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Proceedings of open meetings of working groups on physical sciences of the eighteenth Plenary Meeting of COSPAR, Varna, Bulgaria, 29 May-7 June 1975 and COSPAR Symposium and Workshop on Results from Coordinated Upper Atmosphere Measurement Programs, Varna, Bulgaria, 29-31, May 1975 / organized by the Committee on Space Research - COSPAR and the Bulgarian Academy of Sciences ; edited by M.J. Rycroft
(COSPAR space research ; 16)