# The Sun and Space Weather
## 8 Solar Cycle Variation of Space Weather Events
### Solar Cycle Dependence of Space Weather Consequences  
The reduction of solar activity in SC 24 resulted in mild space weather during that cycle. Figure 23 shows the time variation of the number of CME-associated major geomagnetic storms (Dst \(<-100\) nT) and large SEP events (>10 MeV proton intensity \(\geq 10\) pfu) since 1996. We see that the numbers dropped by 74% and 55%, respectively. When the CIR and CME storms are combined, the reduction is similar. The number of major storms due to CIRs drops from 9 to 2 (or by 78%). The reduction in the number of storms is more than that in SSN and FW CMEs. This can be attributed to the weakened state of the heliosphere in cycle 24. The reduced solar activity results in a weaker heliospheric pressure, which backreacts on CMEs, making them magnetically dilute due to the anomalous expansion. Furthermore, the SC-24 CMEs are slower on average. Since the storm strength is primarily decided by the product of CME speed and the southward IMF, reduction in both factors is responsible for the reduced number of storms. In the case of CIR storms, the reduced heliospheric magnetic field should result in reduced field strength in the compressed interface, contributing to the reduced number of geomagnetic storms. In the case of SEP events, the reduction is primarily due to the reduced number of FW CMEs. The severest reduction is in the number of GLEs: 16 in SC 23 vs. just 2 in SC 24 (i.e., an 87% drop). This has been attributed to the reduced acceleration efficiency in the weakened ambient magnetic field, so particles did not attain high energies [55; 144]. An additional reason could be the presence of fewer seed particles [188].  
Figure 19: The occurrence rate of (**a**) regular (width \(\geq\) 30\({}^{\circ}\)) and (**b**) fast and wide (FW) CMEs, observed by SOHO/LASCO (in red) since 1996 (superposed on the daily SSN in gray). Also shown in blue are the number of major X-ray flares (M and X class) in (**b**). (**c**) CME speed averaged over Carrington rotation periods.  
Figure 21: Scatter plot between the sunspot number and daily CME rate in solar cycles 23 (**top row**) and 24 (**bottom row**). The left, middle, and right columns give the scatter plots in the rise, maximum, and declining phases, respectively. The correlation coefficients and regression lines are shown on the plots. Note that the maximum phases in the two cycles have the lowest correlation between the SSN and CME rate.  
Figure 20: Scatter plot between the sunspot number and CME occurrence rate for the whole SOHO observing period until the beginning of the year 2020 (**left**) and separately for cycles 23 and 24 (**right**). The lower correlation on the left plot is because the relationship changed in cycle 24 compared to cycle 23. For example, the high rate around SSN = 100 is entirely due to cycle-24 CMEs. For a given SSN, the CME rate is much larger in SC 24. For SSN = 100, the regression lines indicate a CME rate of 3.42 in SC 24, compared to 2.16 in SC 23. The scatter plot will be revisited after checking whether CME identification made by different people might have affected the CME rate, especially those with widths close to 30\({}^{\circ}\).