A characteristic in the electric field signature at the ground near the end of a thunderstorm. The electric field changes from upward-directed (foul-weather polarity) to downward-directed (fair-weather polarity) and often retains the latter polarity for several tens of minutes before returning momentarily to foul-weather polarity. Positive ground flashes and spider lightning are occasionally observed during the period of reversed field polarity. The origin of the EOSO is only partly understood.