Empirical models of the electron temperature and density in the Venus ionosphere

Empirical models of the electron temperature and density of the Venus ionosphere have been derived from Pioneer Venus measurements acquired between December 10, 1978, and August 9, 1979. This time interval allowed periapsis to scan a full range of solar zenith angles (SZA) and local times. The models describe the average ionosphere conditions in the vicinity of periapsis (18 degrees N) and from 155-km altitude to an upper boundary altitude, which varies with solar zenith angle. The upper boundary is determined by the average ionopause height, which rises steadily from approximately 340 km at 0 degrees SZA to almost 1100 km at 110 degrees SZA. The electron density in the Venus ionosphere is characterized by nearly uniform values across the dayside followed by a sharp decrease in the vicinity of the terminator (90 degrees SZA). Considerable amounts of ionization persist throughout the nightside ionosphere, a fact which requires that the ionization either be transported to the nightside from the dayside or produced locally by particle precipitation. The electron temperature is characterized by very high temperatures on the dayside followed by a gradual decrease beyond the terminator with increasing solar zenith angle, although significantly elevated temperatures exist throughout the nightside. A great deal of large and small scale structure, evident in both the N-e and T-e measurements, is not reflected in the model, although the distribution of these irregularities is readily revealed by plotting the ratio of the individual measurements to the model values at each point. These plots show that the irregularities in T-e are greatest at low altitudes and in the night side ionosphere.