# The Sun and Space Weather
## 5 Solar Connection to Geomagnetic Storms  
It was recognized a long time ago that geomagnetic disturbances are intimately related to the southward IP magnetic field [82; 83]. When the southward magnetic field component of an IP structure such as an ICME reconnects with the Earth's field in the magnetosphere, a geomagnetic storm ensues. Following the dayside reconnection, a nightside reconnection occurs and particles are injected into the magnetosphere, enhancing the ring current, which affects Earth's magnetic field at ground level [82]. The storm strength is measured by an index such as Dst, which is an average deviation of Earth's horizontal magnetic field measured at four low-latitude magnetometer stations [84] ([[https://wdc.kugi.kyoto-u.ac.jp/dstdir/dst2/onDstindex.html](https://wdc.kugi.kyoto-u.ac.jp/dstdir/dst2/onDstindex.html)]([https://wdc.kugi.kyoto-u.ac.jp/dstdir/dst2/onDstindex.html](https://wdc.kugi.kyoto-u.ac.jp/dstdir/dst2/onDstindex.html)), accessed on 1 October 2022). While the southward magnetic field is necessary for a storm, the storm strength is determined by additional solar wind parameters such as the speed and dynamic pressure [85; 86; 87; 88; 89; 90; 91].  
An IP structure that causes a geomagnetic storm is said to be geoeffective. In the undisturbed solar wind, the IP magnetic field is in the ecliptic plane and hence does not have an out-of-the-ecliptic component (Bz), except for Alfven waves. The CME connection to geomagnetic storms comes from the fact that the IP manifestation of CMEs such as magnetic clouds [92; 93] possess significant Bz, which causes a geomagnetic storm when negative (southward) [94; 95; 96]. Sheath regions behind ICMEs often contain Bz < 0 and cause geomagnetic storms [97; 98].  
Coronal holes are regions of the corona where the density is low and the photospheric magnetic field underlying them is predominantly unipolar, indicating open magnetic flux (see [99] for a review). Plasma is free to escape along the open-field lines, observed as an HSS. The speed of HSS observed at 1 au has been found to depend on the coronal hole area, expansion factor of the magnetic field, and the photospheric magnetic field strength [100; 101; 102; 103; 104; 105]. Coronal holes are generally of limited spatial extent, so an HSS typically presses against a preceding slower wind, forming a SIR. When a SIR is observed for more than one solar rotation, it is called a corotating interaction region (CIR). A CIR/SIR is identified based on the increase in density, temperature, and magnetic field strength during the positive gradient of the solar wind speed [106; 107; 108; 109; 110]. The occurrence rate of SIRs is solar-cycle dependent, with a majority of them occurring during the declining phase of a solar cycle (see e.g., [111] and references therein). The solar cycle variation of CIRs reflect the occurrence pattern of coronal holes on the Sun at low and high latitudes [103; 112]. SIRs possess enhanced density, dynamic pressure, temperature, magnetic field strength, and speed compared to the preceding solar wind. Many of these are important in causing geomagnetic storms [113]. Table 1 compares the SIR parameters with the corresponding ones in the solar wind (from [114]).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multicolumn{2}{c}{**CIR**} & \multirow{2}{*}{**Solar Wind**} & \multirow{2}{*}{**Ratio**} \\  & **Range** & & & \\ \hline Density [cm\({}^{-3}\)] & 2.4–81.0 & 29.3 & 6.7 & 2.93 \\ Dynamic pressure [nPa] & 1.4–57.2 & 10.5 & 2.3 & 4.57 \\ Temperature [10\({}^{5}\) K] & 0.97–26.35 & 4.91 & 1.02 & 4.81 \\ Magnetic field [nT] & 4.6–44.9 & 14.8 & 5.5 & 2.69 \\ Duration [hr] & 2.75–82.10 & 26.47 & — & — \\ Extent [au] & 0.03–0.98 & 0.31 & — & — \\ \hline \hline \end{tabular}
\end{table}
Table 1: Properties CIR and solar wind parameters.