Inverse Transfer in Non-helical 2D Collisionless Magnetic Turbulence: Island-Merger Picture with Kinetic Effects

Magnetic inverse transfer is often invoked to connect small-scale magnetic-field generation to larger coherence scales in high-energy and cosmological plasmas. The underlying magnetohydrodynamic (MHD) arguments combine two logically distinct ingredients: a bulk quantity that is asymptotically conserved in the limit of small resistivity, and a time scale determined by the decay dynamics. In this work, we explore whether this scenario still holds in decaying nonhelical turbulence formed by collisionless plasmas using particle-in-cell simulations. The simulations approximately satisfy B 2 xi B squared const as in the MHD case, and the fitted exponents in B 2 proportional to t-p and xi B proportional to t q obey p 2q. Here B squared B x 2+B y squared is the average in-plane magnetic energy density, and xi b is the magnetic integral scale. However, the decay time scale differs from the MHD case as inferred from the decay exponents. We found p<1 and q<1/2 in all cases with different initial magnetization sigma 0, with both exponents lower than the MHD values and varying systematically with sigma 0. The spectral peak also migrates toward lower wavenumber at a rate faster than the growth of xi B, indicating a broken self-similarity. The broken self-similarity is attributed to the appearance of kinetic scales in the magnetic energy spectrum due to pressure anisotropy and Larmor-scale magnetic structures. These results indicate that in astrophysical collisionless plasmas, including but not restrict to solar wind, pulsar-wind nebulae, interstellar medium, and cosmological plasmas, magnetic coherence can continue to grow by inverse transfer, but extrapolations based on MHD decay-time scaling can overestimate the rate of large-scale field growth.