2 UNUSUAL EPISODES OF APPROXIMATE-TO-13-DAY VARIATIONS

Fourier analysis (DFT/CLEAN) of the international sunspot number (R) s eries since 1932 has revealed two long (250-500 days) and distinct epi sodes of solar activity exhibiting persistent approximate to 13-day va riations. The first episode lasts approximate to 500 days near the max imum of solar cycle 20, and the second, approximate to 250 days near t he end of the curl ent solar cycle 22. The solar radio flux density (F -10.7cm) series since 1947 has also been analyzed. During the first ep isode both solar indices exhibit distinct approximate to 27- and appro ximate to 13-day variations (the first report of approximate to 13-day variations in F-10.7cm) During the second episode neither index exhib its distinct approximate to 27-day variations and only R exhibits appr oximate to 13-day variations. Conditions affecting the appearance of a pproximate to 13-day variations in F-10.7cm are discussed.