Electron Diffusion Regions in Magnetotail Reconnection Under Varying Guide Fields

Using measurements by the Magnetospheric Multiscale spacecraft in the magnetotail, we studied electron distribution functions across an electron diffusion region. The dependence of the nongyrotropic distribution on the energy and vertical distance from the electron diffusion region midplane was revealed for the first time. The nongyrotropic distribution was observed everywhere except for an extremely narrow layer right at the electron diffusion region midplane. The energy of the nongyrotropic distribution increased with growth of the vertical distance from the midplane. For the electrons within certain energy range, they exhibited the nongyrotropic distribution at the distance further away from the midplane than that expected from the meandering motion. The correlation between the crescent-shaped distribution with multiple stripes and the large Hall electric field was established. It appears that the measured nongyrotropic distribution and the crescent-shaped distribution were caused by the meandering motion and the Hall electric field together. Plain Language SummaryThe Magnetospheric Multiscale mission is designed to study electron physics of magnetic reconnection, a key process for many explosive phenomena in solar flares and magnetosphere. Understanding electron motion is highly important in the study of magnetic reconnection, and the electron velocity distribution is an intuitive and effective way to study the electron dynamics. In this paper, we present a complete electron diffusion region crossing by Magnetospheric Multiscale during a nearly symmetric magnetic reconnection in the magnetotail and show the evolution of the electron velocity distributions across the electron diffusion region. In addition, the relationship between Hall electric field and crescent-shaped electron distribution is also shown in this paper. Key Points:• The electron nongyrotropic distribution depended on the energy and the normal distance from the midplane of the electron diffusion region • The electron crescent distributions were caused by the meandering motion and the Hall electric field together • The fine substructures were observed in the narrow electron diffusion region

Section: Discussion and Summarymentioning

confidence: 94%

“…A few EDR events during symmetric reconnection have been reported recently, based on the MMS observations in the magnetotail (Chen et al, ; Genestreti et al, ; Nakamura et al, ; Torbert et al, ;Wang et al, ; Zhou et al, ). However, a direct crossing of the EDR was rare (Wang et al, ; Zhou et al, ).…”

Section: Discussion and Summarymentioning

confidence: 98%

Observation of Nongyrotropic Electron Distribution Across the Electron Diffusion Region in the Magnetotail Reconnection

Wang

et al. 2019

“…It will be interesting to see how the short‐wavelength LHDW can affect electron and reconnection dynamics in the EDR during reconnection with a high guide field. Further systematic research on this topic is warranted both in space (i.e., Chen et al, 2019) and in laboratory (i.e., Stechow et al, 2018).…”

Section: Summary and Discussionmentioning

confidence: 99%

Lower Hybrid Drift Waves During Guide Field Reconnection

Yoo

Ambat

et al. 2020

Self Cite

Generation and propagation of lower hybrid drift wave (LHDW) near the electron diffusion region (EDR) during guide field reconnection at the magnetopause is studied with data from the Magnetospheric Multiscale mission and a theoretical model. Inside the current sheet, the electron beta (β e) determines which type of LHDW is excited. Inside the EDR, where the electron beta is high (β e ∼ 5), the long-wavelength electromagnetic LHDW is observed propagating obliquely to the local magnetic field. In contrast, the short-wavelength electrostatic LHDW, propagating nearly perpendicular to the magnetic field, is observed slightly away from the EDR, where β e is small (∼0.6). These observed LHDW features are explained by a local theoretical model, including effects from the electron temperature anisotropy, finite electron heat flux, electrostatics, and parallel current. The short-wavelength LHDW is capable of generating significant drag force between electrons and ions. Plain Language Summary The lower hybrid drift wave (LHDW) is generated inside the current sheet by the electric current perpendicular to the local magnetic field. With data from the Magnetospheric Multiscale (MMS) mission, we confirm that two types of LHDW are excited in the current sheet during magnetic reconnection with guide field, depending on the local plasma parameter, called beta (ratio between the magnetic field pressure and plasma pressure). One is the short-wavelength, quasi-electrostatic LHDW excited just outside the central reconnection site, called the electron diffusion region. The other is the long-wavelength, electromagnetic LHDW excited in the electron diffusion region. A local theoretical model is developed to explain the excitation of both types of the LHDW in the current sheet. Results from the model agree with MMS observations. The short-wavelength LHDW is capable of generating additional friction between electrons and ions, indicating possible importance of this LHDW on reconnection and electron dynamics.

“…Due to the turbulent nature of the magnetic field, we did not calculate the overall guide field of the reconnecting region. The guide field during magnetotail reconnection can change significantly on the timescale of less than a minute, so this event may not have a consistent overall guide field (Chen et al, 2019). Instead, we calculated the core fields of the observed plasmoids by finding the magnetic field strength along the most invariant direction determined from MDD analysis.…”

Section: Statistical Resultsmentioning

confidence: 99%

Statistical Properties of Magnetic Structures and Energy Dissipation during Turbulent Reconnection in the Earth's Magnetotail

Bergstedt

Jara-Almonte

et al. 2020

Self Cite

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 in situ measurements of the Magnetospheric Multiscale mission in the Earth's magnetotail on 26 July 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 subelectron 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. Plain Language Summary Magnetic reconnection is an important mechanism for generating energetic particles in space and solar environments. Turbulent magnetic reconnection causes the development of many small-scale magnetic structures, such as locally helical or loop-like magnetic fields (plasmoids), or areas where oppositely directed magnetic fields are sandwiched together (current sheets). The exact formation and distribution of the structures, as well as the role the structures play in particle energization and the evolution of magnetic reconnection, is still unknown. Using data from the Magnetospheric Multiscale (MMS) mission, we developed an algorithm that is able to detect and identify the magnetic structures present in a region of turbulent magnetic reconnection. The number of structures was found to decrease with size as a decaying exponential, which is consistent with previous theories. The structures contributed strongly to the energization of particles parallel to the local magnetic field, but were not significant sites of energization overall. Overall energization is dominated by energization perpendicular to the local field outside of these structures. There is also significant energy return from particles to the fields.