A sudden commencement of a geomagnetic storm occurred at 1434 UT on September 18, 1974. A traveling ionospheric disturbance was detected about an hour later by the incoherent scatter radar at Millstone Hill, Massachusetts (42.6¿N, 71.5¿W), and still later by the incoherent scatter radar at Arecibo, Puerto Rico (18.3¿N, 66.7¿W). Measurements of the vertical distribution of the electron temperature, ion temperature, and electron density made during the passage of the disturbance at both stations showed significant perturbations in comparison to the geomagnetic quiet time values obtained on the previous day. We calculated characteristics of the thermospheric gravity wave from a time-dependent dynamic model of the thermosphere. The wave source is assumed to be enhanced electric currents and particle heating over the polar cap region. The gravity wave generated by the impulsive heating propagates equatorward, and its characteristic structure changes as it progresses to lower latitudes. The calculated neutral temperature and wind perturbations are added to the quiet time variations in a diurnal calculated neutral temperature and wind perturbations are added to the quiet time variations in a diurnal model of the mid-latitude ionospheric F region to calculate the time-dependent properties of the ionosphere during the passage of the wave over both incoherent scatter radar stations. By requiring agreement between the calculated and observed ionospheric structure we estimated the magnitude of the gravity wave energy source. For the event on September 18, 1974, we obtained reasonably good agreement between the calculated and observed ionospheric structures for a total energy input of about 2¿1015 J, occurring over a period of 3 hours.