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1. On the Accuracy of Adiabaticity Parameter Estimations Using Magnetospheric Models

   https://doi.org/10.1029/2018JA025916  

   Haiducek, John D. ; Ganushkina, Natalia Y. ; Dubyagin, Stepan ; Welling, Daniel T. ( March 2019 , Journal of Geophysical Research: Space Physics)

   Recent studies have found that even during quiet times, observed proton isotropic boundaries (IBs) are often projected to the region of high adiabaticity parameter (K≈30), whereis the ratio of magnetic field line radius of curvature to the particle gyroradius. This contradicts the accepted hypothesis that current sheet scattering (CSS) is the dominant mechanism of IB formation becauseK≈8 would be expected for this mechanism. We used magnetohydrodynamic simulations and empirical models to computeKfor 30‐keV proton IB observations within 3 hr of local midnight. We found that neither class of model reliably estimatesKunless supported by magnetic field observations in the current sheet. magnetohydrodynamic simulations produced higherKvalues than expected for CSS (K = 15–30), and empirical models gave lower values (K < 4). We obtained reliable estimates ofKby controlling for the accuracy of the normal component and the gradient of the radial component in the neutral sheet, using observations from three Time History of Events and Macroscale Interactions during Substorms satellites. For the first time, we demonstrated that both these variables should be taken into account for the accurate estimation of the curvature radius. This greatly reduced the spread ofKvalues, indicating that much of the previous spread was due to errors in the magnetic field but also that these errors can be controlled. Most of the corrected values fall within the expected range for CSS, supporting the hypothesis that the IB's were formed by CSS. Accounting for all model results, we obtain an average corrected value ofK = 6.0.

    

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2. Finding maximum matchings in random regular graphs in linear expected time

   https://doi.org/10.1002/rsa.20980  

   Anastos, Michael ; Frieze, Alan ( November 2020 , Random Structures & Algorithms)

   In a seminal paper on finding large matchings in sparse random graphs, Karp and Sipser proposed two algorithms for this task. The second algorithm has been intensely studied, but due to technical difficulties, the first algorithm has received less attention. Empirical results by Karp and Sipser suggest that the first algorithm is superior. In this paper we show that this is indeed the case, at least for randomk‐regular graphs. We show that w.h.p. the first algorithm will find a matching of sizein a randomk‐regular graph,k = O(1). We also show that the algorithm can be adapted to find a maximum matching inO(n)time w.h.p. This is to be compared withO(n^3/2)time for the worst‐case.

    

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3. Ballooning‐Interchange Instability in the Near‐Earth Plasma Sheet and Auroral Beads: Global Magnetospheric Modeling at the Limit of the MHD Approximation

   https://doi.org/10.1029/2020GL088227  

   Sorathia, K. A. ; Merkin, V. G. ; Panov, E. V. ; Zhang, B. ; Lyon, J. G. ; Garretson, J. ; Ukhorskiy, A. Y. ; Ohtani, S. ; Sitnov, M. ; Wiltberger, M. ( July 2020 , Geophysical Research Letters)

   Explosive magnetotail activity has long been understood in the context of its auroral manifestations. While global models have been used to interpret and understand many magnetospheric processes, the temporal and spatial scales of some auroral forms have been inaccessible to global modeling creating a gulf between observational and theoretical studies of these phenomena. We present here an important step toward bridging this gulf using a newly developed global magnetosphere‐ionosphere model with resolution capturing30 km azimuthal scales in the auroral zone. In a global magnetohydrodynamic (MHD) simulation of the growth phase of a synthetic substorm, we find the self‐consistent formation and destabilization of localized magnetic field minima in the near‐Earth magnetotail. We demonstrate that this destabilization is due to ballooning‐interchange instability which drives earthward entropy bubbles with embedded magnetic fronts. Finally, we show that these bubbles create localized field‐aligned current structures that manifest in the ionosphere with properties matching observed auroral beads.

    

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4. The Response of Middle Thermosphere (∼160 km) Composition to the November 20 and 21, 2003 Superstorm

   https://doi.org/10.1029/2021JA029449  

   Yu, Tingting ; Wang, Wenbin ; Ren, Zhipeng ; Cai, Xuguang ; Yue, Xinan ; He, Maosheng ( October 2021 , Journal of Geophysical Research: Space Physics)

   TIMED/GUVI limb measurements and first‐principles simulations from the Thermosphere Ionosphere Electrodynamics Global Circulation Model (TIEGCM) are used to investigate thermospheric atomic oxygen (O) and molecular nitrogen (N₂) responses in the middle thermosphere on a constant pressure surface (∼160 km) to the November 20 and 21, 2003 superstorm. The consistency between GUVI observations and TIEGCM simulated composition changes allows us to utilize TIEGCM outputs to investigate the storm‐time behaviors of O and N₂systematically. Diagnostic analysis shows that horizontal and vertical advection are the two main processes that determine the storm‐induced perturbations in the middle thermosphere. Molecular diffusion has a relatively smaller magnitude than the two advection processes, acting to compensate for the changes caused by the transport partly. Contributions from chemistry and eddy diffusion are negligible. During the storm initial and main phases, composition variations at high latitudes are determined by both horizontal and vertical advection. At middle‐low latitudes, horizontal advection is the main driver for the composition changes where O mass mixing ratiodecreases (N₂mass mixing ratioincreases); whereas horizontal and vertical advection combined to dominate the changes in the regions whereincreases (decreases). Over the entire storm period, horizontal advection plays a significant role in transporting high‐latitude composition perturbations globally. Our results also demonstrate that storm‐time temperature changes are not the direct cause of the composition perturbations on constant pressure surfaces.

    

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5. Enhanced spontaneous polarization in double perovskite Bi 2 FeCrO 6 films

   https://doi.org/10.1111/jace.16386  

   Meng, Dehuan ; Xiao, Yunsheng ; He, Hongcai ; Liao, Yulong ; Zhang, Huaiwu ; Zhai, Junyi ; Chen, Zuhuang ; Martin, Lane W. ; Bai, Feiming ( March 2019 , Journal of the American Ceramic Society)

   We report the pulsed‐laser deposition of epitaxial double‐perovskite Bi₂FeCrO₆(BFCO) films on the (001)‐, (110), and (111)‐oriented single‐crystal SrTiO₃substrates. All of the BFCO films with various orientations show theandsuperlattice‐diffraction peaks. The intensity ratios between the‐superlattice and the main 111‐diffraction peak can be tailored by simply adjusting the laser repetition rate and substrate temperature, reaching up to 4.4%. However, both optical absorption spectra and magnetic measurements evidence that the strong superlattice peaks are not correlated with theB‐site Fe³⁺/Cr³⁺cation ordering. Instead, the epitaxial (111)‐oriented Bi₂FeCrO₆films show an enhanced remanent polarization of 92 μC/cm²at 10 K, much larger than the predicted values by density‐functional theory calculations. Positive‐up‐negative‐down (PUND) measurements with a time interval of 10 μs further support these observations. Therefore, our experimental results reveal that the strong superlattice peaks may come fromA‐ orB‐site cation shifts along the pseudo‐cubic [111] direction, which further enhance the ferroelectric polarization of the BFCO thin films.

    

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