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

Abstract

Magnetic reconnection has been suggested to play an important role in the dynamics and energetics of plasma turbulence by spacecraft observations, simulations and theory over the past two decades, and recently, by magnetosheath observations of MMS. A new method based on magnetic flux transport (MFT) has been developed to identify reconnection activity in turbulent plasmas. This method is applied to a gyrokinetic simulation of two-dimensional (2D) plasma turbulence. Results on the identification of three active reconnection X-points are reported. The first two X-points have developed bi-directional electron outflow jets. Beyond the category of electron-only reconnection, the third X-point does not have bi-directional electron outflow jets because the flow is modified by turbulence. In all cases, this method successfully identifies active reconnection through clear inward and outward flux transport around the X-points. This transport pattern defines reconnection and produces a new quadrupolar structure in the divergence of MFT. This method is expected to be applicable to spacecraft missions such as MMS, Parker Solar Probe, and Solar Orbiter.

Magnetic Flux Transport Identification of Active Reconnection: MMS Observations in Earth’s Magnetosphere

“…According to simulation work (Li et al 2021), U ψ will form super-Alfvénic jets in both inflow (N) and outflow (L) directions, indicating strong MFT close to the active X-points. The upstream hybrid Alfvén speed V A is calculated using the L component of the magnetic field and ion density on the two upstream sides (Equation 13, Cassak & Shay 2007).…”

“…They are (1) coexisting magnetic flux (U ψ ) jets in the inflow (N) and outflow (L) directions, and (2) a significantly enhanced divergence of flux transport (∇ • U ψ ) at the X-point. Previous theory and simulation work suggested that U ψ is at least ion Alfvénic (  0.5 V A ) and the divergence on the order of 0.1 electron cyclotron frequency (∇ • U ψ  0.05 f ce ) (Li et al 2021). Observing either signature identifies an encounter of an active reconnection site.…”

“…The MFT velocity U ψ was previously derived in one and two-dimensions (Liu & Hesse 2016;Liu et al 2018). The formula of this velocity can be found in Li et al (2021):…”

“…The trajectory of the spacecraft has a significant influence on the observational profile. In addition, the asymmetry of upstream conditions (Shay et al 2016), the existence of finite guide fields (Ng et al 2011;Bessho et al 2019), and the strong shear flows (Liu et al 2018;Li et al 2021) likely in a turbulent system may all distort the signatures, adding complexity to reconnection.…”

“…Recent studies have analyzed the transport of magnetic flux around an X-point in kinetic simulations (Liu & Hesse 2016;Liu et al 2018). An innovative technique based on Magnetic Flux Transport (MFT) has been developed and carefully analyzed in a two-dimensional gyrokinetic simulation (Li et al 2021), where MFT was applied to both symmetric and asymmetric reconnection X-point regions in turbulence generated by a double-vortex setup (Li et al 2016). MFT successfully captures bidirectional inflows and outflows of magnetic flux around active X-points in a region significantly smaller than the region extended by plasma outflow jets or finite energy conversion, and thus the MFT method can locate the active reconnection sites more accurately than previous methods.…”