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2. Bailey, D. K.: 1964, Planetary Space Sci. 12 495.
3. Hakura, Y.: 1968, NASA Washington D. C., NASA TN D-4473.
4. Quarterly Bulletin on Solar Activity, published by the Eidgen. Sternwarte in Zürich.
5. Mac Math-Hulbert Observatory, Pontiac, unpublished data.