Solar flares or coronal mass ejections (CMEs) from the Sun can cause a disruption in the Earth’s magnetic field. This is known as a magnetic storm, geomagnetic storm or solar storm, and is a form of space weather. The storm begins 24-36 hours after the solar flare or CME, and can last from one day to several days. Intense magnetic storms can cause an interruption in electronics, such as GPS, satellites, radios and power grids, and communications. There are also studies that indicate direct dangers to human health, such as an increase in heart attacks and radiation poisoning. These events could also be extremely dangerous for unshielded astronauts.
Notable magnetic storms occur about once every 10 years. The most severe, however, occur about every century. Magnetic storms are classified in ranges of G1 (minor) to G5 (extreme), depending on the severity of the storm. These storms can result in beautiful auroras, most notably aurora borealis, or northern lights.
The USGS monitors the Earth’s magnetic field to understand the impact of solar events on the Earth’s surface. Their program, called the USGS Geomagnetism Program, has 14 observatories in the United States and its territories.
Sources:
http://www.northernlighthouse.ca/geomagnetic-storms/