Magnetic Waves Excited by Newborn Interstellar Pickup Ions Measured by the Voyager Spacecraft From 1 to 45 AU
Abstract: We have surveyed the Voyager magnetic field data from launch through 1990 in search of low-frequency waves that are excited by newborn interstellar pickup ions (PUIs). During this time the Voyager 1 and 2 spacecraft reached 43.5 and 33.6 au, respectively. The use of daily spectrograms permits us to perform a thorough search of the data.
We have identified 637 different data intervals that show evidence of waves excited by either pickup He+, H+, or both, and these intervals extend to the furthest distances in the years studied. To compare wave features against more typical interplanetary observations, we also employ 1675 data intervals spanning the same years that do not contain wave signatures and use these as control intervals. While the majority of wave events display the classic spectral characteristics of waves due to PUIs, including left-hand polarization in the spacecraft frame, a significant number of the events are right-hand polarized in the spacecraft frame. We have no complete explanation for this result, but we do show that right-handed waves are seen when the local magnetic field is nonradial. We also compare the growth rates for the waves against turbulence rates and find that the waves can only be found when the turbulence is weak enough to allow for the wave energy to accumulate over the relatively long periods required for the weak instability.