(12) had noted an unusual ring-shaped cloud at —35 km (21 mi) over Flagstaff, Arizona. Meinel et al. also connected this cloud with a rocket launch in California. Benech and Dessens (13) observed artificial noctilucent clouds (ANLCs) on 2 evenings (February 23, 1971 and March 18, 1972) when test rockets were accelerating through the upper mesosphere over southern France. On the second occasion, careful observations were made of the evolution of the cloud, which was visible against the darkening twilight sky for more than 30 min. Initially, a bright exhaust
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