A sunrise undulation (SU) is observed by ionosondes when new ionization is produced at sunrise in the upper F region at the geomagnetic equator. It creates a very quick upward transition in the F region peak altitude (hmF2), which subsequently undergoes a sharp descent at a rate far in excess of any electrodynamic drift. The peak density also increases rapidly during the descent. However, the detection of the new plasma with ionosondes is possible only if plasma from the night before has undergone enough recombination, which, on average, can vary from one location to another and one season to the next. With this in mind, we have studied seasonal variations in the SU occurrence in 2010 at two widely separated geomagnetic equatorial stations in longitude, at Jicamarca in Peru and Trivandrum in India. Noticeable differences were found in the characteristics of undulation observed at the two locations. While full undulation (both ascend and descend in hmF2) was observed throughout the year at Trivandrum, only the descending part of undulation was visible at Jicamarca. Plasma density just before sunrise, on average, was 2 times larger at Jicamarca than at Trivandrum. The average hmF2, which peaks during night, was also higher at Jicamarca by as much as 100 km compared to Trivandrum. We traced the origin of these differences to the evolution of zonal electric field between sunset and local midnight at the two locations. The downward drift during this period was steeper at Trivandrum compared to Jicamarca, particularly during equinox. In addition, the strength of the downward drifts at Jicamarca decreased and even underwent a sign reversal prior to sunrise during equinox and winter months. In comparison, the downward motion at Trivandrum only became stronger during early morning hours. The contrast between the vertical drift at the two stations provides a very reasonable qualitative explanation for the differences in the characteristics of SU observed at the two stations.
