North–south (N–S) asymmetry of FI_SXR during solar cycles 21, 22, and 23. The asymmetry has been represented by absolute asymmetry index (panel a)) as well as normalized asymmetry index (panel b)). The FI_SXR is averaged over 13 Carrington rotations for the asymmetry analysis. Vertical dashed lines indicate solar activity minima. The statistical significance of N–S asymmetry is assessed by Student’s t-test and the results are marked by circle/asterisk symbols in top panel. Here asterisks and circles represent N–S asymmetry with significance levels of ≥95% and 80–95%, respectively.