Outflow of ions from Ganymede: A reinterpretation

The cold, dense plasma in the polar mag netosphere of Ganymede has been obs erved to contain one species of positive ions. The usual assumption that th ey are protons leads to severe problems, e.g. finding an adequate ionizatio n mechanism for hydrogen, or explaining why the expected oxygen ions are no t found and what happens to the oxygen. We propose that the observed ions a re in fact singly charged oxygen ions rather than protons, which immediatel y removes all these problems. We show that the assumption of O+ instead of H+ is fully consistent with the plasma ion measurements and, by multiplying the inferred number density by 4 and speed by 1/4, brings them into better agreement with independent estimates of electron density and ion bulk flow .