Ionosphere: Solar Cycle Variations

We summarize here the current state of knowledge of solar cycle variations in the morphology of the Venus dayside and nightside ionospheres, and in the mechanisms for maintaining the nightside ionosphere. The solar cycle variability of the dayside ionosphere is well understood. The dayside peak ion or electron density varies approximately as F _10.7 ^0.35 for short-term or long-term solar variations. The non-Chapman behavior is mostly due to concomitant changes in the neutral atmosphere, and such changes also produce a large amplitude response to solar flux variations well above the peak. Similar behavior is observed on the night side, where the variation of the electron density is a factor of 2 near the peak, but is much larger at high altitudes. On the night side the ionotail extends thousands of kilometers behind the planet. The large response of the ionotail to solar flux variations and to solar wind dynamic pressure indicates that the ions found there originate on the day side. Pioneer Venus measurements have convincingly shown that the major source of nightside ionization at solar maximum is day-to-night plasma transport. Most estimates of the contribution of electron precipitation at high solar activity are in the range 20 to 30%, but the exact value is still not certain. Neither the solar cycle response of the nightside ionosphere, nor its behavior with solar zenith angle, nor its overall variability can be accounted for by electron precipitation as the major ion source.