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This study systematically examines magnetic reconnection in partially ionized plasmas by varying both ion-neutral collisionality and ionization fraction. Using a new three-fluid numerical model, researchers found that reconnection rates scale with ionization fraction to the 1/4 power in strongly coupled regimes, but transition to faster, ionization-independent reconnection at lower collisionalities. The current sheet thickness in weakly coupled regimes approaches ion-inertial scales rather than the theoretically predicted hybrid scale, with results consistent with recent kinetic simulations and laboratory experiments.
Why it matters
Understanding magnetic reconnection in partially ionized plasmas is essential for explaining energy release processes in the solar atmosphere, interstellar medium, and laboratory fusion devices. The identified scaling laws and transition thresholds provide improved predictive models for these environments where neutral particles significantly affect plasma dynamics.
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⚠️ Preprint – Noch nicht peer-reviewed
Dieser Artikel wurde noch nicht von unabhängigen Experten begutachtet. Die Ergebnisse sind vorläufig und sollten mit Vorsicht interpretiert werden.
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Abstract: Magnetic reconnection in partially ionized plasmas plays a crucial role in a wide range of solar, astrophysical, and laboratory environments. While reconnection in such plasmas is commonly characterized by the ion-neutral coupling strength and the ionization fraction $chi=n_{i}/(n_{i}+n_{n})$, most previous studies have focused primarily on the former. A systematic exploration of the ionization fraction, particularly in combination with ion-neutral coupling, is still lacking. This study presents the first systematic scan of the two-dimensional parameter space defined by ion-neutral collisionality and ionization fraction, enabling investigation of the transition from strongly coupled reconnection to faster, decoupled reconnection. To achieve this, we employ a new three-fluid, five-moment numerical model that treats electrons, ions, and neutrals as separate species on an equal footing. We find that in the strongly coupled regime, the reconnection rate is consistent with a $chi^{1/4}$ scaling. As collisionality decreases, the system transitions to a fast, ionization-independent regime. On the other hand, in the weakly coupled and fast-reconnection regimes, the current sheet approaches an ion-inertial-scale thickness rather than the expanded hybrid scale $d_{i}chi^{-1/2}$ predicted by fully coupled analytic fluid theories. The identified critical thickness and the resulting onset of fast reconnection agree reasonably well with recent fully kinetic simulations and laboratory experiments. In addition, we show that, over a wide range of coupling strengths, the ion outflow velocities remain Alfv’enic, scaling with the appropriate ion or hybrid Alfv’en speed, while the hybrid outflow velocity scales as $chi^{1/2}$ when normalized by ion Alfv’en speed.
Source: Transition of Magnetic Reconnection Regimes in Partially Ionized Plasmas