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.568L The Astrophysical Journal, 337:568-578,1989 February 1 © 1989. The American Astronomical Society. All rights reserved. Printed in U.S.A. .337. 9ApJ. 198 INTERMEDIATE-TERM SOLAR PERIODICITIES: 100-500 DAYS J. L. Lean Applied Research Corporation AND G. E. Brueckner E. O. Hulburt Center for Space Research, Naval Research Laboratory Received 1987 July 14; accepted 1988 June 23 ABSTRACT Penodogram analyses of time series of ground-based solar observations during the past three solar cycles confirm that a penodicity near 155 days, originally detected in the occurrence rate of major solar flares is also present in the sunspot blocking function, the 10.7 cm radio flux, and the sunspot number, but not in the plage index. This indicates that the 155 day solar periodicity is not simply a feature of flare activity alone and suggests that it is associated with those regions on the Sun where magnetic fields are concentrated into small areas, such as in sunspots, but not where they are more dispersed, as in plages. Power at 155 days is present m the sunspot blocking function and the 10.7 cm radio flux during each of the three solar cycles, 19, 20 and 21, though it is weaker in cycle 20 than in the other two cycles. The periodogram of the sunspot blocking tunction, 10.7 cm radio flux, sunspot number, and plage index daily data during the entire three-cycle time span each has a peak near 323 days, which supports previous suggestions that this, too, may be a real solar period. However, this period is a prominent feature only in solar cycle 21. Subject headings: Sun: activity — Sun: flares — Sun: radio radiation — Sun: sunspots I. INTRODUCTION Sturrock (1987) disagree that the 155 day flare occurrence That variations in the solar output measured at Earth have period is due to the interaction of active zones rotating at periods of approximately 11 yr and 27 days is well established, slightly different rates and claim that it is a global phenome- the former being related to the solar magnetic activity cycle, non, occurring independently in both solar hemispheres, and the latter reflecting the modulation imposed on the solar flux characteristic of flares produced both inside and outside of at Earth by solar rotation. Establishing the existence of real Bai’s superactive zones. Additionally confusing is the recent periods, other than at 11 yr and 27 days, in solar observational report by Bai (1987b) that, during cycle 19, the global data has long been of interest, for the clues this may provide to occurrence of major flares, as determined from the com- the mechanisms of solar variability. prehensive flare index (CFI > 5), did not exhibit a periodicity Recently, attention has focused on the periodicity near 155 near 155 days. Rather, the dominant periodicity was at 51 days days (~75 nHz) in the occurrence of flares, first reported by which, it was noted, may be a harmonic ofthat at 155 days. Rieger et al. (1984). Subsequent efforts characterizing the Evidence for a real solar periodicity near 155 days in other nature of this cycle have continued to utilize, primarily, records indicators of solar activity is, to date, less convincingly than in which measure directly, or infer, flare occurrence. From the the flare records. In the power spectrum of the monthly mean analysis of SMM and GOES satellite data (Rieger et al. 1984; sunspot numbers from 1874 to 1979 Wolff (1983) did detect, Kiplinger, Dennis, and Orwig, 1984; Bai and Sturrock 1987) among a multitude of peaks with similar power, a peak near and ground-based microwave and Ha data (Bogart and Bai 155 days. Delarche, Laclare, and Sadsaoud (1985) also 1985; Ichimota et al. 1985), the picture which has thus far observed this period in the solar diameter record and Zürich emerged is of a period between 152 and 158 days which was sunspot numbers during cycle 21. However, Hughes and stronger in cycle 21 than in cycle 20 and which is associated Kestevan (1981) make no mention of such a period in their predominantly, but not exclusively, with active complexes, analysis of the daily 10.7 cm radio flux from 1947 to 1978. Nor, especially those with large sunspots. according to Hudson (1987), is it striking in the synoptic data While it is generally agreed that the 155 day period is indeed that display active regions clearly, such as the Kitt Peak syn- of solar origin, the associated physical mechanisms are still optic magnetic data. being debated. Wolff (1983) has suggested that rotation coup- There is, generally, even less confidence in the reality of ling of active bands generated by the solar 0-mode oscillations intermediate-term solar periodicities longer than at 155 days, with spherical harmonics l = 2 and l = 3 may generate this because of the brevity of the time series and the presence of period. Bogart and Bai (1985) attribute it to the interaction of considerable nonoscillatory power (Hudson 1987), although active regions or active Carrington longitudes, rotating at dif- their existence has been speculated from the analysis of ground ferent rates. Ichimota et al. (1985) suggest that it originates based solar activity data. Both Wolff (1983) and Delarche, from strong “magnetized streams” which last for several rota- Laclare, and Sadsaoud (1985) detected a peak near 320 days tions and are easily recognized in magnetic field synoptic (36 nHz) in the Zürich sunspot number variations, and this charts. These streams are, according to Bai and Sturrock peak is also present in Delarche et al. 