A solar storm is an event in which activity on the Sun interferes with the Earth's magnetic field. Because the Sun is so far away, many people believe that these storms are not capable of causing very much damage, but they can in fact be quite devastating. Research in the early 2000s created a number of potential worst case scenarios that illustrated the destructive power of solar storms, including scenarios showing that powerful nations in the developed world could be brought to their knees for a decade or more by a storm of large magnitude.
Solar storms occur as a result of events such as solar flares and coronal mass ejections. These events generate a so-called “solar wind,” a gust of charged particles that can slam into the Earth's magnetic field in hours if the solar wind happens to be traveling in the direction of Earth. The collision can generate quite a light show as a geomagnetic storm erupts, and the storm can last for several days under the right conditions.
The most severe storm in recorded history occurred in 1859. Known as the Carrington Event, it caused electromagnetic disruptions all over the world, including auroras over the equator and the interruption of scientific devices that relied on the Earth's magnetic field to function properly. Lesser storms were also recorded in 1958, 1989, and 2000, and these events were associated with interruptions of the electrical grid and damage to satellites.
Concerns about solar storms revolve around their ability to disrupt electrical systems. A severe storm could effectively knock out the electricity grid across a large swath of the world, which would have very serious consequences, as it would be necessary to replace large portions of it. Storms can also disrupt communications and satellites, and they can be very dangerous for astronauts if they happen to be in the way of the solar wind.
A solar storm can also involve the introduction of tremendous amounts of radiation, which is dangerous for many living organisms. The disruption of the Earth's magnetic field can also be damaging. Some animals, for example, appear to be sensitive to magnetic fields and they could become disoriented in a storm.
Due to concerns about this phenomena, a number of agencies monitor solar activity for signs of events which could trigger a storm. Periodically, warnings or advisories may be issued to alert people to events that could cause disruption, such as interference with radio signals. In the event of a major storm, however, there is little that could be done to prepare or prevent damage.