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The Halloween solar storms were a series of solar flares and coronal mass ejections that occurred from mid-October to early November 2003, peaking around October 28–29. Satellite-based systems and communications were affected, aircraft were advised to avoid high altitudes near the polar regions, and a one-hour-long power outage occurred in Sweden as a result of the solar activity. Aurorae were observed at latitudes as far south as Texas and the Mediterranean countries of Europe.
The SOHO satellite failed temporarily, and the Advanced Composition Explorer (ACE) was damaged by the solar activity. Numerous other spacecraft were damaged or experienced downtime due to various issues. Some of them were intentionally put into safe mode in order to protect sensitive equipment. Astronauts aboard the International Space Station (ISS) had to stay inside the more shielded parts of the Russian Orbital Segment to protect themselves against the increased radiation levels. Both the Ulysses spacecraft which was near Jupiter at the time, and Cassini, approaching Saturn, were able to detect the emissions. In April 2004, Voyager 2 was also able to detect them as they reached the spacecraft. One of the solar storms was compared by some scientists in its intensity to the Carrington Event of 1859.
These events occurred during Solar cycle 23, approximately three years after its peak in 2000, which was marked by another occurrence of solar activity known as the Bastille Day Flare.