over the same radial range, L ≳ 6, where energetic ring current ions exhibit a pronounced depletion in phase space density owing presumably to strong pitch‐angle scattering loss to the atmosphere. While the average wave intensity observed by Voyager was typically an order of magnitude smaller than the level E′ SD ²/Δf ∼ 10 −7 to 10 −6 (v/m)²Hz required for strong diffusion there were several prolonged intervals where the waves surpassed this critical value. Because the Voyager trajectory was far from the predicted source region (λ ≳ 15°) for ion‐cyclotron instability, the lower intensity of most waves reaching the spacecraft could be evidence for significant resonant absorption by thermal electrons and ions in the torus. If the more intense emissions, observed intermittently by Voyager, are representative of waves in the high latitude source region, the resulting pitch‐angle scattering would be rapid enough to explain the observed energetic ion losses and the excitation of intense Jovian auroral emissions of field lines passing through the torus.