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Hydromagnetic waves near O + (OR H 2 O + ) ion cyclotron frequency observed by Sakigake at the closest approach to comet Halley
Author(s) -
Yumoto Kiyohumi,
Saito Takao,
Nakagawa Tomoko
Publication year - 1986
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/gl013i008p00825
Subject(s) - physics , comet , ion , cyclotron , atomic physics , excited state , magnetic field , amplitude , halley's comet , astrophysics , optics , quantum mechanics
Long‐period HM waves with 150‐ 380 s periods were observed by Sakigake at 7 × 10 6 km upstream from comet Halley. The wave amplitudes were as great as ∼2 nT peak to peak. The long‐period waves were of an approximately linearly polarized transverse mode and propagated parallel to the ambient magnetic field. Dominant periods of the waves were found to be consistent with the local O + (or H 2 O + ) cyclotron period and to change as a function of the ambient magnetic field. The long‐period HM waves are believed to be excited by a resonant instability with high‐energy cometary heavy ions. The necessary cometary ion density for excitation at a distance of 7 × 10 6 km from the comet can be inferred to be of the order of 10 −3 ‐ 10 −4 cm −3 for 10¹ ‐ 10² keV O + (or H 2 O + ) ions.