Gamma radiation

 was the name of a type of radiation that could originate in a solar flare and be collected by a collector which included components of Dalekanium. The Daleks called the expected surge of gamma radiation a gamma strike. When the solar flare and the surge happened, it was either concurrent with or took the form of a bolt of lightning. (TV: Evolution of the Daleks)

Behind the scenes
Either there was a second type of energy released by solar flares with the same name, or gamma radiation behaved differently in the DWU. In the real world, gamma radiation was the type of roentgen radiation used in X-ray machines, making this the type of radiation amplified and used on the Slab in TV: Smith and Jones. However, this type of radiation was invisible and travelled in a straight line, was largely absorbed by the atmosphere, and could be blocked by a significant layer of rocks and metals. The gamma radiation from a solar flare at night would be blocked by the bulk of the earth and the atmosphere, and would not manifest as a bolt of lightning.