A geomagnetic storm is a temporary disturbance of the Earth's magnetosphere caused by a disturbance in space weather. Associated with solar coronal mass ejections (CME), coronal holes, or solar flares, a geomagnetic storm is caused by a solar wind shock wave which typically strikes the Earth's magnetic field 24 to 36 hours after the event. This only happens if the shock wave travels in a direction toward Earth.

The solar wind pressure on the magnetosphere will increase or decrease depending on the Sun's activity. These solar wind pressure changes modify the electric currents in the ionosphere. Magnetic storms usually last 24 to 48 hours, but some may last for many days.

In 1989, an electromagnetic storm disrupted power throughout most of Quebec—it caused auroras as far south as Texas. From August 28 until September 2, 1859, numerous sunspots and solar flares were observed on the sun, the largest flare occurring on the 1st. A massive CME headed directly at Earth due to the solar flare and made it within eighteen hours—-a trip that normally takes three to four days.

On September 1 and 2nd, the largest recorded geomagnetic storm occurred. The horizontal intensity of geomagnetic field was reduced by 1600 nT as recorded by the Colaba observatory near Bombay, India. Telegraph wires in both the United States and Europe shorted out, some even causing fires.

Auroras were seen as far south as Hawaii, Mexico, Cuba, and Italy—phenomena that are usually only seen near the poles. This was the 1859 solar superstorm. On 13 March 1989 a severe geomagnetic storm caused the collapse of the Hydro-Québec power grid in a matter of seconds as equipment protection relays tripped in a cascading sequence of events.

Six million people were left without power for nine hours, with significant economic loss. The storm even caused auroras as far south as Texas. The geomagnetic storm causing this event was itself the result of a Coronal Mass Ejection, ejected from the Sun on March 9, 1989.

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