IT HAS long been assumed that aurorae, like stars, are present during daylight, but hidden by the high surface brightness of the sky, The brightest aurorae are discrete curtains of light created by beams of electrons accelerated in the Earth’s magnetosphere. Many theories’ have been proposed to explain discrete aurorae, but the one that provides the best match to recent observations(2) invokes a positive feedback mechanism between the conductivity of the ionosphere, which is enhanced by the injection of electrons, and the current carried by these same electrons(3-5). Here we present a statistical study of electron precipitation events by using nine years of charged-particle data from weather satellites, We find that the beams of accelerated electrons that cause the discrete aurorae occur mainly in darkness : the winter hemisphere is favoured over the summer hemisphere, and night is favoured over day. We also find that discrete aurorae rarely occur in the presence of diffuse aurorae, which provide sufficient conductivity to support the electric currents between the magnetosphere and the ionosphere. Taken together, these observations strongly support the view that ionospheric conductivity is the key factor controlling the occurrence of discrete aurorae.