Electron-neutrinos are created in the thermonuclear reactions at the solar core.

BBC: SNO, SNO

One explanation for the solar-neutrino deficit (only a third of the predicted number have been detected) is that electron-neutrinos produced by nuclear-fusion reactions in the sun's core mutate into the other two types, which are harder to detect.

ECONOMIST: The news that neutrinos have mass is both good and bad