How important are vibrationally excited molecular nitrogen and oxygen for the negative phase of ionospheric storms? In order to answer this question we performed a numerical simulation of an ionospheric storm which occurred in summer during a period of high solar activity. We find that during such conditions, vibrationally excited N2and O2 are important and cannot be neglected. The negative phase of the ionospheric storm, however, can almost entirely be explained by the observed perturbations of the neutral gas composition.