Browsing by Author "Giles, B. L."
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Item Electron vorticity indicative of the electron diffusion region of magnetic reconnection(American Geophysical Union, 2019-06-27) Hwang, K.‐J.; Choi, E.; Dokgo, K.; Burch, J. L.; Sibeck, D. G.; Giles, B. L.; Goldstein, M. L.; Paterson, W. R.; Pollock, C. J.; Shi, Q. Q.; Fu, H.; Hasegawa, H.; Gershman, D. J.; Khotyaintsev, Y.; Torbert, R. B.; Ergun, R. E.; Dorelli, J. C.; Avanov, L.; Russell, C. T.; Strangeway, R. J.Abstract While vorticity defined as the curl of the velocity has been broadly used in fluid and plasma physics, this quantity has been underutilized in space physics due to low time resolution observations. We report Magnetospheric Multiscale (MMS) observations of enhanced electron vorticity in the vicinity of the electron diffusion region of magnetic reconnection. On 11 July 2017 MMS traversed the magnetotail current sheet, observing tailward‐to‐earthward outflow reversal, current‐carrying electron jets in the direction along the electron meandering motion or out‐of‐plane direction, agyrotropic electron distribution functions, and dissipative signatures. At the edge of the electron jets, the electron vorticity increased with magnitudes greater than the electron gyrofrequency. The out‐of‐plane velocity shear along distance from the current sheet leads to the enhanced vorticity. This, in turn, contributes to the magnetic field perturbations observed by MMS. These observations indicate that electron vorticity can act as a proxy for delineating the electron diffusion region of magnetic reconnection. Plain Language Summary Magnetic reconnection, causing explosive magnetic energy conversion into particle energy, is one of the most fundamental physical processes occurring both within the heliosphere and throughout the universe. The multiscale kinetic structures associated with reconnection have long been a focus in space plasma physics. We investigated how electron vorticity, a physical quantity widely used in fluid physics, but underutilized in the plasma, in particular, reconnection physics, enables us to delineate multiscale kinetic boundaries of reconnection sites using the unprecedented time resolution data from National Aeronautics and Space Administration's Magnetospheric Multiscale spacecraft. The magnitude of electron vorticity to be compared with the electron gyrofrequency provides a frame‐independent indicator of the electron diffusion region, therefore, greatly advancing our ability to delineate the multiscale reconnection boundaries. This study, directly relevant to plasma/reconnection physics, will improve our understanding of fundamental physics with far‐reaching implications in astrophysicsItem Hybrid Kinetic Model of the Interaction Between the Dense Plasma Clouds and Magnetospheric Plasma on Large Time and Spatial Scales, and Comparison With MMS Observations(AGU, 2022-06-30) Lipatov, Alexander; Avanov, L. A.; Giles, B. L.We present a new simulation results of the cloud dynamics in the ambient magnetospheric plasma on the large time and spatial scales. It was assumed that these impulsive structures observed by the MMS spacecraft originally were created because of the reconnection at the magnetopause. Our new 3-D hybrid kinetic modeling on the large time and spatial scales captures several of these processes: an excitation of the electromagnetic waves (whistler and shear-Alfvén waves) and plasma instabilities (mirror and flute); a formation of shock waves, and collapsing diamagnetic cavity; particle acceleration. A strong overshoot in plasma density profile was observed in the modeling and MMS observation at the interface between the cloud and magnetospheric plasma. The cloud expansion into ambient magnetospheric plasma causes the flute waves connected with excitation of the Rayleigh-Taylor instability observed at the overshoot in plasma density profile across the external magnetic field. The modeling demonstrates a formation of the whistler waves at the initial stage which propagate in the external magnetic field direction. At the later stage, a formation of shear-Alfvén waves was observed.Item On the origin of the crescent-shaped distributions observed by MMS at the magnetopause(AGU, 2017-02-18) Lapenta, G.; Berchem, J.; Zhou, M.; Walker, R. J.; El-Alaoui, M.; Goldstein, Melvyn; Paterson, W. R.; Giles, B. L.; Pollock, C. J.; Russell, C. T.; Strangeway, R. J.; Ergun, R. E.; Khotyaintsev, Y. V.; Torbert, R. B.; Burch, J. L.MMS observations recently confirmed that crescent-shaped electron velocity distributions in the plane perpendicular to the magnetic field occur in the electron diffusion region near reconnection sites at Earth's magnetopause. In this paper, we reexamine the origin of the crescent-shaped distributions in the light of our new