's power spectrum of the (1987), equivalent to the superactive zones of higher than solar diameter record during cycle 21. In their analysis of the average flare occurrence rate, described by Bai (1987a). Bai and area and numbers of sunspot groups from 1969 to 1986, 568 © American Astronomical Society • Provided by the NASA Astrophysics Data System .568L INTERMEDIATE-TERM SOLAR PERIODICITIES 569 .337. Akioka et al (1987) detected a peak near 12 solar rotations, in Figure 1. Over this total time interval of 11323 days, data which may correspond to the 320 day periodicity. They also are available for Ps, F10.7, Rz, and PI on, respectively, 92.0%, reported a 17 month (~510 day) periodicity. Hudson (1987), 98.6%, 92.9%, and 77.5% of all days. That there is a linear 9ApJ. however, cautions that power spectrum peaks at long periods, relation between each of these time series is well known and is 198 for example at 36 nHz, may be partly artifacts of running mean demonstrated by the high values of the linear regression coeffi- smoothing and normalization of the time series, such as were cients which are given in Table 1 for both the raw, daily data, used in Wolffs analysis. and the data detrended by removing a 365 day running mean In this paper we investigate the existence of periodicities in from the daily values. None of the four time series are, ground-based records of solar activity during the past three however, identical (see, for example, Donnelly, Hinteregger, solar cycles. We analyze the sunspot blocking function, the and Heath 1986). 10.7 cm radio flux, the Zürich sunspot number, and the plage III. PERIODOGRAM ANALYSIS index. Our primary focus is the period near 155 days. If this period in solar flare occurrence is, as speculated by Bogart and Each of the four data sets are analyzed for periodicities Bai (1985), associated with complexes of solar active regions, using, first, data for the entire time span of three solar cycles, then it should be detectable in at least some of the ground- and then the data for each separate cycle. An overview of the based solar activity time series, unless it is a feature of flare procedures is given in this section, and the results of the activity alone. Furthermore, since the magnetic fields which analysis are given in § IV. form an activity complex manifest a variety of solar pheno- We use the technique of Horne and Bahúnas (1986), based mena such as sunspots, plages, and hot coronal loops, then the on Scargle (1982), to calculate the modified periodogram for detection of this cycle in one or other of these active region unevenly sampled data. Missing data in the time series of daily phenomena may yield physical insight as to its origin. solar observations shown in Figure 1 preclude the use of the We also examine, in the ground-based data, the reality of Fast Fourier Transform for calculating spectral density func- solar periodicities other than at 155 days, concentrating on tions directly from the time series, unless the missing data periods between 100 and 500 days. Presumably, peaks which points are filled in or the data are binned into wider time are common to the power spectra of a variety of solar activity intervals. Interpolating missing data points and binning indicators give greater confidence that the corresponding unevenly sampled observations onto an evenly sampled grid cycles may be intrinsic solar variability, rather than artifacts of may, however, alter the perceived frequency and significance of the analysis. a periodic signal (Horne and Bahúnas 1986). Initially, the periodograms are determined from the raw II. SOLAR ACTIVITY TIME SERIES daily data, transformed to zero-mean time series, but without Each of the four records of solar variability, the sunspot smoothing, binning or removing long-term trends. The power blocking function (Ps), the 10.7 cm radio flux (F101), the is calculated at 100 equally spaced frequencies over the two Zürich sunspot number (Rz) and the Ca n K plage index (PJ), separate frequency intervals, 55-129 nHz (~ 100-200 days) reflects different solar active region phenomena.

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