Simultaneous observations of neutral and ionic magnesium in the thermosphere

Earth‐limb observations by the GLO instrument aboard space shuttle mission STS‐53 include the first simultaneous resonant‐scattering detection of Mg + (279.6–280.3 nm) and neutral Mg (285.2 nm) in the thermosphere. Although Mg + emissions are observed in approximately 45% of GLO scans over a wide geographic and local‐time range, neutral Mg is observed in fewer than 1% of the measurements, and most of these sightings occur on two consecutive orbits in a near‐equatorial patch at local dusk. Neutral Mg is observed at tangent heights between 150 and 230 km, with an instrument‐view‐column density of 0.4–1.4 × 10 8 cm −2 , compared with simultaneous Mg + values of 1.4–5.8 × 10 9 cm −2 . Calculated in‐cloud densities are [Mg + ] ∼250 cm −3 and [Mg] ∼5 cm −3 , and the average ratio of [Mg + ] to [Mg] in the individual measurements is 34:1.