The directional dependence of magnetic fluctuations generated by cometary ion pickup

The properties of low frequency magnetic fluctuations generated by cometary ion pickup are examined by means of one‐dimensional hybrid simulations, in which newborn ions are created at a constant rate. The helicity and direction of propagation of magnetic fluctuations are investigated for various cometary ion injection angles, α, relative to the solar wind magnetic field. The parameter η represents the relative contribution of wave energy density propagating in the direction away from the comet, parallel to the beam. For small (quasi‐parallel) injection angles, α ∼ 0° and η is of order unity, while for larger (quasi‐perpendicular) angles, α ∼ 90° and η is about 0.5. At intermediate angles, α ∼ 60°, η can vary between 0 and 1, depending on the wave number. The wave properties are consistent with the instabilities (right‐hand cyclotron resonant modes at small α, left‐hand Alfven/ion cyclotron modes at large a) expected from linear theory. Consequences of these results for particle acceleration are also discussed.