Simultaneous Photoelectron and Ion Measurements in the Martian Tail

The Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) experiment on the Mars Express spacecraft contains an Electron Spectrometer (ELS), and an Ion Mass Analyzer (IMA). While orbiting Mars, the ELS is able to observe peaks in the photoelectron spectrum due to photoionization of carbon dioxide and atomic oxygen by a solar He 30.4 nm photon. The source of these peaks is the dayside Martian ionosphere, with the majority of photoelectrons created at the exobase where the density is greatest. A fraction of these photoelectrons is transported to spacecraft altitudes. Since atmospheric photoelectrons originating from the dayside ionosphere are observed in the tail of Mars, by charge conservation there should also be low energy planetary ions at these locations. Recent improvements in the sensitivity of IMA have made it possible to study cases in which ions are detected simultaneously with the ionospheric photoelectrons. In this paper we survey the cases in which photoelectrons are observed in the Martian tail region, describe the characteristics of the simultaneously observed ions, and discuss the observations in terms of ion escape from the planet.