A geomagnetic storm is a temporary disturbance in the magnetic field surrounding Earth. Flares on the sun and/or coronal mass ejections (aka CMEs) cause these storms. When a flare erupts on the sun’s surface – and a CME, or cloud of charged particles, goes hurtling outward from the sun – a geomagnetic storm might follow a few days later. Earth would need to be in the path of the CME in order for the geomagnetic storm to take place.
The frequency of geomagnetic storms increases and decreases with the 11-year cycle of activity on the sun. During solar maximum, geomagnetic storms occur more often.
During geomagnetic storms, people at far northern and southern latitudes on Earth see increased displays of the beautiful aurorae, or northern and southern lights.
The last solar cycle – cycle number 24 – peaked in April 2014. It was a substantially quieter peak than other recent solar cycles, with sunspot numbers and other activity on the sun down to a level that hadn’t seen since cycles 12 to 15 (1878-1923).
The coming solar cycle – cycle number 25 – might start around late 2019 and continue through 2030.
Bottom line: Geomagnetic storms are disturbances in Earth’s magnetic field, caused by activity on the sun.