Session GM14: Mini-Conference on Reconnection: Magnetosphere

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Advances in magnetic reconnection in the Earth's magnetosphere over the past five years result in large part from the implementation of electron-scale measurements from four closely-spaced probes by the NASA Magnetospheric Multiscale (MMS) mission. These new capabilities are focused primarily on magnetic reconnection in the boundary regions of the magnetosphere, most notably the dayside magnetopause and the neutral sheet in the magnetic tail. On the day side the predictions of asymmetric reconnection and effects of guide fields have been tested and, in many cases, resolved. Tail events provide detailed studies of symmetric reconnection and energetic particle acceleration. Various plasma wave modes have been identified within reconnection events and their importance determined both as results and drivers of reconnection. In addition to providing our first look at reconnection in space at the electron scale, MMS reveals a much more ubiquitous nature of reconnection than was previously supposed as reconnection is being found in many new places, including bow shocks, Kelvin-Helmholtz vortices, magnetic flux ropes, flux transfer events, and near-tail dipolarization fronts.   

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The unprecedented high resolution measurements from NASA’s Magnetospheric Multiscale (MMS) mission uncover a never-before-seen connection between new turbulence and reconnection regimes. For the first time, lower hybrid drift turbulence is observed in the electron-scale reconnection layer, driving nongyrotropic electron heating. The perpendicular heating occurs within lower hybrid drift vortices and may lead to locally enhanced tearing growth, triggering further reconnection on scales between the electron and ion inertial lengths. At the terrestrial bow shock and its downstream, the intense current structures generated by the shock turbulence are prone to reconnection. As indicated by measurements from the MMS mission and PIC simulations, electron-scale turbulence gives rise to reconnection without ion dynamics, while the typical reconnection with both electron and ion participation occurs in ion-scale turbulence. We will discuss how these MMS discoveries conceptually change the existing picture about reconnection and turbulence, and inspire a new frontier of research for both simulations and laboratory experiments.\\In collaboration with: Shan Wang, NASA Goddard Space Flight Center/University of Maryland; Jonathan Ng, NASA Goddard Space Flight Center/University of Maryland   

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The launch of NASA’s Magnetospheric Multiscale (MMS) mission in 2015 transformed research into magnetic reconnection, the fundamental physics process during which the often abrupt change of magnetic topology leads to particle energization, plasma heating, and large-scale energy release from magnetic fields. This crucial process impacts a diverse set of phenomena on Earth and elsewhere in the universe, making major contributions to space weather, causing astrophysical eruptions, hampering magnetically confined fusion, and having potential applied uses in spacecraft thrusters. The veritable plasma physics laboratories embedded on the four identical MMS spacecraft have temporal resolution approximately 100 times faster than previous missions. This has ushered in a new era in which kinetic-scale physics, at and below the scale of the ion and electron gyroradius, is regularly measured during magnetic reconnection using Earth’s magnetosphere as a laboratory. In this presentation, numerous aspects of what has been learned in the MMS era about kinetic-scale physics of magnetic reconnection will be reviewed. Then, remaining theoretical challenges for understanding magnetic reconnection will be outlined and discussed.\\In collaboration with: Michael Hesse, University of Bergen; Haoming Liang, University of Alabama-Huntsville; Hasan Barbhuiya, West Virginia University   

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We study magnetic reconnection at the Earth's bow shock using observations by the Magnetospheric Multiscale (MMS) mission. The reconnecting current sheets exist both in the foreshock and in the deep shock transition region. The current sheets may contain the electron outflow jet, Hall fields and Hall currents, and show energy conversion between the fields and particles, while ions do not have response. There also exists reconnecting current sheets where the ion exhaust is observed with ion acceleration and heating. The compression of the current sheets originated from the foreshock waves appears to be one mechanism of generating thin current sheets subject to reconnection.   

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We present the first statistical study of magnetic structures and associated energy dissipation observed during a single period of turbulent magnetic reconnection, by using the \textit{in-situ} measurements of the Magnetospheric Multiscale mission in the Earth's magnetotail on July 26, 2017. The structures are selected by identifying a bipolar signature in the magnetic field and categorized as plasmoids or current sheets via an automated algorithm which examines current density and plasma flow. The size of the plasmoids forms a decaying exponential distribution ranging from sub-electron up to ion scales. The presence of substantial number of current sheets is consistent with a physical picture of dynamic production and merging of plasmoids during turbulent reconnection. The magnetic structures are locations of significant energy dissipation via electric field parallel to the local magnetic field, while dissipation via perpendicular electric field dominates outside of the structures. Significant energy also returns from particles to fields.   

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MMS observations have revealed reconnecting current sheets in the Earth's bow shock and the magnetosheath. These current sheets may play an important role in shock heating and particle acceleration. We show results from 2D particle-in-cell simulations with parameters similar to Earth's quasi-parallel bow shock. In the shock transition region, we identified two types of waves: a long-wavelength mode (wavelength $=$ a few ion skin depths) and a short-wavelength mode (wavelength $<$ ion skin depth). The long-wavelength mode is propagating toward the shock in the shock frame, obliquely to the magnetic field. This is a right-handed wave excited by an ion beam instability. The obliquity of the wave results in a longitudinal electric fluctuation, which accelerates electrons. The secondary instability due to the electron beam produces the short-wavelength mode, a right-handed wave propagating in the electron flow direction. These waves bend the magnetic field allowing reconnection to occur. Reconnection can occur in two different scales: the scale of several ion skin depths, due to the long-wavelength mode, and the scale of a sub-ion skin depth, due to the short-wavelength mode. In the latter, electron-only reconnection occurs because ions cannot respond to the small-scale structures.   

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Reconnection in Earth’s magnetotail transfers magnetic energy to thermal and kinetic energy in ions and electrons. These particles are injected both Earthward and tailward from the reconnection region. The Earthward particles are transported to the inner magnetosphere where they drive the ring current and radiation belts. The injections are observed in the plasma sheet in conjunction with dipolarizations of the magnetic field. The particles have been found to travel within narrow flow channels, rather than broadly across the magnetotail, in spatially and temporally localized events known as bursty bulk flows (BBF). Simulations of such events show these narrow flow channels moving from the reconnection region to the injection region. However, global observations are needed to understand how BBFs connect the reconnection region and the inner magnetosphere during storms and substorms. Ion heating has been observed with in situ measurements at the reconnection region and within the dipolarization fronts and BBFs. Using energetic neutral atom (ENA) imaging, ion temperature maps can be calculated to provide such global observations. We describe the 3 August 2016 storm-time substorm using observations from TWINS, MMS, and AMPERE along with MHD simulations.   

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Magnetic reconnection plays an important role in the energization of particles in collisionless plasmas. We apply an established field-particle correlation technique to explore the energization of ions and electrons in collisionless magnetic reconnection simulations. The goal is to determine the characteristic velocity-space signatures of energy transfer in a collisionless plasma due to magnetic reconnection using single-point measurements of the electromagnetic fields and particle velocity distributions. We compare kinetic wave-particle energization to energy in bulk flows at specific spatial locations. The comparisons and characterization of energy outflows due to magnetic reconnection will help in understanding the impact of this phenomena on collisionless plasma energization. This work utilizes a diagnostic suite developed to analyze field-particle correlations from the gyrokinetic simulation code AstroGK. Understanding the entire phase-space energy budget in single point measurements may provide novel insight into kinetic plasma energy transfer. Developments of novel spacecraft measurement techniques to identify particle energization due to magnetic reconnection may gain insight from understanding these energy transfer signatures.