The vast space around the Earth is not a vacuum but is filled with plasma that is composed of positively-charged and negatively-charged particles. This space is called "geospace." In geospace, a space station (400 km altitude), GPS satellites (20,000 km altitude), and a geostationary satellite "Himawari" (36,000 km altitude) are flying, and our society depends on its use. Natural phenomena like aurora (100 km altitude) also occur there. The motion of plasma induces electric currents, which cause magnetic field variations according to the Biot-Savart law. Thus, we can monitor electromagnetic disturbances in geospace from ground geomagnetic field observations. In this article, the geomagnetic field, its observations, geospace environmental disturbances estimated from data, and geomagnetic indices will be reviewed.
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