Exact system-size threshold for transition to the asymptotic disruption regime in magnetized pair plasmas

Determine the exact system size, expressed as the flux-tube radius R normalized by the cold electron skin depth d_e0, at which collisionless reconnection in magnetically dominated electron–positron pair plasmas transitions from a skin-depth-limited disruption thickness (a_* ∼ d_e0) to the asymptotic regime characterized by a_* ≫ d_e0.

Background

The study uses 2D particle-in-cell simulations of driven collisionless reconnection between two force-free magnetic flux tubes in a magnetically dominated electron–positron pair plasma. Across system sizes up to R ≈ 1600 d_e0, the current sheet thins exponentially and disrupts at a thickness comparable to the electron skin depth (a_* ∼ d_e0), with plasmoid formation when the aspect ratio A_* ≳ 30.

Theory anticipates that in asymptotically large domains the disruption thickness should satisfy a_* ≫ d_e0. The simulations suggest that reaching this asymptotic regime likely requires R ≫ 10^3 d_e0, but only a lower bound is established. Pinpointing the exact transition size is important for extrapolating onset physics to astrophysical scale separations and for validating asymptotic theories of reconnection onset.