The lower ionosphere at high latitudes

Current understandings of the ionospheric E and D regions at high latitudes are reviewed with particular emphasis on differences from the lower ionosphere at lower latitudes. The ion composition of the E region is examined based on in situ mass spectrometer measurements, and possible mechanisms controlling the production and loss of NO and other odd nitrogen compounds below about 140 km under auroral conditions are considered. Attention is then given to sources of ionization, the positive ion composition and the presence of negative ion species in the D region, and to the characteristics of the D region during conditions of low mesospheric temperatures in summer leading to the formation of noctilucent clouds through the hydration of water cluster ions.