During the six-years of Demeter satellite, electron density perturbations at around 650km observed at the equator during night-time are analyzed. Those perturbations are classified following morphological criteria in 4 classes D1 to D4 [while there is no expected increase of Ne at night-time, D1 : a simple increase of Ne, D2 : 2 increases of Ne, D3: a plateau of electron density and D4 : a plateau with some decreases]. The most common perturbations are the D3 ones. The D3 perturbations occur mainly around 22.30 MLT, at latitudes /- 15° from the geomagnetic equator. The D3 type exhibits maximum at the equinoxes. The vernal maximum is greater than the autumnal one. There are more events ( 18%) during the years of the deep minimum (2006-2009) that during the year 2005 and 2010 . In this paper the authors will present the climatology and interpretation of those D-type perturbations.