The North-South asymmetry of soft X-ray flare index during solar cycles 21, 22 and 23

In this paper the N-S asymmetry of the soft X-ray flare index (FISXR) during solar cycles 21, 22 and 23 has been analyzed. The results show the existence of a real N-S asymmetry which is strengthened during solar minimum. The slope of regression lines fitted to the daily values of asymmetry time series is negative in all three cycles. The yearly asymmetry curve can be fitted by a sinusoidal function with a period of eleven years. The power spectral analysis of daily asymmetry time series reveals significant periods of around 28.26 days, 550.73 days and 3.72 years.