A theory for low-frequency waves observed at comet Giacobini-Zinner

As the ICE spacecraft approached comet Giacobini-Zinner, low-frequency waves in two frequency ranges were observed in the magnetic field data. 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 percent. Whenever that angle exceeds about 30 percent, the gyrating ring distribution formed by newly ionized water is also unstable to the generation of whistler waves with 3-s periods.