Scaling of Magnetic Reconnection With a Limited X‐Line Extent

Abstract

Contrary to all the 2-D models, where the reconnection x-line extent is infinitely long, we study magnetic reconnection in the opposite limit. An internal x-line asymmetry along the current direction develops because of the transport of reconnected magnetic flux by electrons beneath the ion kinetic scale, resulting in a suppression region identified in Liu et al. (2019, https://doi.org/10.1029/ 2019JA026539). In this letter, we incorporate the length scale of this suppression region ≃10d i to quantitatively model the reduction of the reconnection rate and the maximum outflow speed observed in the short x-line limit. The average reconnection rate drops because of the limited active region (where the current sheet thins down to the electron inertial scale) within an x-line. The outflow speed reduction correlates with the decrease of the J × B force, that can be modeled by the phase shift between the J and B profiles, also as a consequence of the flux transport. Plain Language Summary Magnetic reconnection is a fundamental physical process that is responsible for releasing the magnetic energy during substorms of planetary magnetotails. Previous studies of magnetic reconnection usually take the two-dimensional (2-D) approach, which assumes that reconnection is uniform in the third direction out of the 2-D reconnection plane. However, observations suggest that reconnection can be limited in the third direction, such as reconnection at Mercury's magnetotail. It turns out that reconnection can be suppressed when reconnection region is very limited in the third direction. Under the guidance of a series of 3-D kinetic simulations, in this work, we write down quantitative models to describe how the reconnection rate and reconnection outflow speed drop in this limit. Notably, these two quantities are most essential in defining the well-being of magnetic reconnection, which can tell us when reconnection shall be suppressed. The models are formulated by considering the transport of reconnected magnetic flux in the third direction, which can weaken the driver of the reconnection process.

Auroral Spiral Structure Formation Through Magnetic Reconnection in the Auroral Acceleration Region

“…The simulated process resembles the observed evolution of a real auroral arc, providing an explanation to the auroral spiral structure formation. These secondary tearing modes are pronounced only in the lower altitude; this altitude dependence is consistent with the 3-D nature of reconnection x-line in Y. H. Liu et al (2019) and Huang, Liu, et al (2020). Moreover, this study predicts that when the auroral acceleration region is too short, the auroral arc will be stable, and no spiral structure can form.…”

“…In Figure 2d, it is evident that most of the flux ropes only develop at the lower altitude (Huang, Liu, et al, 2020;Y. H. Liu et al, 2019).…”

Identification of Active Magnetic Reconnection Using Magnetic Flux Transport in Plasma Turbulence

“…Thus, the conditions for applying MFT is expected to be realistic for reconnection in heliospheric turbulence. Recent 3D PIC simulations further show that a long extended X-line, satisfying k k ⊥ , easily arising in sub-ion-scale current sheets in 3D (Li et al 2020), also favors reconnection activity (Liu et al 2019;Huang et al 2020).…”

The Effect of Thermal Pressure on Collisionless Magnetic Reconnection Rate

“…Fourth, pressure anisotropy instabilities (e.g., mirror instability) may arise in high-β plasmas and was suggested to distort the field geometry and the current sheet (Alt & Kunz 2019). Finally, we do not take 3D physics into account, for example, self-generated turbulence (Daughton et al 2011;Liu et al 2013; or localized reconnection layer along the third dimension (Huang et al 2020;Liu et al 2019). Nevertheless, this present model extends the reconnection rate model to the high-β regime and provides new insights into the reconnection rate problem in high-β plasmas, which can be applicable to the outer heliosphere, hot intracluster medium of galaxy clusters, and the Galactic center.…”