Solar flares were bursts of energy given off by active stars at periodic or random intervals. Solar flares contained gamma radiation which could be used as a power source and could devastate entire planets.
Historical significance
The greatest solar flare for a thousand years hit the Earth in 1930 and, attracted by dalekanium, struck the Empire State Building in the form of a lightning strike and was used by the Cult of Skaro to create human-Daleks. Martha Jones and Lazlo were also able to channel its energy to kill the pig slaves. (TV: Evolution of the Daleks)
Earth was saved from a solar flare in the early 21st century by a global forest which grew in a day. (TV: In the Forest of the Night)
The Ghosties were forced to leave their home planet when it became threatened by solar flares. They created a fleet of spaceships, one of which landed in Cardiff in the 21st century. (AUDIO: Asylum)
On 21 November 2059, a solar flare caused Bowie Base One on Mars to lose contact with Mission Control on Earth for ten hours. (TV: The Waters of Mars)
The Eleventh Doctor claimed that the threat of solar flares roasting the Earth caused the people of the United Kingdom to depart for a new home on the Starship UK around the 29th or 30th century. (TV: The Beast Below)
According to one account, in 51st century Earth was ravaged by solar flares and the rich left the planet in a fleet of space arks. (COMIC: The Keep)
In 6087, human scientists predicted that due to solar flare activity the Earth would be uninhabitable for 5000 years, and froze a group of carefully selected humans in an "Ark" in space to repopulate the Earth after the climate had repaired itself. The rest of the humans were evacuated to Thermic shelters but were apparently not expected to survive through the entire period. (TV: The Ark in Space)
In 200,100, the Controller of the Game Station exploited a cycle of solar flares, which occurred every eleven years, to warn the Ninth Doctor and his allies about her "masters", the Daleks. During this time, the Daleks could not monitor her. (TV: Bad Wolf)
By 500,002 Earth had again been deserted due to solar flare activity. (AUDIO: The Girl Who Never Was)
Circa 2,000,000 AD, the Time Lords transported the entire solar system using a magnetron, devastating Earth. Less-informed people attributed the devastation to solar flares. (TV: The Mysterious Planet)
Some time prior to 10,000,000 AD, intense solar flares caused the Refusians of Refusis II to lose their physical forms. (TV: The Ark)