A reconsideration of the effects of terminator ionopause height on the nightward ion transport at venus

Abstract

The ionopause altitude near the terminator is a crucial parameter for studies dealing with the maintenance of the nightside ionosphere of Venus. It is generally thought that during high solar wind dynamic pressures (P_sw) or during solar minimum conditions the ionopause comes down to very low altitudes so that the dayside ionosphere is not able to supply sufficient plasma to maintain the observed nightside densities. However, there are a number of workable definitions of the ionopause. Near the terminator, the altitude of the ionopause differs considerably depending upon the definition. The ionopause deduced from the radio occultation experiment (used to study the nightward transport for solar minimum conditions) as well as the pressure ionopause can be significantly lower than the density ionopause deduced from the Langmuir probe at these locations. The latter refers to the altitude where the electron density falls to 100 cm^-3. Using in situ data from the Pioneer Venus Orbiter, it is shown that the density ionopause remains fairly high even for high P_sw conditions. Simple quantitative estimates indicate that significant flow of plasma is still possible under these conditions. Thus nightward transport of plasma during high P_sw conditions may be more efficient than has been assumed so far. Since such conditions are more prevalent during solar cycle minimum, it is argued that transport may be relevant in the maintenance of nightside ionosphere at that time also.