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1. The Role of Current Sheet Scattering in the Proton Isotropic Boundary Formation During Geomagnetic Storms

   https://doi.org/10.1029/2018JA026290  

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

   Abstract There is considerable evidence that current sheet scattering (CSS) plays an important role in isotropic boundary (IB) formation during quiet time. However, IB formation can also result from scattering by electromagnetic ion cyclotron waves, which are much more prevalent during storm time. The effectiveness of CSS can be estimated by the parameter, the ratio of the field line radius of curvature to the particle gyroradius. Using magnetohydrodynamic and empirical models, we estimated the parameterKassociated with storm time IB observations on the nightside. We used magnetic field observations from spacecraft in the magnetotail to estimate and correct for errors in theKvalues computed by the models. We find that the magnetohydrodynamic and empirical models produce fairly similar results without correction and that correction increases this similarity. Accounting for uncertainty in both the latitude of the IB and the threshold value ofKrequired for CSS, we found that 29–54% of the IB observations satisfied the criteria for CSS. We found no correlation between the correctedKand magnetic local time, which further supports the hypothesis that CSS played a significant role in forming the observed IBs. 

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2. Kinetic-scale Current Sheets in the Solar Wind at 1 au: Scale-dependent Properties and Critical Current Density

   https://doi.org/10.3847/2041-8213/ac4fc4  

   Vasko, I. Y.; Alimov, K.; Phan, T.; Bale, S. D.; Mozer, F. S.; Artemyev, A. V. (February 2022, The Astrophysical Journal Letters)

   Abstract We present analysis of 17,043 proton kinetic-scale current sheets (CSs) collected over 124 days of Wind spacecraft measurements in the solar wind at 11 samples s⁻¹magnetic field resolution. The CSs have thickness,λ,from a few tens to one thousand kilometers with typical values around 100 km, or within about 0.1–10λ_pin terms of local proton inertial length,λ_p. We found that the current density is larger for smaller-scale CSs,J₀≈ 6 nAm⁻²· (λ/100 km)^−0.56, but does not statistically exceed a critical value,J_A,corresponding to the drift between ions and electrons of local Alvén speed. The observed trend holds in normalized units: $J_0/J_A\approx 0.17·{\left(\lambda /{\lambda }_p\right)}^{-0.51}$. The CSs are statistically force-free with magnetic shear angle correlated with CS spatial scale: $\mathrm{\Delta }\theta \approx 19°·{\left(\lambda /{\lambda }_p\right)}^{0.5}$. The observed correlations are consistent with local turbulence being the source of proton kinetic-scale CSs in the solar wind, while the mechanisms limiting the current density remain to be understood. 

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3. Numerical Simulations of the Geospace Response to the Arrival of an Idealized Perfect Interplanetary Coronal Mass Ejection

   https://doi.org/10.1029/2020SW002489  

   Welling, Daniel T.; Love, Jeffrey J.; Rigler, E. Joshua; Oliveira, Denny M.; Komar, Colin M.; Morley, Steven K. (February 2021, Space Weather)

   Abstract Previously, Tsurutani and Lakhina (2014,https://doi.org/10.1002/2013GL058825) created estimates for a “perfect” interplanetary coronal mass ejection and performed simple calculations for the response of geospace, including. In this study, these estimates are used to drive a coupled magnetohydrodynamic‐ring current‐ionosphere model of geospace to obtain more physically accurate estimates of the geospace response to such an event. The sudden impulse phase is examined and compared to the estimations of Tsurutani and Lakhina (2014,https://doi.org/10.1002/2013GL058825). The physics‐based simulation yields similar estimates for Dst rise, magnetopause compression, and equatorialvalues as the previous study. However, results diverge away from the equator.values in excess of 30 nT/s are found as low asmagnetic latitude. Under southward interplanetary magnetic field conditions, magnetopause erosion combines with strong region one Birkeland currents to intensify theresponse. Values obtained here surpass those found in historically recorded events and set the upper threshold of extreme geomagnetically induced current activity at Earth. 

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4. 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)

   Abstract 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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5. High and Variable Drag in a Sinuous Estuary With Intermittent Stratification

   https://doi.org/10.1029/2021JC017327  

   Bo, Tong; Ralston, David K.; Kranenburg, Wouter M.; Geyer, W. Rockwell; Traykovski, Peter (October 2021, Journal of Geophysical Research: Oceans)

   Abstract In field observations from a sinuous estuary, the drag coefficientbased on the momentum balance was in the range of, much greater than expected from bottom friction alone.also varied at tidal and seasonal timescales.was greater during flood tides than ebbs, most notably during spring tides. The ebb tidewas negatively correlated with river discharge, while the flood tideshowed no dependence on discharge. The large values ofare explained by form drag from flow separation at sharp channel bends. Greater water depths during flood tides corresponded with increased values of, consistent with the expected depth dependence for flow separation, as flow separation becomes stronger in deeper water. Additionally, the strength of the adverse pressure gradient downstream of the bend apex, which is indicative of flow separation, correlated withduring flood tides. Whilegenerally increased with water depth,decreased for the highest water levels that corresponded with overbank flow. The decrease inmay be due to the inhibition of flow separation with flow over the vegetated marsh. The dependence ofduring ebbs on discharge corresponds with the inhibition of flow separation by a favoring baroclinic pressure gradient that is locally generated at the bend apex due to curvature‐induced secondary circulation. This effect increases with stratification, which increases with discharge. Additional factors may contribute to the high drag, including secondary circulation, multiple scales of bedforms, and shallow shoals, but the observations suggest that flow separation is the primary source. 

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