# The Sun and Space Weather
## 9 Extreme Space Weather Events  
The mild space weather discussed in the previous section is a case of extreme event. In the opposite end of the spectrum, there are large events in the recent history as well as in the natural archives, such as tree rings and polar ice cores (see [78] for a review). A rough idea on the size of extreme events can be obtained by looking at the cumulative distribution of known events. Figure 24 shows the cumulative distributions of event sizes in large SEP events and intense geomagnetic storms. The distributions are obtained using modern data available in the space age. From the distributions, we can see that the one-in-100 year and one-in-1000 year SEP events have sizes of \(\sim\)2 \(\times\) 10\({}^{5}\) pfu and 1 \(\times\) 10\({}^{6}\) pfu, respectively. The largest event plotted in Figure 24a occurred on 23 March 1991 and has a size of \(\sim\)4.3 \(\times\) 10\({}^{4}\) pfu [189], about five times smaller than a 100-year event. The 23 July 2012 event was estimated to have a similar size observed at STEREO ahead [157]. The tree-ring event of AD 774 identified by [PERSON] et al. [190] is indeed a 1000-year event. Inspired by this event, further investigations have resulted in several tree-ring and ice core events that qualify for a 1000-year event [78]. Figure 23: # means Number in Figure 23. The number of CME-associated major geomagnetic storms (**a**) and large SEP events (SEPEs) (**b**) summed over Carrington rotation periods. In (**a**), the numbers in parentheses are the total number of storms including those due to CIRs. SSN is shown in gray. Figure 22: (**left**) Microwave butterfly diagram (contours) showing the 17 GHz brightness temperature at low latitudes due to active regions and at high latitudes due to the polar magnetic field. The red circles represent locations of prominence eruptions detected automatically in 17 GHz images of the Sun. (**right**) Comparison between CME and prominence eruption (PE) rates. The PEs from the Northern and Southern Hemispheres are distinguished by different colors. The gray plot in the background is the smoothed sunspot number. The CME and PE rates have been smoothed over 13 Carrington rotations. The cumulative distribution of intense geomagnetic storms is based on the Dst index recorded since 1957. The Weibull fit to the cumulative distribution shows that 100-year and 1000-year event sizes are \(-603\) nT and \(-845\) nT, respectively. The 14 March 1989 storm is the largest event (Dst = \(-589\) nT), plotted in Figure 24b, which is clearly a 100-year event. The intensity of the Carrington 1859 event has been estimated to be between \(-850\) nT [192] and \(-1600\) nT [193], indicating that it clearly is a 1000-year storm. Several new events have been identified based on sightings of low-latitude overhead auroras: about six 100-year storms and three 1000-year storms have been identified over the past five centuries [78]. Extreme SEP events and geomagnetic storms are exclusively due to energetic CMEs. Figure 25 shows the cumulative distribution of CME speeds with the average speeds of various CME populations. CMEs associated with purely metric type II bursts have a speed of only \(\sim\)600 km/s, still faster than the general population (\(\sim\)400 km/s). On the other hand, GLE-associated CMEs are the fastest (\(\sim\)2000 km/s). Halo CMEs and CMEs associated with magnetic clouds (MC), non-cloud ejecta (EJ), and IP shocks (S) are similar to those associated with geomagnetic (GM) storms. This is understandable because all these CME populations indicate CMEs directly impacting Earth and causing GM storms. The next two populations are CMEs causing decameter-hectometric (DH) type II bursts and large SEP events. These events are due to electrons and ions accelerated by CME-driven shocks, similar to GLE events but accelerated to lower energies. This figure shows that a couple of thousand CMEs with speed exceeding \(\sim\)600 km/s have significant space weather consequences.