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

BBC: SNO, SNO

It is unclear how solar particles affect rates of radioactive decay, but Prof Jenkins asserts that "either neutrinos are affecting the decay rate or perhaps an unknown particle is".

BBC: System could warn of solar storms

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

Besides being a useful source of dark matter, it may explain the solar-neutrino deficit (the gap between prediction and observation would be caused by a different mutation, from electron neutrinos into undetectable tau neutrinos).

ECONOMIST: Neutrinos