This study examines the causes of changes in observed and modeled nitric oxide density at midlatitudes during magnetic storms in September 1974. Results are presented for two altitudes where the behavior of nitric oxide is different because of the different chemical lifetimes. At 110 km altitude, the lifetime is a day or more, while at 150 km altitude the lifetime is a few hours. In addition, vertical diffusion is a net sink of NO at 150 km but an important source at 110 km. The Atmosphere Explorer-C (AE-C) satellite data show that the midlatitude NO density increased by a factor of 3 above ~150 km following a large increase in magnetic activity on 15 September 1974 and this increase is well modeled. In the model the storm time increase begins when the Sun illuminates the disturbed thermosphere about 6 hours after the onset of the storm. The model NO density increases above 150 km are the direct result of increased NO production through solar production on storm-induced enhancements in O2 density and neutral temperature. At 150 km the storm-enhanced NO density decreases within a day. Downward vertical diffusion of this enhanced NO then causes a large increase in model NO density at 110 km following the storm with a time delay of about 1 day and the enhancement lasts for more than a day. Model calculations indicate that at and above 150 km, any NO transported from high latitudes is likely to be swamped by local chemical processes once the Sun rises. At lower altitudes near 110 km, where the NO density peaks, the aurorally produced NO lasts long enough to be transported from high latitudes to midlatitudes. Thus for horizontal transport from high latitudes to be effective in producing midlatitude increases in NO density, it must take place at low altitudes. Additional calculations for a magnetic storm in the high solar activity period of April 2002 show similar NO behavior to the low solar activity period in September 1974. Since NO radiation is an important sink of thermospheric energy, the midlatitude source associated with enhanced O2 density and neutral temperature is likely to be important in speeding the recovery of the thermosphere and ionosphere following magnetic storms.