Features of nightside ULF wave activity in the ionosphere

International audienceWe investigate the features of nightside ULF waves in the ionosphere using electric field data in the DC/ULF range observed by the DEMETER satellite over a ~5.5 year period from May 2005 to November 2010. The ULF wave events are recognized by an automatic detection algorithm and a superposed epoch analysis is performed to study the characteristics of ULF waves during geomagnetic storms. The results show that (1) ULF waves of both non‐magnetospheric and magnetospheric origins occur in the ionosphere; (2) non‐magnetospheric ULF waves present seasonal variations but have no clear response to storms; (3) magnetospheric ULF waves show clear association with the recovery phase of storms but have no obvious seasonal variations; and (4) an interhemispheric asymmetry with higher magnetospheric ULF wave occurrence rate in the southern hemisphere particularly around storms can be partly explained by the fact that more of selected isolated storms occurred during summer in the northern hemisphere (NH), hence higher conductivity in the NH. However, an analysis of few storms during equinoxes also shows a minor asymmetry. These results still indicate that the ionospheric conductivity in the southern hemisphere would be lower than in the northern hemisphere at nighttime