"All the outer planets were found by Voyager to have auroral lights. This was a surprise to space scientists, because they have assumed that auroras on Earth are caused by the impact of the solar wind on the atmosphere above the Poles. However, with Voyager's discoveries of auroras on the outer planets which are located so far from the sun that any solar wind reaching them is far too weak to cause their auroras, scientists have begun looking for another power source that could cause the auroras."
Uranus is Hollow by Rod M. Cluff.
Perhaps the auroras are caused by energies emanating from the hollow portion of planets through the polar orifices. ( See: Magnetic Lines Enter drawing. )