Abstract Two of the many sounding rockets launched as part of the MAC/EPSILON campaign are particularly suited to test the validity or quality of D-region models under night-time conditions. The measurements covered atomic oxygen, neutral temperatures and fluxes of charged particles. With these parameters as inputs, one is in the position to assess their relevance for the plasma parameters ion composition, transition heights of clusters and negative ions, and total plasma density. The two IOMAS payloads are believed to be the only night-time flights with associated measurements of atomic oxygen extending below the ledge at about 80 km. The relevance of atomic oxygen for the ion chemistry of the lower D-region could be experimentally demonstrated.