blue flash

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun meteorology, astronomy A very rare phenomenon observed in morning or evening when the sun is crossing or immediately below the horizon, in which a momentary flash of blue light appears above the upper rim of the solar disk, caused by refraction of light in the atmosphere.
• noun nuclear physics A phenomenon sometimes observed when a criticality incident occurs in nuclear materials, inducing a discharge of radiation into the environment.