Abstract
As the ICE spacecraft approached comet Giacobini‐Zinner, low‐frequency waves in two frequency ranges were observed in the magnetic field data. We show that, under certain conditions, a gyrating beam of water group ions is capable of generating electromagnetic waves with periods near 100 s (in the spacecraft frame of reference) and shorter‐period whistler branch waves with periods near 3 s. The 100‐s wave can arise from either a nonresonant firehose instability or a resonant beam driven instability so long as the angle between the directions of the local magnetic field and solar wind velocity is less than 90°. Whenever that angle exceeds about 30°, the gyrating ring distribution formed by newly ionized water is also unstable to the generation of whistler waves with 3‐s periods.
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