LAUSR

Abstract Prior to its discovery, O+ from the Earth's high-latitude ionosphere was not expected to be present in any significant amount in the magnetosphere. It is now known that O+ can dominate ionospheric outflow and dominate magnetotail composition during geomagnetically active times. Observations from the Magnetospheric Multiscale (MMS) mission are used here to illustrate properties of O+ in the dayside outer magnetosphere and to identify gaps in the knowledge of this ionospheric ion. When the interplanetary magnetic field (IMF) is southward, O+ from the high-latitude ionosphere convects to the magnetotail then from the magnetotail to the dayside. The two primary O+ populations observed in the dayside magnetosphere are the ring current and warm plasma cloak. They are sometimes distinguishable by their energy, although velocity-space distribution functions are often needed to make a true distinction. There are several sinks for these populations in the dayside magnetosphere. One important sink is the dayside magnetopause. While much is understood about O+ in the dayside magnetosphere, there are several open questions detailed in the final section of the paper.