The space environment near Earth is constantly subjected to changes in the solar wind flow generated at the Sun. Examples of effects resulting from this variability are the occurrence of powerful solar disturbances, such as coronal mass ejections (CMEs). The impact of CMEs on the Earth's magnetosphere very often greatly perturbs the geomagnetic field causing the occurrence of geomagnetic storms. Such extremely variable geomagnetic fields trigger geomagnetic effects measurable not only in the magnetosphere but also in the ionosphere, upper atmosphere, and at the ground. For example, during extreme cases, rapidly changing geomagnetic fields generate intense geomagnetically induced currents (GICs). Intense GICs can cause dramatic effects on human technological systems, such as damage to high-voltage power transmission transformers leading to interruption of power supply, and/or corrosion of oil and gas pipelines. These space weather effects can in turn lead to severe economic losses. In this chapter, we supply the reader with theoretical concepts related to GICs as well as their general consequences. As an example, we discuss the GIC effects during the June 22, 2015 geomagnetic storm.
