MOSCOW (Reuters) - Swathes of Russia and Ukraine were bathed in some of the strongest scarlet and green "northern lights" for years on Monday due to solar flares, according to pictures posted on social media and Russian media.

The so-called "aurora borealis" bathed swathes of Siberia, the Urals, southern Russia and Ukraine in green, scarlet and purple overnight.

Pictures posted on social media showed the night sky across Russia shining red and green.

The lights are generated by streams of charged particles from the sun which penetrate the earth's atmosphere and collide with gas molecules which then release photons of light.

The New Scientist magazine said in September that the northern lights are expected to be stronger this year than for at least a decade due to a surge in activity in the sun.

(Reporting by Reuters; editing by Guy Faulconbridge)