# The Sun and Space Weather
## 5 Solar Connection to Geomagnetic Storms
### CMEs and Geomagnetic Storms
In the second MC, the sheath has a large Bz < 0 with mostly Bz > 0 in the cloud. The reason for the different appearance of MCs is that they originate from different neutral lines in the source active region. The neutral line/filament is a first good indicator of the expected orientation of the flux rope axis in the IP medium [127]. Figure 12: Two super-intense magnetic storms from the Halloween 2003 period, one caused by an FS MC, the other caused by a sheath. The underlying CMEs originated from the same active region, one day apart (from [115]). The speed and magnetic content of ICMEs are ultimately connected to the free energy in the source magnetic region on the Sun. One of the parameters that is readily measured during an eruption is the total reconnected flux, which is highly correlated with the CME speed [128; 129], CME kinetic energy, and flare fluence [130]. By combining the reconnected flux and the geometrical parameters of the CME obtained from flux rope fit to white-light images, it has been shown that the axial field strength near the Sun is correlated with the CME speed [130]. Such a relationship was obtained in MCs at 1 au [122; 131]. These studies suggest that faster CMEs are likely to cause stronger geomagnetic disturbances when Bz < 0. Figure 13 shows three observational facts about CMEs causing intense (Dst \(\leq-100\) nT) geomagnetic storms: (i) The CMEs are two times faster than typical CMEs (955 km/s vs. 416 km/s). (ii) Two thirds of the storm-causing CMEs are halos; the average width of non-halo CMEs is 175\({}^{\circ}\), much larger than the typical width of CMEs, viz, 38\({}^{\circ}\). (iii) The heliographic locations of the storm-causing CMEs are concentrated near the solar disk center, especially the ones causing more intense storms. The geoeffectiveness of CMEs decreases as a function of the central meridian distance [132]. (i) and (ii) imply that the CMEs are very energetic (wider CMEs are more massive, and hence the kinetic energy is high--see [55]). (iii) implies that CMEs heading directly toward Earth are more impactful in causing geomagnetic storms. Note that almost all storms with intensities \(<-200\) nT are within \(\pm 30^{\circ}\) longitude. This was recognized long time ago by [PERSON] [133], who found that the locations of the flares associated with great storms are close to the central meridian, with a slight bias to the Western Hemisphere (see also [1] for details). The slight western bias has been demonstrated using CME data by [134]. The western bias is related to the fact that CMEs are deflected slightly eastward due to solar rotation [135]. The source locations are also distributed around N15 and S15 latitudes, which correspond to the active region belt. Active regions possess the highest levels of magnetic energy needed to power these energetic CMEs. Another implication of the source locations close to the disk center is that the CME speeds are underestimated because of projection effects. If we apply the empirical relationship V3D = 1.1 Vsky + 156 ([136]), we see that for Vsky = 955 km/s, the average 3D speed (V3D) of storm-producing CMEs becomes 1207 km/s. Occasionally, CMEs originating close to the limb also cause intense storms. There are five limb halos (CMD \(\geq\) 60\({}^{\circ}\)) in Figure 13c that produced intense storms. These CMEs are geoeffective because they are very energetic and their sheath with significant Bz < 0 component is intercepted by Earth. One of these storms (Dst = \(-288\) nT) is due to the 4 April 2000 west limb CME with a sky-plane speed of 1188 km/s [137; 138; 139]. The deprojected speed is \(\sim\)1450 km/s. Figure 13: # means Number in Figure 13a,b. Distributions of speed (**a**), width (**b**), and source locations (**c**) of CMEs that resulted in intense geomagnetic storms (Dst \(\leq-100\) nT), observed over a period of \(\sim\)23 years. In (**c**) three storm intensity levels are distinguished by the size of circles. Geoeffective CMEs are fast and wide, and they originate from close to the solar disk center.