Electric currents induced in the ionosphere generate magnetic fields (ionospheric dynamo region). Variability in irradiance influences conditions in the ionosphere and thermosphere. He conducted key measurements which revealed the presence of a Jovian ionosphere and magnetosphere. The ionosphere almost coincides with the thermosphere. The increasing terms fit the external sources (currents in the ionosphere and magnetosphere). The ionosphere is ionized by solar radiation. The ionized air also disrupts radio traffic that would normally bounce off the ionosphere. Within it lie layers of increased electron and ion density that form the ionosphere. Radio waves reflected off the ionosphere can change the wave's polarization. Observations show that the ionosphere occupies altitudes from . There are also mechanisms that disturb the ionosphere and decrease the ionization. HAARP also enables studies of how the natural ionosphere affects radio signals. This led to the discovery of HF radio propagation via the ionosphere in 1923. Strength of radio propagation changes between day- and night-side of the ionosphere. The region below the ionosphere is called neutral atmosphere, or neutrosphere.