Simultaneous measurement of the 5200-¿ dayglow from the N I (4S-2D) transition, neutral, and ion densities and photoelectron fluxes by Atmosphere Explorer is used to examine the production and loss mechanisms of N(2D) atoms. The branching ratio for production of N(2D) from N+2+e is found to be larger than the branching ratio for the sum of the processes NO++e, N+2+O, and e+N2. If it is unity, then the effective branching ratio for their latter processes lies in the range 0.34-1.0; the rate for quenching of N(2D) by O lies in the range 1.4-6.3¿10-13 cm3 s-1, the rate for the reaction of N(2D) with O2 lies in the range 0-2.7¿10-12 cm3 s-1; and the rate for quenching of N(2D) by electrons is ~1¿10-9(Te/300)1/2 cm3 s-1. Values nearer the upper ends of these ranges are favored by a comparison between calculated and measured NO. The reaction N++O2→N+O+2 is a small source of N in the thermosphere. A calculation shows that the ratio of N(4S) to O is 0.01-0.02 at 250 km.