The critical solar wind pressure for IMF penetration into the Venus ionosphere

Early observations and simulations have revealed that the occurrence of IMF penetration into the Venus ionosphere depends on the upstream solar wind pressure, and that IMF is transported into the ionosphere by the downward convection when the solar wind dynamic pressure is relatively large. In this paper, we investigated the critical solar wind pressure for the IMF penetration, by using a two-dimensional global MHD model. The simulations showed that variations in the upstream solar wind pressure (P-SW) affect plasma dynamics and chemistry in the subsolar ionosphere. The chemical plasma loss rate increases as P-SW increases, and the amount of plasma loss exceeds that of plasma production when P-SW exceeds a certain critical level (P-crtc). On this occasion, the plasma convection in the ionosphere connects to the solar wind flow, and as a result the connected downward convection transports IMF into the ionosphere. The simulations also showed that IMF penetration occurs only when P-SW exceeds P-crtc, even if the solar wind pressure changes abruptly. The critical pressure is not necessarily equal to the ionospheric peak thermal pressure (P-peak). Our simulations suggested that P-crtc is smaller than P-peak.