SIMULTANEOUS OBSERVATIONS OF NEUTRAL AND IONIC MAGNESIUM IN THE THERMOSPHERE

Earth-limb observations by the GLO instrument aboard space shuttle mis sion STS-53 include the first simultaneous resonant-scattering detecti on of Mg+ (279.6-280.3 nm) and neutral Mg (285.2 nm) in the thermosphe re. Although Mg+ emissions are observed in approximately 45% of GLO sc ans over a wide geographic and local-time range, neutral Mg is observe d in fewer than 1% of the measurements, and most of these sightings oc cur on two consecutive orbits in a near-equatorial patch at local dusk . Neutral Mg is observed at tangent heights between 150 and 230 km, wi th an instrument-view-column density of 0.4-1.4x10(8) cm(-2), compared with simultaneous Mg+ values of 1.4-5.8 x 10(9) cm(-2). Calculated in -cloud densities are [Mg+] similar to 250 cm(-3) and [Mg] similar to 5 cm(-3), and the average ratio of [Mg+] to [Mg] in the individual meas urements is 34:1.