# The Sun and Space Weather
## 5 Solar Connection to Geomagnetic Storms
### CMEs and Geomagnetic Storms  
CMEs are thus a major source of southward IMF (Bz \(<0\)) owing to their flux rope nature and shock-driving capability ([98] depending on the location(s) of Bz \(<0\) within the ICME, the storm can start anytime from sheath arrival to the arrival of the back of the ICME [115]). One can think of the following geoeffectiveness scenarios depending on the location of Bz \(<0\) interval within the overall structure of ICMEs: (1) both sheath and cloud are geoeffective, (2) sheath alone is geoeffective, and (3) cloud alone is geoeffective, and (4) neither sheath nor cloud is geoeffective. When both the sheath and cloud portions are geoeffective, the Dst (disturbance storm time) profile can be complex, leading to multistep storms [116; 117]. Figure 11 shows a shock-driving CME and a schematic of its IP manifestation. The flux rope is a bundle of helical field lines that are rooted on either side of the neutral line in the source region on the Sun. In the cross-sectional plane, the field lines appear circular, with the front and back field lines pointing in opposite directions. In reality, the cross-section can be elliptical or heavily deformed due to interaction with the ambient solar wind. If the MC arrives at Earth in the flux rope configuration shown Figure 11, it will be termed as south-north (SN) MC. If the rotation is reversed, it represents a north-south (NS) MC, indicating that the leading edge now has a north-pointing magnetic field component. The NS and SN MCs are known as bipolar, as opposed to the unipolar MCs in which the axial field is in the north-south direction, while the field rotates in the east-west direction. Unipolar MCs are called fully-north (FN) or fully-south (FS) to indicate that the axial field points to the north or south, respectively. More details can be found in [118; 119; 120; 121; 122; 123; 124]. The onset of a geomagnetic storm can be delayed with respect to the arrival time of the MC, depending on the MC type and the presence of a sheath [115]. The SN, FS, and NS type MCs have average delays of about 6, 9, and 19 h, respectively, from the cloud arrival to the Dst minimum time. The SN and NS type MCs have a similar storm strength (Dst \(\sim-107\) and \(-104\) nT, respectively), and the FS MCs result in stronger storms (average \(\sim-125\) nT). Sheath storms attain their peak strengths about 3-4 h before the MC arrival because the shock arrives earlier. When the back of MCs contain high-density material due to filaments [36; 91] or when compressed by a CIR [125], the geoeffectiveness can be enhanced. Figure 11: Illustration of the CME structure responsible for geomagnetic storms. The SOHO/LASCO image of a CME with the flare location shown in the superposed SOHO/EIT image **(top left)**. TheCME is observed in the IP medium as a magnetic cloud (MC) driving a shock. The blue straight arrow points to the direction of motion of the flux rope with the shock. The blue concentric circles represent the cross-section of the MC flux rope, with the arrows indicating direction of field lines in a direction perpendicular to the flux rope axis (**right**). The downward (**upward**) arrows denote field direction pointing southward (**northward**) in the IP medium. When the field points southward, a geomagnetic storm ensues. When the flux rope axis is in the ecliptic plane, the azimuthal field becomes the Bz component. When the axis is highly inclined with respect to the ecliptic, the axial field becomes Bz. When shock-driving (see Figure 1), the sheath ahead of the flux rope contains Bz. CIRs can also be a source of Bz because they amplify the solar wind Alfven waves in the compression region. Examples of a double-dip storm and a sheath storm are shown in Figure 12. The underlying CMEs occurred on 28 and 29 October 2003 at the Sun [126]. The two CMEs are fast halo CMEs (speed > 2000 km/s) that ended up as dissimilar ICMEs. The first ICME has Bz < 0 in the sheath, followed by a large Bz < 0 in the cloud.