Solar flare count periodicities in different X-ray flare classes

Using the Monet wavelet transform and the Hilbert Huang transform (HHT), we investigate the periodic behaviours of C, M and X-class flare counts, respectively, recorded by the Geo-stationary Operational Environmental Satellites (GOES) from 1983 May to 2014 December, which cover the two complete solar cycles (SCs) 22 and 23 as well as the part of declining phase of SC 21 and rise and maximum phases of SC 24. Analyses show that the periodic behaviours of various class flare counts are different. (1) Not all periods of various class flare counts appear dominant during the cycle maxima. For C-class flares, during SC 23, periods appear dominant during the maximum phase, however, compared to those during SC 23, there are more periods during the declining phase of SC 22; for M-class flares, during SCs 22 and 23, periods appear dominant during the cycle maxima; for X-class flares, during SC 22, almost all periods appear during the maximum phase; however, during SC 23, there are more periods during the declining phase compared to those during SC 22. (2) For C-class flares, the appearance of periods do not follow the amplitude of C-class flare cycles; while, for M and X-class flares, the appearance of periods follows the amplitude of the investigated corresponding class flare cycles. (3) From the overall trends, the 10 yr and longer time-scale trends of the monthly numbers of M and X-class flares, we can infer that the maximum values of the monthly M and X-class flare numbers would increase during SC 25.