With all that mass in the immediate neighborhood, the gravitational forces were ferocious. The young planets raised enormous tides on one another, and the friction generated by these rising and falling bulges of material raised their temperatures far higher than normal for their sizes and rates of rotation. This heating went on until the violent tidal action pushed the moon out to near its present location and slowed the earth's rotation period from close to 4 hours to 24 hours. The moon's relationship to the aurora is trickier to explain, but also relates to the high heat levels of the earth's core. The explanation requires a few steps.
Auroras begin with the sun.
Our star spits out vast quantities of electrons and protons, which stream away as the solar wind. When these particles are captured by Earth's magnetic field, they are pulled into the atmosphere along the magnetic field lines. The molecules of atmospheric gases glow with auroral light when they are struck by the cascading particles. So, to have auroras, a planet needs a strong magnetic field. Our magnetic field is 100 times stronger than it should be, because the earth's core is so hot.