Scaling of magnetic reconnection with a limited x-line extent

<p>Magnetic reconnection is a fundamental physical process that is responsible for releasing the magnetic energy during substorms of planetary magnetotails. Previous studies of magnetic reconnection usually take the two-dimensional (2D) approach, which assumes that reconnection is uniform in the 3rd direction out of the 2D reconnection plane. However, observations suggest that reconnection can be limited in the 3rd direction, such as reconnection at Mercury's magnetotail. It turns out that reconnection can be suppressed when reconnection region is very limited in the 3rd direction. An internal x-line asymmetry along the current direction develops because of the transport of reconnected magnetic flux by electrons beneath the ion kinetic scale, resulting in a suppression region identified in Liu et al., 2019. Under the guidance of a series of 3D kinetic simulations, in this work, we incorporate the length-scale of this suppression region ~10d<sub>i</sub> to quantitatively model the reduction of the reconnection rate and the maximum outflow speed observed in the short x-line limit. The average reconnection rate drops because of the limited active region (where the current sheet thins down to the electron inertial scale) within an x-line. The outflow speed reduction correlates with the decrease of the <strong>J</strong>×<strong>B</strong> force, that can be modeled by the phase shift between the <strong>J</strong> and <strong>B</strong> profiles, also as a consequence of the flux transport. Notably, these two quantities are most essential in defining the well-being of magnetic reconnection, which can tell us when reconnection shall be suppressed.</p>

Transmission, trapping and filtering of waves in periodically constrained elastic plates

The paper discusses properties of flexural waves in elastic plates constrained periodically by rigid pins. A structured interface consists of rigid pin platonic gratings parallel to each other. Although the gratings have the same periodicity, relative shifts in horizontal and vertical directions are allowed. We develop a recurrence algorithm for constructing reflection and transmission matrices required to characterize the filtering of plane waves by the structured interface with shifted gratings. The representations of scattered fields contain both propagating and evanescent terms. Special attention is given to the analysis of trapped modes which may exist within the system of rigid pin gratings. Analytical findings are accompanied by numerical examples for systems of two and three gratings. We show geometries containing three gratings in which transmission resonances have very high quality factors (around 35 000). We also show that controlled lateral shifts of three gratings can give rise to a transmission peak with a sharp central suppression region, akin to the phenomenon of electromagnetic-induced transparency.

IMPROVED BEAMFORMER WITH WEIGHTED SOURCE REGION SUPPRESSION FOR COHERENT MEG SOURCE LOCALIZATION

Beamformer is one of the main techniques for spatio-temporal neuroelectromagnetic source reconstruction. However, the classical Beamformer is extremely sensitive to strongly coherent sources, thereby encountering difficulty in localizing the highly correlated bilateral auditory cortices in auditory evoked field (AEF) or auditory steady state evoked potential. The multiple constrained minimum-variance Beamformer with coherent source region suppression (Beamformer-CS) can potentially overcome such difficulties. However, when coherent interferer is located close to the edges of the suppression region, Beamformer-CS has localization bias and the closer it is, the larger it will be. Here, we present an improved Beamformer-CS that can localize coherent sources with much less localization bias, especially in the case of the interferer close to the edges of the suppression region. First, based on approximate information about source energy distribution from other neuroimaging techniques, a region encompassing the coherent interfering sources is defined. Then, the dominant eigenvectors of the lead field matrix, weighted using source energy information obtained by other imaging method, for the suppression region is incorporated into Beamformer design as hard null constraints. Such weighting strategy is able to improve the localization performance. Simulation test shows that, compared to Beamformer-CS, the new weighting approach is of much smaller localization bias, sharper peak of the estimated sources, more robust against noise, and less sensitiveness to the number of the eigenvector components for the suppression region, as is also confirmed by real AEF data test.

RF Magnetic Signal Localization at Very High Magnetic Field Systems

The impetus of this work originated from the advent of high magnetic field magnetic resonance imaging scanners with B0 fields of 4T, 7T, and 9.4T. These ultrahigh magnetic field systems generally improve the signal to noise ratios. However, B1 field non-uniformity also occurs due to the increased RF field frequencies when wavelengths in the head become shorter than its size. As interest in multiple channel transmission line coils increases, the control of the amplitude and phase of individual coil elements is required in order to develop desired B1 field. The choice of the excitation of the coil elements may be determined by convex optimization. Convex optimization is used provides results very fast, when the problem is formulated globally. In addition, convex optimization provides better signal to noise (SNR) ratio when anatomic specific regions are investigated. In this paper, simulation and experimental results are discussed at 9.4T systems based on the number of elements. The primary objective of this study is to increase the signal in a specific target region and decrease the signal and noise in the outside region termed the suppression region. The convex formulations are minimizing the maximum field point in the suppression region while keeping the center of target maximum. Based on this min-max optimization criterion, an iteration method which modifies the selection of suppression fields is also performed to produce better results. The results of the localization on FDTD human data at 9.4T are shown in Fig. 1. In these figures, the axial slices of the center of human head model provided by XFDTD are used after manipulating with MATLAB and the 16 channel head coil is excited. Figure 1 shows an improvement of the homogeneity in the suppression region when the target region is at center. In Fig. 2, received signal localizations are obtained for three different regions of interest (ROI) after using the convex optimization. Note that the selection of ROI is limited by the geometric setting of phantom in the 8-channel TEM head coil. Convex optimization with an iterative method was performed on both the human head and phantom models with operating frequency 400 MHz to design coil channel excitation parameters. By applying the iterative method to the convex optimization, more homogeneous B1 fields are obtained in the suppression region for 9.4T system.

BASP1 Is a Transcriptional Cosuppressor for the Wilms' Tumor Suppressor Protein WT1

ABSTRACT The Wilms' tumor suppressor protein WT1 is a transcriptional regulator that plays a key role in the development of the kidneys. The transcriptional activation domain of WT1 is subject to regulation by a suppression region within the N terminus of WT1. Using a functional assay, we provide direct evidence that this requires a transcriptional cosuppressor, which we identify as brain acid soluble protein 1 (BASP1). WT1 and BASP1 associate within the nuclei of cells that naturally express both proteins. BASP1 can confer WT1 cosuppressor activity in transfection assays, and elimination of endogenous BASP1 expression augments transcriptional activation by WT1. BASP1 is present in the developing nephron structures of the embryonic kidney and, coincident with that of WT1, its expression is restricted to the highly specialized podocyte cells of the adult kidney. Taken together, our results show that BASP1 is a WT1-associated factor that can regulate WT1 transcriptional activity.