Quest for Waves Excited by Interstellar Helium Pickup Ions

‘Torus-like’ velocity distribution of the helium pickup ions of interstellar origin has directly been observed by the GEOTAIL spacecraft. These distributions possess a free energy to excite an electromagnetic wave owing to the anisotropic distribution. Although there have been several reports on the quest of waves induced by hydrogen pickup ions based on the observation beyond ~4AU, there has been no report on the waves of helium pickup ions. In this brief report, power spectrum analysis for helium pickup ion events obtained by GEOTAIL is carried out to search for such a wave excitement. We report, however, that the expected wave could not be found owing to the low density of the helium pickup ions at 1AU. The upper limit of the waves is determined to be 24.3% above the background level.