Determining L‐M‐N Current Sheet Coordinates at the Magnetopause From Magnetospheric Multiscale Data

Abstract

We discuss methods to determine L‐M‐N coordinate systems for current sheet crossings observed by the Magnetospheric Multiscale (MMS) spacecraft mission during ongoing reconnection, where eL is the direction of the reconnecting component of the magnetic field, B, and eN is normal to the magnetopause. We present and test a new hybrid method, with eL estimated as the maximum variance direction of B (MVAB) and eN as the direction of maximum directional derivative of B, and then adjust these directions to be perpendicular. In the best case, only small adjustment is needed. Results from this method, applied to an MMS crossing of the dayside magnetopause at 1305:45 UT on 16 October 2015, are discussed and compared with those from other methods for which eN is obtained by other means. Each of the other evaluations can be combined with eL from MVAB in a generalized hybrid approach to provide an L‐M‐N system. The quality of the results is judged by eigenvalue ratios, constancy of directions using different data segments and methods, and expected sign and magnitude of the normal component of B. For this event, the hybrid method appears to produce eN accurate to within less than 10°. We discuss variance analysis using the electric current density, J, or the J × B force, which yield promising results, and minimum Faraday residue analysis and MVAB alone, which can be useful for other events. We also briefly discuss results from our hybrid method and MVAB alone for a few other MMS reconnection events.

Localized and Intense Energy Conversion in the Diffusion Region of Asymmetric Magnetic Reconnection

“…However, v eN is typically much smaller than either v eM or v eL near X-points, which makes its measurement susceptible to errors arising from small variations in the determination of the LMN coordinate system. Denton et al (2018) have shown that variations up to 25 ∘ are possible, which is more than sufficient to explain the discrepancy.…”

Polynomial Reconstruction of the Reconnection Magnetic Field Observed by Multiple Spacecraft

“…Here L may be the direction of the reconnection magnetic field, N may be the normal direction across the current sheet, and M is the other direction. In the common two-dimensional description of magnetic reconnection, M is assumed to be the direction of invariance, but sometimes the most invariant direction has a different orientation (Denton et al, , 2018.…”

“…The gold dotted curves in Figures 1e-1j show the values of B and J averaged over the spacecraft. The coordinate system L-M-N was determined for t = 31 s to 35.5 s after 12:18 UT using the hybrid method ofDenton et al (2018), which combines the best determined MDD direction, N, with the best determined minimum variance analysis (MVA) direction, L(Sonnerup & Cahill, 1967;Sonnerup & Scheible, 1998). For more details, see Text S1.The result was e L = [0.98721, −0.13055, −0.09149], e M = [0.15797, 0.8782, 0.45146], e N = [0.02141, −0.46014, 0.88759] in geocentric solar ecliptic (GSE) coordinates.…”

MMS Study of the Structure of Ion‐Scale Flux Ropes in the Earth's Cross‐Tail Current Sheet

“…The spatiotemporal difference (STD) method (Shi et al, 2006) is used in combination of the MDD to obtain a reference frame for each flux rope. The MDD and STD techniques have been applied to multipoint observations of Cluster (e.g., Li et al, 2016;Sun et al, 2010) and MMS (e.g., Denton et al, 2018;Hasegawa et al, 2017;Yao et al, 2017). Local magnetic curvature of magnetic field lines is obtained by magnetic field rotation analysis (Shen et al, 2007).…”