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Magnetic reconnection is a fundamental process providing topological changes of the magnetic field, reconfiguration of space plasmas and release of energy in key space weather phenomena, solar flares, coronal mass ejections and magnetospheric substorms. Its multiscale nature is difficult to study in observations because of their sparsity. Here we show how the lazy learning method, known as K nearest neighbors, helps mine data in historical space magnetometer records to provide empirical reconstructions of reconnection in the Earth’s magnetotail where the energy of solar wind-magnetosphere interaction is stored and released during substorms. Data mining reveals two reconnection regions (X-lines) with different properties. In the mid tail ( ∼ 30 R E from Earth, where R E is the Earth’s radius) reconnection is steady, whereas closer to Earth ( ∼ 20 R E ) it is transient. It is found that a similar combination of the steady and transient reconnection processes can be reproduced in kinetic particle-in-cell simulations of the magnetotail current sheet. 

Wind spacecraft measurements are analyzed to obtain a current sheet (CS) normal widthd_csdistribution of 3374 confirmed magnetic reconnection exhausts in the ecliptic plane of the solar wind at 1 au. Thed_csdistribution displays a nearly exponential decay from a peak atd_cs= 25d_ito a median atd_cs= 85d_iand a 95th percentile atd_cs= 905d_iwith a maximum exhaust width atd_cs= 8077d_i. A magnetic fieldθ-rotation angle distribution increases linearly from a relatively few high-shear events toward a broad peak at 35° <θ< 65°. The azimuthalϕangles of the CS normal directions of 430 thickd_cs≥ 500d_iexhausts are consistent with a dominant Parker-spiral magnetic field and a CS normal along the ortho-Parker direction. The CS normal orientations of 370 kinetic-scaled_cs< 25d_iexhausts are isotropic in contrast, and likely associated with Alfvénic solar wind turbulence. We propose that the alignment of exhaust normal directions from narrowd_cs∼ 15–25d_iwidths to well beyondd_cs∼ 500d_iwith an ortho-Parker azimuthal direction of a large-scale heliospheric current sheet (HCS) is a consequence of CS bifurcation and turbulence within the HCS exhaust that may trigger reconnection of the adjacent pair of bifurcated CSs. The proposed HCS-avalanche scenario suggests that the underlying large-scale parent HCS closer to the Sun evolves with heliocentric distance to fracture into many, more or less aligned, secondary CSs due to reconnection. A few wide exhaust-associated HCS-like CSs could represent a population of HCSs that failed to reconnect as frequently between the Sun and 1 au as other HCSs.

 

This white paper is on the HMCS Firefly mission concept study. Firefly focuses on the global structure and dynamics of the Sun's interior, the generation of solar magnetic fields, the deciphering of the solar cycle, the conditions leading to the explosive activity, and the structure and dynamics of the corona as it drives the heliosphere.