The ratio of auroral [NI> (2D3/2,5/2 0→4S3/2 0) 5198.5 ¿ to 5200.7 ¿ emissions at Svalbard (&Lgr; = 75¿N) is found to vary between 1.56¿0.06 and 1.20¿0.08. The observed variations can be explained if quenching of N2(2Dj0) by atomic oxygen is constrained by the conservation of total angular momentum as follows: N(2D3/2 0) +O(3P2.1.0)→N(4S3/2 0)+O(3P2.1.0); N(2D5/2 0)+O(3P1.0)→N(4S3/2 u0)+ O(3P2.1). The ratio Jj = 0 1n(O(3PJ))/Jj = 0 2n(O(3PJ)) in the atmosphere, which affects steady state populations of N(2D3/2 0) and N(2D5/2 0) through the above quenching reactions, depends on kinetic temperature; the latter varies with atmospheric height. Consequently, the ratio of the steady state populations of N(2D3/2 0) and N(2D5/2 0) and hence the intensity ratio I(5198.5 ¿)/I (5200.7 ¿) would vary in an aurora as changes in particle characteristic energy lead to auroral emissions from different heights in the atmosphere. We have calculated the ratio of the volume emission rates of 5198.5 ¿ and 5200.7 ¿ lines over the altitude range 100--350 km, taking into account the quenching effects of O, O2, and e interacting with N(2DJ 0), the variation in [O(3P1.0)>O(3P2,1,0)> with height and the radiative transitions of N(2DJ 0), This ratio decreases from 1.5 to 100 km to around 1.1 to 250 km and then increases to 1.45 at 350 km.