blue flash

Short definition from Wiktionary

Noun

1. 1
   A very rare phenomenon observed in the 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.
2. 2
   A phenomenon sometimes observed when a criticality accident occurs in nuclear materials, inducing a discharge of radiation into the environment.