Umpolung Research Articles

Visual cortical area contributions to the transient, multifocal and steady-state VEP: A forward model-informed analysis

Abstract Central to our understanding of how visual-evoked potentials (VEPs) contribute to visual processing is the question of where their anatomical sources are. Three well-established measures of low-level visual cortical activity are widely used: the first component (“C1”) of the transient and multifocal VEP, and the steady-state VEP (SSVEP). Although primary visual cortex (V1) activity has often been implicated in the generation of all three signals, their dominant sources remain uncertain due to the limited resolution and methodological heterogeneity of source modelling. Here, we provide the first characterisation of all three signals in one analytic framework centred on the “cruciform model”, which describes how scalp topographies of V1 activity vary with stimulus location due to the retinotopy and unique folding pattern of V1. We measured the transient C1, multifocal C1, and SSVEPs driven by an 18.75 Hz and 7.5 Hz flicker, and regressed them against forward models of areas V1, V2, and V3 generated from the Benson-2014 retinotopy atlas. The topographic variations of all four VEP signals across the visual field were better captured by V1 models, explaining between 2 and 6 times more variance than V2/V3. Models with all three visual areas improved fit further, but complementary analyses of temporal dynamics across all three signals indicated that the bulk of extrastriate contributions occur considerably later than V1. Overall, our data support the use of peak C1 amplitude and SSVEPs to probe V1 activity, although the SSVEP contains stronger extrastriate contributions. Moreover, we provide elaborated heuristics to distinguish visual areas in VEP data based on signal lateralisation as well as polarity inversion.

The Relationships among Solar Flare Impulsiveness, Energy Release, and Ribbon Development

We develop the impulsiveness index, a new classification system for solar flares using the Solar Dynamics Observatory/Extreme Ultraviolet Experiment 304 Å Sun-as-a-star light curves. Impulsiveness classifies events based on the duration and intensity of the initial high-energy deposition of energy into the chromosphere. In stellar flare U-band light curves, Kowalski et al. found that impulsiveness is related to quantities such as a proxy for the Balmer jump ratio. However, the lack of direct spatial resolution in stellar flares limits our ability to explain this phenomenon. We calculate impulsiveness for 1368 solar flares between 2010 April and 2014 May. We divide events into categories of low, mid, and high impulsiveness. We find, in a sample of 480 flares, that events with high maximum reconnection rate tend to also have high impulsiveness. For six case studies, we compare impulsiveness to magnetic shear, ribbon evolution, and energy release. We find that the end of the 304 Å light-curve rise phase in these case studies corresponds to the cessation of polarity inversion line (PIL)-parallel ribbon motion, while PIL-perpendicular motion persists afterward in most cases. The measured guide-field ratio for low- and mid-impulsiveness case-study flares decreases about an order of magnitude during the impulsive flare phase. Finally, we find that, in four of the six case studies, flares with higher, more persistent shear tend to have low impulsiveness. Our study suggests that impulsiveness may be related to other properties of the impulsive phase, though more work is needed to verify this relationship and apply our findings to stellar flare physics.

Rearrangement of sunspot magnetic field caused by an X1.5 solar flare

ABSTRACT Solar flares will cause the change of the photospheric magnetic field and sunspot structure. However, the exact physical processes involved remain unclear. Here, we study the changes of photospheric magnetic field before and after an X1.5 flare caused by a circular filament eruption in the active region NOAA 13006. The magnetic field structure of this active region is a fan-spine structure with a circular polarity inversion line (PIL). We found that the sunspot structure contracted towards the PIL as a whole after the flare. The penumbra away from the PIL gradually disappears, and the umbra and penumbra near the PIL gradually enhances. By analysing the local magnetic field, the Lorentz force (LF), and the photospheric velocity field in these three regions, we find that the magnetic flux in the region of the disappearing penumbra converges and contracts towards the PIL, leading to an enhancement of the umbra, while the enhancement of the penumbra potentially indicates that this region may be the footpoint of a reconnected magnetic field system. We suggest that this contracting motion is driven by the horizontal LF.

