Ulysses and Voyager Observations of Waves Due to Interstellar Pickup H+ and He+

Interstellar pickup ions, in scattering to isotropy, should generate waves in particular frequency ranges, but observations of such waves are very rare. To date, only proton-generated waves at Ulysses have been reported, and only sporadically during times of quasi-radial field. Here, we present two sets of wave observations. The first set are waves due to pickup H+ seen by the Ulysses spacecraft as part of a new study to gather and build a data base of observations that can better be questioned regarding the fundamental physics of wave generation by interstellar pickup ions. There we demonstrate that the waves may have small signatures in the power spectrum while showing strong evidence of wave-particle dynamics in the polarization spectra. The second set is a spectral enhancement measured at the Voyager 2 spacecraft during 4.5 hours on DOY 7, 1979 at a heliocentric radial position of 4.5 AU. The large-scale IMF was nearly radial, and the frequency range and polarization of the enhanced fluctuations are consistent with pickup ion-generated waves from inflowing interstellar H and He. This event is the first indication of pickup ion-generated waves seen at Voyager. It is also the first identification of pickup He+ waves on any spacecraft, and prompts further searches for such waves in data sets closer to the Sun.