On December 27, 2004, the radiation from an explosion on the surface of SGR 1806-20 reached Earth. In terms of gamma rays the burst was brighter than a full moon… It was the brightest event known to have been sighted on this planet from an origin outside our solar system.
The gamma rays struck the ionosphere and created more ionization which briefly expanded the ionosphere.
The magnetar released more energy in one-tenth of a second than our sun has released in 100,000 years.
Such a burst is thought to be the largest explosion observed by humans in the galaxy since the SN 1604 supernova observed by Johannes Kepler in 1604.