The Nonorthogonal X-line in a Small Guide-field Reconnection Event in the Magnetotail

Abstract

Magnetic reconnection is a fundamental plasma process that has been studied with analytical theory, numerical simulations, in situ observations, and laboratory experiments for decades. The models that have been established to describe magnetic reconnection often assume a reconnection plane normal to the current sheet in which an antiparallel magnetic field annihilates. The annihilation points, also known as the X-points, form an x-line, which is believed to be perpendicular to the reconnection plane. Recently, a new study using Magnetospheric Multiscale mission observations has challenged our understanding of magnetic reconnection by providing evidence that the x-line is not necessarily orthogonal to the reconnection plane. In this study we report a second nonorthogonal x-line event with similar features as that in the previous case study, supporting that the sheared x-line phenomenon is not an aberrant event. We employ a detailed directional derivative analysis to identify the x-line direction and show that the in-plane reconnection characteristics are well maintained even with a nonorthogonal x-line. In addition, we find the x-line tends to follow the magnetic field on one side of the current sheet, which suggests an asymmetry across the current sheet. We discuss the possibility that the nonorthogonal x-line arises from an interplay between the two aspects of reconnection: the macroscopic magnetic field topology and microscopic particle kinetics.

Reconnection Inside a Dipolarization Front of a Diverging Earthward Fast Flow

“…One feature to note is that the X-line is not parallel to M but oblique to it (Figures4k-4n, which shows the reconstruction in the LM plane of the current sheet). A similar slanted X-line was recently observed byPathak et al (2022) andQi et al (2023). As a consequence, the position of the X-line depends on M and the choice of the appropriate M plane for the reconstruction is therefore crucial.…”

Determining the Orientation of a Magnetic Reconnection X Line and Implications for a 2D Coordinate System

“…(2022) examined this issue for an MMS event and reported that the direction of least gradient was tilted between 40° and 60° from e M as defined above, and their work motivated our study (See also work by Qi et al. (2023). ).…”

“…For the purposes of a 2D description, the minimum gradient would ideally be orthogonal to the reconnection plane, L-N; so an L-N plane defined using MVA may not best represent the plane of predominant spatial variation. Recently Pathak et al (2022) examined this issue for an MMS event and reported that the direction of least gradient was tilted between 40° and 60° from e M as defined above, and their work motivated our study (See also work by Qi et al (2023). ).…”

Investigation of a Magnetic Reconnection Event with Extraordinarily High Particle Energization in Magnetotail Turbulence

“…Turbulence, however, adds difficulty in establishing the LMN system, which is vital for determining EDR characteristics. Here, we follow a process outlined in Qi et al (2023). The N direction is derived from the maximum variance of E and from the maximum directional derivative of B and E. N is derived repeatedly using low-pass filters of 1, 2, and 4 Hz, employing time periods centered on the EDR (∼04:38:58.5) of 2, 4, and 8 s, and isolating the most active components of B and E. The 36 derived values are in good agreement with a standard deviation of less than 5°.…”

“…The position of MMS1 in N is derived from Ampere's law. Starting from J M ∼ ∂B L /∂N − ∂B N /∂L, ΔN can be written as Pathak et al 2022 andQi et al 2023 for details). B N (Figure 3(a)) has a small and nearly constant slope suggesting a smooth progression in L so ∂B N /∂L is a small correction.…”