A solar flare is term used to describe an explosion on the sun. Flare is a homophone of flair. Flair means to have an inclination or tendency towards something. The scientists in this joke have an inclination for research, and the word "flare" is used because its a homophone of "flair" and the scientists are studying explosions on the sun, or solar flares.
Scientists studying the sun have a flare for research .