The role of saturated thermal conduction in shaping electric current dynamics in jet-launching disks

Abstract

This study investigates the dynamics of jet formation from a viscous, resistive accretion disk around a young stellar object using MHD simulations with the PLUTO code. We conducted an in-depth analysis of electric currents from various perspectives to elucidate how their dynamics, influenced by saturated thermal conduction, impact jet acceleration and collimation processes. Our simulations confirm the butterfly topology of electric current lines, as predicted by earlier analytic models. Crucially, increasing the saturation parameter ${\varphi }_s$ alters the current line topology: higher saturation enhances collimation in the inner disk regions while inducing decollimation in the outer regions. By the end of our simulations ($t=700$, ∼112 rotational periods), higher ${\varphi }_s$ results in a faster and more collimated jet compared to its lower ${\varphi }_s$ counterpart, whereas at earlier times ($t=200$, ∼32 rotational periods), the jet exhibits lower acceleration despite enhanced collimation. In our reference case, we find that the electric current lines connect the launching region to the Alfvén and fast magnetosonic surfaces from early times, confirming the effectiveness of the acceleration mechanism at the onset of jet formation. Our study reveals that the upper jet regions are transitioning toward Kelvin–Helmholtz instability (KHI), while the jet base continues to exhibit wave activity. At $t=700$, doubling the saturation parameter reduces these oscillations and suppresses KHI, indicating faster plasma propagation. Analysis of the vertical profile of the current density, $J_z$, shows a clear correlation between increased saturation and both enhanced jet acceleration and current density. Furthermore, the ratio of electric currents at the Alfvén point, $I_A$, and sub-magnetosonic point, $I_{sm}$, indicates that higher saturation significantly improves the efficiency of matter propulsion and energy transfer within the jet.