 Venus has an extended ionosphere located at altitudes 120 – 300 km. The ionosphere almost coincides with the thermosphere. The high levels of the ionization are maintained only over the dayside of the planet. Over the nightside the concentration of the electrons is almost zero. The ionosphere of Venus consists of three layers: v1 between 120 and 130 km, v2 between 140 and 160 km and v3 between 200 and 250 km. There may be an additional layer near 180 km. The maximum electron volume density (number of electrons in a unit of volume) 3 × 1011 m − 3 is reached in the v2 layer near the subsolar point. The upper boundary of the ionosphere — ionopause is located at altitudes 220 – 375 km and separates the plasma of the planetary origin from that of the induced magnetosphere. The main ionic species in the v1 and v2 layers is O2 + ion, whereas the v3 layer consists of O + ions. The ionospheric plasma is observed to be in motion; solar photoionization on the dayside, and ion recombination on the nightside, are the processes mainly responsible for accelerating the plasma to the observed velocities. The plasma flow appears to be sufficient to maintain the nightside ionosphere at or near the observed median level of ion densities.
