A simulation study of the current-voltage relationship of the Io tail aurora

The Io tail aurora extends for approximately 100 degrees downstream in longitude from the Io footprint aurora. Observations indicate that the brightness of the Io tail aurora continuously decreases along the footpath while its peak altitude remains constant. According to the quasi-steady theoretical frame, this suggests that the field-aligned voltage is constant while the parallel current density decreases in the downstream direction. The mechanism that realizes the current-voltage relationship of the Io tail aurora remains unresolved. In this paper, we apply a new multimagnetofluid code to the Io-Jupiter system to clarify the origin of the current-voltage relationship. The code solves a set of equations that includes the electron convection term in Ohm's law, which enables us to simulate the current-driven ion acoustic instability in the fluid frame. The instability forms a transition layer at a high altitude, which accelerates the magnetospheric electrons and blocks the magnetospheric ions, leading to the formation of a density depleted region called an auroral cavity. We find that if the ionospheric proton density decreases at the same rate as the parallel current density, the timescale on which the transition layer disappears is consistent with the longitudinal extent of the tail aurora, and the potential gap is constant all along the tail. We discuss the possibility that the fringe, wideband repetitive bursts of the Io-related Jovian decametric radiation, is excited in the auroral cavity.