In this letter we report new modeling results that show the generation of ion sound waves in the topside low latitude ionosphere. The waves are excited at sunrise and sunset; they persist ~1--3 hrs at altitudes above the O+/H+ transition altitude (~1000 km) with periods on the order of 10's min. The waves are the result of the rapid heating and cooling of the lower ionosphere that occurs at sunrise and sunset. At sunrise, photo-electron heating produces strong upward plasma flows along the geomagnetic field. These flows lead to a local compression and heating of the plasma at the apex of the field line which in turn generates ion sound waves. At sunset, the waves are produced by a rapid cooling of the plasma. We suggest that this phenomenon may be observable with the Arecibo radar facility. ¿ 2000 American Geophysical Union