Ion and Electron Sensor Observations on Photoelectrons and Coma Development at Comet 67P/Churyumov-Gerasimenko
Thursday, 18 December 2014
The Ion and Electron Sensor (IES) on Rosetta measures 3D fluxes of electrons and total ions over an energy/charge range from 4 eV to 30 keV. The field of view is swept electrostatically over 2.8 pi steradians with energy resolution of 4% and angular resolution of 5 x 22.5 deg. for electrons and 5 x 45 deg. for ions except over the 45-deg. sector that typically views the solar wind, within which the resolution is 5 x 5 deg. As Rosetta approached to and within 100 km of the comet, enhanced fluxes of electrons were observed at energies below a few hundred eV. In one case on Aug. 2, 2014, the enhanced fluxes were observed during a time when the comet activity (gas and/or dust ejections) was observed by the multispectral imager (OSIRIS), suggesting the start of coma development. In another instance on Aug. 7, 2014, a large enhancement of electron fluxes was at energies in the tens of eV range, indicative of possible photoelectrons emitted from the nucleus. These, and expected subsequent observations, are described in this paper.