Simultaneously realizing reversal of piezoelectric coefficient and enhancement of piezoresponse by chromium-doping in aluminum nitride films

Polarity inversion is an interesting phenomenon in non-centrosymmetric wurtzite-structured aluminum nitride (AlN), which offers an important platform to establish acoustic devices with heteropolar junctions. However, previous studies showed that switching polarity generally resulted in only reversing the piezoelectric coefficient (d33) in AlN films. Here, we discovered that appropriate Cr-doping would not only allow to reverse d33 but also improve piezoelectric response in the c-axis oriented AlN films by co-sputtering dual targets of Cr and Al. Specifically, the d33 was reversed from +2.6 pC/N for the undoped AlN to −2.6 pC/N for the 6.2 at. % Cr-doped CrxAl(1−x)N films. As the Cr-doping ratio increased to 9.3 at. %, d33 was −7.0 pC/N, which was 1.7 times higher than that of the undoped AlN films. Independent PFM phase image measurement offered further evidence of the polarity inversion by comparing the undoped and 9.3 at. % Cr-doped CrxAl(1−x)N films. This work offers a simple doping strategy that allows for simultaneous reversal of piezoelectric coefficient and enhancement of piezoresponse. As a result, it establishes a promising foundation for the design and development of acoustic wave devices featuring heteropolar junctions.

Formation of Polar Crown Filament Magnetic Fields by Supergranular Helicity Injection

To understand the magnetic fields of the polar crown filaments (PCFs) at high latitudes near polar regions of the Sun, we perform magnetofrictional numerical simulations on the long-term magnetic evolution of bipolar fields with roughly east–west polarity inversion lines (PILs) in a 3D spherical wedge domain near polar regions. The Coriolis-effect-induced vortical motions at the boundaries of several supergranular cells inject magnetic helicity from the photospheric boundary into the solar atmosphere. Supergranular-scale helicity injection, transfer, and condensation produce strongly sheared magnetic fields. Magnetic reconnections at footpoints of the sheared fields produce magnetic flux ropes (MFRs) with helicity signs consistent with the observed hemispheric helicity rule. The cross-sectional area of MFRs exhibits an uneven distribution, resembling a “foot-node-foot” periodic configuration. Experiments with different tilt directions of PILs indicate that the PCFs preferably form along PILs with the western end close to the polar region. The bending of PILs caused by supergranular flows, forming S-shape (Z-shape) PIL segments, promotes the formation of dextral (sinistral) MFRs. The realistic magnetic models we obtained can serve as starting points for the study of the plasma formation and eruption of PCFs.

Comprehensive Analysis of a Filament-embedding Solar Active Region at Different Stages of Evolution

Active regions are the brightest structures seen in the solar corona, so their physical properties hold important clues to the physical mechanisms underlying coronal heating. In this work, we present a comprehensive study for a filament-embedding active region as determined from observations from multiple facilities including the Chinese Hα Solar Explorer. We find three types of dynamic features that correspond to different thermal and magnetic properties, i.e., the overlying loops—1 MK cool loops, the moss region—2–3 MK hot loops’ footprints, and the sigmoidal filament. The overlying cool loops, which have a potential field, always show Doppler blueshifts at the east footprint and Doppler redshifts at the west, indicating a pattern of “siphon flow.” The moss-brightening regions, which sustain the hot loops that have a moderate sheared field, always show downward Doppler redshifts at the chromosphere, which could be a signature of plasma condensing into the inner region adjacent to the filament. The sigmoidal filament, which has strongly sheared field lines along the polarity inversion line, however, shows a different Doppler velocity pattern in its middle part, i.e., an upward Doppler blueshift at the double-J-shaped stage indicating tether-cutting reconnection during the filament channel formation and then a downward redshift showing the plasma condensation for the sigmoidal filament formation. The present work shows overall properties of the filament-embedding active region, constraining the heating mechanisms of different parts of the active region and providing hints regarding the mass loading of the embedded filament.

