The Arecibo Observatory was home to a unique sensitive geospace radar that probed Earth’s ionosphere—the ionized upper atmosphere—by scattering radio waves off free electrons. Thanks to the 305 m spherical dish, the radar at Arecibo was 20–50 times more sensitive than any other radar that has been used for geospace studies. Arecibo was more than one instrument: it was a multi-instrument facility that also included radars, an ionospheric modification facility, lidars, optical sensors, and satellite-based systems. This unique combination of instruments led to profound discoveries in geospace science, which are reviewed here. One major topic is ionospheric plasma physics via the study of plasma lines (a specific feature in the radar echo), which provided new insights, especially during experiments using high-power radio wave transmissions at very-high-frequency bands. Another is the ultraprecise measurements of the F-region electron concentration profiles in the ionosphere that revealed previously hidden photoelectron processes possible only because of the sensitivity of the incoherent scatter radar at the Arecibo Observatory. Nighttime plasma line detection has also offered clues about the coupling between the ionosphere and the magnetosphere. Other topics are discoveries from uniquely positioned radars operating in the same atmospheric volume and studies of atomic metals observed via lidar. A notable example is the highly sensitive lidar study of calcium ions (${\mathrm{Ca}}^{+}$) in the ionosphere.

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• Atmospheric science
• Geophysics
• Space science
• Geophysical networks