Detection of high-altitude explosions by observation of air fluorescence

X-rays radiated from a nuclear explosion in space excite fluorescence of air molecules in the upper atmosphere. The fluorescence has a characteristic space-time signature which can be recognized from the ground even against a bright daylight sky. The criteria determining detectability and range are discussed, along with a number of practical systems of detection. False alarms, caused mainly by lightning flashes, can cause excessive triggering of single-station detectors unless discrimination techniques are employed. They also compel reliance on a net of stations for reliable recognition of true, nuclear explosion induced fluorescence.