Ion-wave instabilities at VLF in the polar wind

Considerations based on plasma kinetic theory suggest that a variety of ion waves in the ELF and VLF bands can become unstable in the polar wind when the relative flow velocity between the hydrogen ions and oxygen ions enters the range 5–10 km/sec. Peak growth rates of ∼100 sec−1 seem achievable for wavelengths typically 10 times the Debye length (wavelengths of ∼2 meters in the polar wind). The threshold flow velocity for instability generally decreases as altitude increases within the altitude range 2000–5000 km, where the waves are expected to appear. These numerical estimates are based on a heuristic calculation for a specific instability (ion-acoustic). Other ionic two-stream instabilities (e.g., ion-cyclotron) belonging to the more general class may be equally important in the earth's polar wind. Electrostatic waves in the appropriate frequency range have been observed at altitudes 2500–4500 km over the polar regions by a VLF receiver on the OV3-3 satellite. These waves may be important in impeding the escape of polar-wind hydrogen and helium ions.