Intermediate-term periodicities in relativistic solar electron fluences during solar cycles 22 and 23 Partha Chowdhury a,b , Manoranjan Khan c , P.C. Ray d, * a University of Calcutta, Office of Controller of Examinations, 87/1 College Street, Calcutta 700073, India b Departments of Natural Sciences, West Bengal University of Technology, BF-142, Salt Lake, Kolkata 700064, India c Department of Instrumentation Science & Centre for Plasma Studies, Jadavpur University, Calcutta 700032, India d Department of Mathematics, Government College of Engineering and Leather Technology, LB-Block : Sector III, Calcutta 700098, India Received 13 February 2008; received in revised form 9 June 2008; accepted 19 June 2008 Abstract In this paper, we have investigated the intermediate-term periodicities of the relativistic (E > 10 MeV) solar electron flares measured by IMP-8 satellite of NASA for the time period of 1986–2001. This period of investigation includes the entire solar cycle 22; ascending, maximum and a part of descending phase of the current solar cycle 23. To determine accurately the occurrence rate of electron flux, we have employed three different spectral decomposition techniques, viz. fast Fourier transformation (FFT); maximum entropy method (MEM) and Lomb–Scargle periodogram analysis method. For solar cycle 22, in the low frequency range, power spectrum analysis exhib- its statistically significant periodicities at 706, 504 and 392 days. In the intermediate frequency range, we have found a series of significant periodicities 294, 221, 153, 86, 73 and 66 days. For short term, periodicities of 21–23, 31 and 37 days were found in power spectrum. When solar cycle 23 is considered the significant periodicities are 20, 23, 29, 39, 54, 63, 118, 133 and 154 days. These results provide evidence that the best known Rieger period (153 days), appeared in the high energetic elec- tron flux data for cycle 22 and also likely during maxima of cycle 23. The existence of these periodicities has been discussed in the light of earlier results. Ó 2008 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Sun: activity; Flares; Electron emission; Solar wind 1. Introduction Solar activities appear to exhibit extremely complex time dependent periodic and quasi-periodic variations ranging from weeks to decades. Study of periodicities in solar observational data, both ground-based and satellite obser- vations have long been of high interest. Existence of a long- term cycle around 11 year (Schwabe cycle) and short term of 26–27 days periods are related to the solar magnetic activity and to the modulation of solar wind features due to rotation of the Sun, respectively. A search for additional possible periodicities in solar data is necessary as any absolute detection of periodicity in active phenomenon may be significant for our understanding of solar activity. Periodic variations of solar activity in the intermediate term have been studied extensively, starting with the dis- covery of 153 days periodicity in c-ray and X-ray flare data (Rieger et al., 1984; Dennis, 1985) taken from the SMM and GOES satellites in solar cycle 21. During the same cycle it was also found in proton flares (Bai and Cliver, 1990), microwave flares (Bogart and Bai, 1985; Kile and Cliver, 1991), energetic interplanetary elec- trons (Dro ¨ge et al., 1990; Chowdhury and Ray, 2006; Chowdhury et al., 2008), interplanetary magnetic field intensity (Cane et al., 1998), photospheric magnetic flux (Ballester et al., 2002). It has also been found in H a flare data of combined solar cycles 20 and 21 (Ichimota et al., 1985) and in the solar flare index (O ¨ zgu ¨c and Atac, 0273-1177/$34.00 Ó 2008 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2008.06.008 * Corresponding author. Tel.: +91 33 2334 0735; fax: +91 33 2335 6977. E-mail addresses: partha240@yahoo.co.in (P. Chowdhury), mkhan_ ju@yahoo.com (M. Khan), raypratap1@yahoo.co.in (P.C. Ray). www.elsevier.com/locate/asr Available online at www.sciencedirect.com Advances in Space Research 43 (2009) 297–307