The relationship between sunset terminators and the onset of radar backscatter plumes associated with equatorial spread F (ESF) observed with the 47-MHz Equatorial Atmosphere Radar from October 2002 to April 2004 is discussed. Almost all irregularity echoes began to appear at or before sunset time at the altitude of the apex of the geomagnetic field line connected with the observed area, and the onset time of more than half of the events corresponded exactly to apex sunset time. This tendency should be due to two causes: (1) the rapid change of the evening zonal electric field and (2) damping of 3-m scale irregularities by the solar radiation before the apex sunset.