Low‐frequency plasma waves and ion pitch angle scattering at large distances (>3.5 × 105 km) from Giacobini‐Zinner: Interplanetary magnetic field α dependences

Abstract

The properties of 50‐ to 100‐s low‐frequency plasma waves and the parent 65‐ to 95‐keV H2O group ion pitch angle distributions are presented for spatial regions distant (3.5‐7.0 × 105 km) from comet Giacobini‐Zinner where the cometary ion densities are low and the solar wind interaction is weak. The presence of interplanetary discontinuities in this interval causes the interplanetary magnetic field angle α to vary over a range from ∼35° to 90°, allowing the possible generation of several new wave modes which are theoretically expected, but as yet undetected. It is determined that when x = 45° ± 20° (Parker spiral orientations), the waves are typically elliptically polarized and propagate at 20°‐50° relative to B0. The wave properties are, however, not limited to the above range. They can exhibit even more extreme polarizations and propagation directions. The waves have periods near the H2O group ion cyclotron period and are left‐hand polarized in the spacecraft frame. In the relatively rare instances when wave steepening is detected, the steepened edge is near the trailing portion of the wave. The wave period, sense of polarization, and steepening are consistent with the properties of waves detected closer to the comet and identify them as being generated by the right‐hand resonant instability. The high degree of ellipticity is in sharp contrast to waves detected closer to the comet, however. The energetic ions are not isotropic in pitch angle (in the plasma frame), in general agreement with recent numerical modeling. When α increases to 70°‐90°, the right‐hand resonant mode is observed to cut off, as theoretical studies indicate. However, the left‐hand cyclotron resonant mode is not detected. Although the detected energetic 65‐ to 95‐keV ion fluxes are enhanced during these intervals (because of the instantaneous pickup by the solar wind), particle pitch angle scattering away from the initial ring distribution is very weak, consistent with the observed absence of resonant cyclotron waves.