Fabry-Perot interferometer determinations of thermospheric temperatures from 630.0 nm nightglow line width measurements have been carried out for the period April-August 1983 from Arequipa, Peru (16.4 ¿S, 71.5 ¿W geographic; 4.4 ¿S magnetic). The nightly variation of the thermospheric temperature Tn measured on 62 nights is compared with MSIS model predictions and found to agree occasionally with the model but, on average, to exceed model predictions by ~180 K. The largest differences, 400--500 K, often occur during strongly increasing geomagnetic activity such as sudden commencements. The rapid increases in Tn may result from energetic neutrals precipitating at low latitudes from the ring current or from energy carried to equatorial regions from high-latitude (auroral oval) heat sources by gravity waves and equatorward neutral winds.