Professor Martinis studies the low and midlatitude ionosphere using all-sky optical imagers built in-house and distributed worlwide. Naturally occurring chemical reactions in the nighttime upper atmosphere produce very tenuous light at different wavelength. These emissions, known as airglow, can be studied with very sensitive cameras and narrow band filters to give information on processes occurring at different heights. The optical analysis is complemented and augmented with in-situ satellite measurements and ground-based GPS and radar data. One of the processes studied, known as equatorial Spread-F, is related to large scale plasma irregularities that can have serious effects on propagation of radio signals. The research involves the investigation of the climatology of low and midlatitude ionospheric processes, electrodynamical coupling between ionospheric regions, and inter-hemispheric studies of processes affected by the presence of Earth’s magnetic field.