A MAVEN investigation of O⁺⁺ in the dayside Martian ionosphere

O⁺⁺ is an interesting species in the ionospheres of both the Earth and Venus. Recent measurements made by the Neutral Gas and Ion Mass Spectrometer (NGIMS) on board the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft provide the first firm detection of O⁺⁺ in the Martian ionosphere. This study is devoted to an evaluation of the dominant O⁺⁺ production and destruction channels in the dayside Martian ionosphere, by virtue of NGIMS data accumulated over a large number of MAVEN orbits. Our analysis reveals the dominant production channels to be double photoionization of O at low altitudes and photoionization of O⁺ at high altitudes, respectively, in response to the varying degree of O ionization. O⁺⁺ destruction is shown to occur mainly via charge exchange with CO₂ at low altitudes and with O at high altitudes. In the dayside median sense, an exact balance between O⁺⁺ production and destruction is suggested by the data below 200 km. The apparent discrepancy from local photochemical equilibrium at higher altitudes is interpreted as a signature of strong O⁺⁺ escape on Mars, characterized by an escape rate of 6×10²² s^–1.