Why is at high altitude more light from oxygen in the auroras while it is heavier than nitrogen?
The visible emissions from atomic oxygen take place between the fine structure levels of the ground state, and transitions between these states are not quantum-mechanically allowed by electric-dipole radiation. These "forbidden" transitions take much more time to occur, proceeding by magnetic dipole or electric-quadripole radiation. As a result the atom spends a long time in the excited state (~1sec for the 557.7 nm transition, and ~200 s for the 630 nm transition), during which time the atom can make collisions and deactivate without radiating. However, the emissions from molecular nitrogen take place between dipole-allowed transitions, and spend only ns to micro-seconds in the excited state before radiating. Thus, they survive to radiate even in high collision conditions (i.e. low altitudes).

The frequency of collisions an atom or molecule makes increases with decreasing altitude. In addition, the higher the energy of the auroral electrons (2 keV to 20 keV), the lower in the atmosphere they deposit the energy (150 km to 90 km). If there are low energy electrons in the aurora, the emission occurs at very high altitude and both the excited atomic oxygen and the molecular nitrogen make few collisions. Thus they both radiate, but at these altitudes there is more atomic oxygen than molecular nitrogen and the reds and greens of atomic oxygen dominate. 

As the auroral energies increase and the electrons deposit their energy at lower altitudes, collisions are much more frequent and the atomic oxygen excited states do not live long enough to radiate. This kills off the green and red emissions at low altitudes. However, the molecular nitrogen, with its short excited state lifetimes, continues to radiate and the lower border of the aurora turns pinkish-purple with the nitrogen emissions.