An international collaboration is forged to investigate the triggers of the Equatorial Spread-F (ESF) phenomenon, using balloon and ground-based measurements. The high-altitude balloons permit measurement of the resonant 280 nm airglow emission of ionized Magnesium originating from the upper mesosphere, near 85 km. These measurements are coupled with OI 555.7 nm and OI 630.0 nm airglow measurements to sample at altitudes of approximately 115 km and 230 km, respectively. Periodic variations of these airglow feature intensities are associated with gravity waves, which in turn may provide the Rayleigh-Taylor instability trigger of ESF. To investigate the possible gravity wave triggering mechanism, afternoon and twilight measurements of these emissions are necessary, so that the hypothesized gravity wave catalyst is isolated before ESF onset - rather than postulated after ESF onset by correlation with possible wave structures in the F-region plasma. The daytime measurements are best made above the altitude where Rayleigh scattering contaminates the entire daytime sky with background sunlight, in this instance by flying low-cost spectrometers on balloons.
