Ion energetics in the Venus nightside ionosphere

The transport time required for the ionospheric ion gas to flow through an arc of one radian across the Venus terminator at the observed horizontal speed is short in comparison with the time required for the electron gas to transfer to the ion gas an amount of heat comparable to its internal energy. The transport time is approximately equal to the time required for the internal energy of the ion gas to be lost through heat conduction down into the neutral atmosphere. We define this flow condition in which the ion gas gains insignificant heat from the electron gas and loses only a moderate amount through conduction in flowing through an arc of one radian as quasiadiabatic. One important implication of this flow condition is that steady state, vertical, one dimensional energy balance models for the nightside ionosphere must be used with caution. A second implication is that bulk kinetic energy is being converted into heat energy through some dissipation mechanism. Inasmuch as we expect the plasma to expand across the terminator and thereby suffer some adiabatic cooling, we suggest that conversion of bulk kinetic energy into heat is responsible for maintaining the observed nightside ion temperature. The bulk kinetic energy at the terminator is large compared with the internal energy.