Astronomy
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Item Lower-Hybrid Wave Structures and Interactions With Electrons Observed in Magnetotail Reconnection Diffusion Regions(Wiley, 2022-04-22) Wang, Shan; Chen, Li-Jen; Bessho, Naoki; Ng, Jonathan; Hesse, Michael; Graham, Daniel B.; Le Contel, Olivia; Gershman, Daniel; Giles, BarbaraWe investigate waves close to the lower-hybrid frequency in 12 magnetotail reconnection electron diffusion region (EDR) events with guide field levels of near-zero to 30%. In about half of the events, the wave vector has a small component along the current sheet normal, consistent with known lower-hybrid drift wave properties, but the perpendicular magnetic field fluctuations can be comparable or greater than the parallel component, a feature unique to the waves inside and adjacent to EDRs. Another new wave property is that the wave vector has a significant component along the current sheet normal in some events and completely along the normal for one event. In 1/4 of the events, the 𝐴𝐴∇⋅𝑷𝑷𝑒𝑒 term has a significant contribution to the wave electric field, possibly a feature of lower-hybrid waves more likely to exist in the diffusion region than further away from the X-line. Electron temperature variations are correlated with the wave potential, due to wave electric field acceleration and crossings at the corrugated separatrix region with different amounts of mixing between reconnection inflowing and outflowing populations. The latter also leads to the anti-correlation between parallel and perpendicular temperature components. Using four-spacecraft measurements, the magnetic field line twisting is demonstrated by the correlated fluctuations in 𝐴𝐴(∇×𝑽𝑽𝐸𝐸×𝐵𝐵)|| and 𝐴𝐴(∇×𝐁𝐁)||. The lower-hybrid wave in the EDR of weak guide field reconnection may be generated near separatrices and penetrate to the mid-plane or locally generated, and the latter possibility is beyond the prediction of previous reconnection simulations.Item Dataset for "Reconstruction of electron and ion distribution functions in magnetotail reconnection diffusion region"(2020-02) Ng, Jonathan; Chen, Li-Jen; Hakim, Ammar; Bhattacharjee, AmitavaIn the diffusion region of magnetotail reconnection, particle distributions are highly structured, exhibiting triangular shapes and multiple striations that deviate dramatically from the Maxwellian distribution. Fully kinetic simulations have been demonstrated to be capable of producing the essential structures of the observed distribution functions, yet are computationally not feasible for 3D global simulations. The fluid models used for large-scale simulations, on the other hand, do not have the kinetic physics necessary for describing reconnection accurately. Our study aims to bridge fully kinetic and fluid simulations by quantifying the information required to capture the non-Maxwellian features in the distributions underlying the closures used in the fluid code. We compare the results of fully kinetic simulations to observed electron velocity distributions in a magnetotail reconnection diffusion region, and use the maximum entropy model to reconstruct electron and ion distributions using various numbers of moments obtained from the simulation. Our results indicate that using only local moments, the maximum entropy model can reproduce many of the features of the distributions: (1) the anisotropic electron distributions inside the ion diffusion region but outside the current-sheet can be modelled with 10-14 moments, (2) the electron-outflow distribution with a tilted triangular structure is reproduced with 21-35 moments and (3) counterstreaming distributions can be captured with the 35-moment model when the separation in velocity space between the populations is large.