Authors: Shiva Bikram Thapa (Department of Physics and Astronomy, Dartmouth College, Hanover, NH 03755, USA), Yi-Hsin Liu (Department of Physics and Astronomy, Dartmouth College, Hanover, NH 03755, USA), Xiaocan Li (Los Alamos National Laboratory, Los Alamos, NM 87545, USA), Adam Stanier (Los Alamos National Laboratory, Los Alamos, NM 87545, USA), Nengyi Huang (Department of Physics, New Jersey Institute of Technology, Newark, New Jersey 07102, USA), Fan Guo (Los Alamos National Laboratory, Los Alamos, NM 87545, USA)

Magnetic reconnection onset marks the transition from a slow energy-storage phase to the rapid conversion of magnetic energy into plasma heating, flows, and particle energization. However, the mechanism that controls this transition in strongly inhomogeneous plasmas remains unclear. We use fully kinetic VPIC simulations with self-consistent Coulomb collisions to study reconnection in a transition-region-like system containing a cold, dense plasma adjacent to a hot, rarefied plasma. Because the classical Spitzer resistivity scales as η∝ T_e^(-3/2), the temperature gradient produces a strong spatial resistivity gradient. We show that this gradient introduces an additional induction contribution,   -(∂_x η)J_x, which localizes magnetic-flux processing on the high-resistivity side of the transition layer, resulting in a Petschek-type configuration. We find that resistivity gradients trigger much earlier reconnection onset than uniform-resistivity and collisionless reference cases, especially for thicker current sheets relevant to large-scale astrophysical plasmas. The measured onset times are consistent with a scaling law derived from classical resistive diffusion. These results identify resistivity gradients as a self-consistent pathway for rapid reconnection onset in stratified plasmas, with potential application to the lower solar atmosphere and spicules.