E-field at a low-activity comet derived from cometary ion velocity distributions

The ion spectrometer ICA onboard the Rosetta mission has provided us with detailed measurements of the plasma environment around comet 67P/Churyumov-Gerasimenko. The distribution of cometary ions is an important indicator for the cometary plasma environment and its interaction with the solar wind. The cometary ion production at a comet decreases with increasing radial distance from the comet. The fluxes of observed ions are therefore also expected to fall off proportional to their point of origin relative to the comet. Due to electric fields, ions that are born further away are observed at higher energies at the spacecraft. The exact relation between the flux and the observed energy depends on the density distribution of cometary ions and the electric field around the comet.
We derive the bulk flow properties of the cometary pickup ion population with a fitting procedure. In our study, cometary pickup ions are all heavy ions with an energy above 40 eV as observed by ICA. The particle drift speed of the bulk flow can be used to estimate the average local electric field. This gives us a 3D estimate for the electric field close to the comet nucleus. Using this E-field estimate, we can back-trace the observed particles to determine an approximate location of where they were born. This ionisation point is further away for particles with higher energies. The relation between the flux of the observed ions and their origin provides us with information about the inhomogeneity of the cometary plasma environment between the observation point (30 km from the nucleus) and the ionisation point of the particle (hundreds of km away from the nucleus). 
We will present results of the ion distribution of cometary pickup ions above 40eV on a selected day (April 19th, 2016) of the Rosetta mission, along with the derived electric field estimate close to the nucleus. We will also show the results and implications of the particle trajectory backtracing.