Suppression of Inverse Magnetic Energy Transfer in Collisionless Marginally Magnetized Plasmas

Abstract We investigate the inverse cascade of magnetic energy in decaying, collisionless pair plasmas with moderate to high β values via first-principles numerical simulations and analytical theory. We find that pressure-anisotropy-driven instabilities, in particular the firehose instability, suppress reconnection-driven coalescence of magnetic structures (i.e., inverse transfer) by nullifying magnetic tension. The presence of a magnetic guide field of sufficient strength restores the system’s ability to inverse transfer magnetic energy. These results reveal that inverse energy transfer in collisionless plasmas is not guaranteed, but rather depends sensitively on magnetization. In the astrophysical context, this identifies a kinetic mechanism by which Weibel-generated seed fields may fail to merge consistently, potentially limiting their role in cosmic magnetogenesis.