Bridging the Kinetic-Fluid Gap: Ion-Driven Magnetogenesis to Prime Cosmic Dynamos

Abstract

The origin of cosmic magnetic fields is widely attributed to the amplification of weak seed fields by turbulent dynamos. However, a critical understanding gap remains between the microscopic generation of these seeds and the macroscopic onset of the dynamo. Current kinetic models, often constrained to electron scales, predict premature saturation via magnetic trapping, leaving the generated fields potentially too weak and small-scale to effectively prime magnetohydrodynamic (MHD) processes. Here, using high-resolution kinetic simulations with a realistic mass ratio, we reveal the physics of this unexplored ion-kinetic regime. Under generalized continuous shear driving, used to simulate ubiquitous macroscopic flows, we demonstrate that the saturation of electron instabilities is not the endpoint but a precursor to a distinct, ion-dominated evolution. Massive ions, sustaining the velocity shear, trigger a subsequent filamentation instability that accesses the vast ion kinetic energy reservoir. This mechanism amplifies the magnetic energy by orders of magnitude beyond the electron-saturation limit, expanding the field coherence to ion scales. Our results establish ion kinetics as the essential ''missing link'' that bridges the divide between microscopic plasma instabilities and macroscopic cosmic dynamos.

Figures (5)

• Figure 1: Temporal evolution of various parameters. Colored dashed lines represent theoretical values for the unmagnetized stage; gray lines represent the magnetic field evolution for the electron-positron pair case. Black vertical dashed lines indicate three typical moments.
• Figure 2: Magnetic field distributions at $\tau_{\rm{e}}$, $\tau_{\rm{sat,e}}$, $\tau_{\rm{i}}$, and the final simulation moment.
• Figure 3: Evolution of magnetic energy spectra for (a) the electron-positron pair system and (b) the proton-electron system. The vertical dashed line in (b) indicates the theoretical wave number of maximum growth rate.
• Figure 4: Magnetic energy spectra (blue solid line) and Larmor radius distributions for electrons (pink) and ions (green) at (a) $\tau_{\rm{e}}$, (b) $\tau_{\rm{sat,e}}$ and (c) the final simulation moment. Black dashed lines indicate the characteristic scales of electrons and ions, respectively. The blue vertical dashed line represents the mean magnetic field scale $\xi_M$.
• Figure 5: (a)(b) Proton phase space distributions at different moments; white dashed lines indicate the locations of the maximum acceleration region and the intermediate region. (c) Proton momentum distribution at $z=L/2$.