finding that ions and electrons are drifting in opposite directions when displayed in magnetopause boundary-normal coordinates. Therefore, E × B drifts cannot cause the crescent shapes. We performed a high-resolution multiscale simulation capturing subelectron skin-depth scales. The results suggest that the crescent-shaped distributions are caused by meandering orbits without necessarily requiring any additional processes found at the magnetopause such as the highly asymmetric magnetopause ambipolar electric field. We use an adiabatic Hamiltonian model of particle motion to confirm that conservation of canonical momentum in the presence of magnetic field gradients causes the formation of crescent shapes without invoking asymmetries or the presence of an E × B drift. An important consequence of this finding is that we expect crescent-shaped distributions also to be observed in the magnetotail, a prediction that MMS will soon be able to test.Item Particle Acceleration by Dense Impulsive Structures Moving in Ambient Magnetospheric Plasma. 3-D Hybrid Kinetic Modeling and MMS Observations(AGU, 2021-01-25) Lipatov, Alexander; Avanov, L. A.; Giles, B. L.High resolution observations of dense plasma impulsive structures moving through an ambient background magnetospheric flows were captured by the Magnetospheric Multiscale mission. The observations show particle heating and acceleration, shock-like wave formation, and whistler wave excitation inside the interface between the dense impulsive plasma structures and the ambient plasma. A multiscale hybrid kinetic simulation provides an explanation of the observed wave-particle interactions with the assumption that the dense plasma structures may be represented by plasma clouds which are formed at the magnetopause layer due to reconnection processes.Item Structures in the terms of the Vlasov equation observed at Earth’s magnetopause(Nature, 2021-07-05) Shuster, J. R.; Gershman, D. J.; Dorelli, J. C.; Giles, B. L.; Wang, S.; Bessho, N.; Chen, L.-J.; Cassak, P. A.; Schwartz, S. J.; Denton, R. E.; Uritsky, V. M.; Paterson, W. R.; Schiff, C.; Viñas, A. F.; Ng, J.; Avanov, L. A.; da Silva, Daniel; Torbert, R. B.The Vlasov equation describes collisionless plasmas in the continuum limit and applies to many fundamental plasma energization phenomena. Because this equation governs the evolution of plasma in six-dimensional phase space, studies of its structure have mostly been limited to numerical or analytical methods. Here terms of the Vlasov equation are determined from observations of electron phase-space density gradients measured by the four Magnetospheric Multiscale spacecraft in the vicinity of magnetic reconnection at Earth’s magnetopause. We identify which electrons in velocity space substantially support the electron pressure divergence within electron-scale current layers. Furthermore, we isolate and characterize the effects of density, velocity and temperature gradients on the velocity-space structure and dynamics of these electrons. Unipolar, bipolar and ring structures in the electron phase-space density gradients are compared to a simplified Maxwellian model and correspond to localized gradients in density, velocity and temperature, respectively. These structures have implications for the ability of collisionless plasmas to maintain kinetic Vlasov equilibrium. The results provide a kinetic perspective relevant to how the electron pressure divergence may develop to violate the electron frozen-in condition and sustain electron-scale energy conversion processes, such as the reconnection electric field, in collisionless space plasma environments.Item Temporal, Spatial, and Velocity-Space Variations of Electron Phase Space Density Measurements at the Magnetopause(AGU, 2023-03-21) Shuster, J. R.; Gershman, D. J.; Giles, B. L.; Bessho, N.; da Silva, Daniel; et al.Temporal, spatial, and velocity-space variations of electron phase space density are measured observationally and compared for the first time using the four magnetospheric multiscale (MMS) spacecraft at Earth's magnetopause. Equipped with these unprecedented spatiotemporal measurements offered by the MMS tetrahedron, we compute each term of the electron Vlasov equation that governs the evolution of collisionless plasmas found throughout the universe. We demonstrate how to use single spacecraft measurements to improve the resolution of the electron pressure gradient that supports nonideal parallel electric fields, and we develop a model to intuit the types of kinetic velocity-space signatures that are observed in the Vlasov equation terms. Furthermore, we discuss how the gradient in velocity-space sheds light on plasma energy conversion mechanisms and wave-particle interactions that occur in fundamental physical processes such as magnetic reconnection and turbulence.