Reconnection Onset in Overstretched Magnetotail Current Sheets

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As the solar wind flows past the Earth, it stretches our planet’s magnetic field lines away from the Sun, storing vast amounts of energy within the magnetic field while forming the magnetotail. When this stretching reaches a critical level, the magnetic field lines reconnect, rapidly releasing the stored energy while powering important space weather effects, such as magnetospheric substorms, that can be hazardous to satellites and the power grid on Earth. Understanding the physics describing the stretched magnetotail and reconnection onset is crucial to modeling and predicting space weather phenomena. Reconnection onset requires the magnetotail current sheet (CS) to thin down to the scales of the plasma particles, such as protons or even electrons orbits, which are nearly microscopic compared to the vast expanse of the magnetotail. This project aims to investigate the structure, stability, and reconnection regimes of these overstretched CSs to understand the mechanism of reconnection onset in the magnetotail and explain its observed location far outside the near-Earth region. The data-mining component of the research will contribute to the relatively under-investigated area of machine learning dealing with little and sparse rather than big data. The broader impacts of this project include hands-on research training for a graduate student, the career development of early career scientists, and interdisciplinary applications to the methodology developed.The onset of reconnection in the magnetotail requires its current sheet (CS) to thin down to the thermal ion gyroradius (or thinner) to demagnetize ions (or even electrons) and to provide their Landau dissipation. However, in isotropic plasmas, the ion-scale CSs inflate too rapidly with the distance from Earth to remain ion-scale at and beyond ~20 Earth’s radii, where most X-lines are observed. A key to solving this problem was recently found due to the discovery of “overstretched” thin CS (OTCS): If an ion-scale CS is embedded into a much thicker CS with even a weak field-aligned ion anisotropy, its current density iso contours can be stretched way more than the magnetic field lines. The goal of this project is to investigate the structure, stability, and reconnection regimes of OTCSs to answer the following fundamental science question: What is the mechanism of the reconnection onset in the magnetotail, which explains its observed location far outside the near-Earth region, and what are its global implications? The effects of the negative CS charging and electron current domination suggested by recent observations, interplay between tearing and ballooning/interchange modes, as well as roles of external driving and local magnetic flux accumulation, will be investigated using new OTCS equilibria in 2D and 3D PIC simulations. The impact of OTCS on the global scale structure and dynamics of the magnetosphere will be assessed by adding their empirical reconstructions obtained by mining a multi-mission and multi-decade database of spaceborne magnetometers and new-generation magnetic field architectures to a global MHD model of the magnetosphere.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.