Evaluation of the Asymmetry in Photoelectron Distribution Around the GEOTAIL Spacecraft

We examine photoelectron distributions detected by the low-energy-particle (LEP) instrument onboard the GEOTAIL spacecraft by means of both data analysis and numerical simulations. Statistical data analysis shows asymmetries in the photoelectron distributions. For photoelectrons incident normal to the spacecraft spin axis, a higher flux is observed in the dawnward than in the duskward sector of the LEP. The distribution significantly depends on the ratio of the photoelectron energy to the spacecraft potential. Our numerical simulations reveal that the asymmetry is caused by the electrostatic potential around the thin antenna located at +18^deg anticlockwise (viewed from the top) relative to the LEP. Photoelectrons in the dawnward sector are preferentially carried from the sunlit surface by this potential. For upward/downward incident photoelectrons, a higher flux of upward photoelectrons is observed in the antisunward than in the sunward sector, whereas downward photoelectrons show a weak asymmetry. Our numerical simulations demonstrate that the greater flux of upward photoelectrons is caused by the electrons emitted from the sunlit surface; they are attracted to the antisunward sector. Based on these results, the asymmetries in the photoelectron distribution measured around GEOTAIL are found to be caused by the asymmetric positioning of the thin antennas relative to the LEP.