Fair weather electric field measurements of the global electric circuit at the Mt. Washington Observatory.
Dr. Earle Williams stands next to an electric field mill sensor optimized for low-level electric field measurements under severe cold-weather conditions.
NASA/Langley Research Center Lear Jet configured for the Airborne Field Mill (ABFM) program. See Lightning and the Space Program (.pdf) (page 2).
The determination of the vector electric field from an aircraft (X, Y, Z, and Q- charge on the aircraft) requires at least 4 independent measurements. In practice, five or more mills are used to make the problem more tractable. Aircraft mills must be able to accurately resolve the fair weather field (~100v/m) to calibrate the aircraft geometric form factor and be able to measure thunderstorm fields at altitude (~ 200kv/m).
The NASA ER-2 is the successor to the U-2.
With a ceiling of 70,000 ft, electric field measurements over thunderstorms are possible.