New observations and reanalysis of previous measurements suggest an upward revision of the measured thermospheric nitric oxide density. Our previous model of NO production by fast N(4S) atom collisions with O2 has been updated. It includes the effect of soft solar X rays, Auger electrons, and a detailed treatment of N2 dissociative ionization channels. In addition, new calculations indicate that the transfer of translational energy in N+N2 collisions is less efficient than in the hard sphere approximation. This result leads to reevaluation of the energy dependent relaxation cross section and to an upward revision of the reacting efficiency of collisions of N with O2 to form nitric oxide. The calculated peak NO density increases by a factor of ~2 when the effect of superthermal nitrogen atoms is included. The model response of the N(4S) energy distribution function and NO density to solar cycle variations is presented. The NO density at 110 km changes from 5.4¿107 to 1.3¿108 cm-3 when the solar F10.7 index varies from 70 to 245, but its response depends on the magnitude of the soft X ray increase with solar activity.¿ 1997 American Geophysical Union