Cosmic rays at high latitudes and altitudes covering four solar maxima

Since 1937 balloon flights have been made in which instruments measuring cosmic rays were carried to high altitudes at high latitudes. The earlier flights involved only the years 1937, 1938, 1940, and 1946, but since 1951 only 1952 and 1953 have been missed. The instruments used were ionization chambers, which were compared with standards kept at Pasadena. Checks on absolute values of ionization at intervals indicate that experimental results over the 32-year period can be closely compared. In addition to giving a summary of already published results, data from balloon flights at Thule for the years 1966 through 1969 are presented for the first time. In comparing cosmic rays at high altitudes during the 1930's with similar measurements in the 1960's, it is shown that the intensity of cosmic rays was at least 10% higher at 20 gm cm−2 during the earlier periods for similar solar activity. It is suggested that this long-term modulation effect probably takes place where the solar wind encounters the interstellar medium and may be determined by the average solar activity of several preceding solar cycles.