The solar particles that enter the atmosphere and produce the luminous aurora are known to include protons, which also ionize the atmosphere. As suggested by Bates, their ionizing action may extend below the level of their own penetration, by the Lyman α-photons they emit. These can penetrate to about 75 km, and ionize nitric oxide. Recent rocket researches by Van Allen and his colleagues prove that the primary auroral particles also include electrons, with energies up to 100 keV. These penetrate to about 80 km, thus directly extending the auroral ionization well below the level of auroral luminosity. Indirectly these electrons ionize the atmosphere down to far lower levels, by the X-rays emitted by a small fraction of the electrons. Winckler's balloon results show that such ionization extends at least down to 32 km. This ionization below the level of the visible aurora accounts for most of the absorption, in auroral regions, of high frequency radio waves. Though the primary electron flux at night probably exceeds that by day, the secondary electrons are often more numerous (and absorbent) by day than by night. This is because, by day, photodetachment prolongs the free life of these electrons, despite their ready attachment to oxygen.