On the transition radiation interpretation of anomalous ANITA events

The Antarctic Impulsive Transient Antenna (ANITA) detector has observed several radio pulses coming from the surface of the ice cap at the South Pole. These pulses were attributed to upward-going atmospheric particle showers instead of the downward-going showers induced by cosmic rays that exhibit a characteristic polarity inversion of the radio signal due to reflection in the ice. Coherent transition radiation from cosmic-ray showers developing in the atmosphere and intercepting the ice surface has been suggested as a possible and alternative explanation of these so-called “anomalous” events. To test this interpretation, we have developed an extension of ZHS, a program to calculate coherent pulses from electromagnetic showers, to deal with showers that transit a planar interface between two homogeneous and dielectric media, including transition radiation. By considering different geometries, it is found that all pulses from air showers intercepting the ice surface and detected at the height of ANITA, display the same polarity as pulses emitted by ultra-high-energy cosmic-ray showers that fully develop in the atmosphere and are reflected on the ice. We find that transition radiation is disfavored as a possible explanation of the anomalous ANITA events.

Inferring Fundamental Properties of the Flare Current Sheet Using Flare Ribbons: Oscillations in the Reconnection Flux Rates

Magnetic reconnection is understood to be the main physical process that facilitates the transformation of magnetic energy into heat, motion, and particle acceleration during solar eruptions. Yet, observational constraints on reconnection region properties and dynamics are limited due to a lack of high-cadence and high-spatial-resolution observations. By studying the evolution and morphology of postreconnected field-lines footpoints, or flare ribbons and vector photospheric magnetic field, we estimate the magnetic reconnection flux and its rate of change with time to study the flare reconnection process and dynamics of the current sheet above. We compare high-resolution imaging data to study the evolution of the fine structure in flare ribbons as ribbons spread away from the polarity inversion line. Using data from two illustrative events (one M- and X-class flare), we explore the relationship between the ribbon-front fine structure and the temporal development of bursts in the reconnection region. Additionally, we use the RibbonDB database to perform statistical analysis of 73 (C- to X-class) flares and identify quasiperiodic pulsation (QPP) properties using the Wavelet Transform. Our main finding is the discovery of QPP signatures in the derived magnetic reconnection rates in both example events and the large flare sample. We find that the oscillation periods range from 1 to 4 minutes. Furthermore, we find nearly cotemporal bursts in Hard X-ray (HXR) emission profiles. We discuss how dynamical processes in the current sheet involving plasmoids can explain the nearly cotemporal signatures of quasiperiodicity in the reconnection rates and HXR emission.

Nonvolatile and reconfigurable two-terminal electro-optic duplex memristor based on III-nitride semiconductors

With the fast development of artificial intelligence (AI), Internet of things (IOT), etc, there is an urgent need for the technology that can efficiently recognize, store and process a staggering amount of information. The AlScN material has unique advantages including immense remnant polarization, superior temperature stability and good lattice-match to other III-nitrides, making it easy to integrate with the existing advanced III-nitrides material and device technologies. However, due to the large band-gap, strong coercive field, and low photo-generated carrier generation and separation efficiency, it is difficult for AlScN itself to accumulate enough photo-generated carriers at the surface/interface to induce polarization inversion, limiting its application in in-memory sensing and computing. In this work, an electro-optic duplex memristor on a GaN/AlScN hetero-structure based Schottky diode has been realized. This two-terminal memristor shows good electrical and opto-electrical nonvolatility and reconfigurability. For both electrical and opto-electrical modes, the current on/off ratio can reach the magnitude of 104, and the resistance states can be effectively reset, written and long-termly stored. Based on this device, the “IMP” truth table and the logic “False” can be successfully reproduced, indicating the huge potential of the device in the field of in-memory sensing and computing.

Survey of Magnetic Field Parameters Associated with Large Solar Flares

Until now, how the magnetic fields in M/X-class flaring active regions (ARs) differ from C-class flaring ARs remains unclear. Here, we calculate the key magnetic field parameters within the area of high photospheric free energy density (HED region) for 323 ARs (217 C- and 106 M/X-flaring ARs), including total photospheric free magnetic energy density E free, total unsigned magnetic flux ΦHED, mean unsigned current helicity h c , length of the polarity inversion lines L PIL with a steep horizontal magnetic gradient, etc., and compare these with flare/coronal mass ejection (CME) properties. We first show the quantitative relations among the flare intensity, the eruptive character, and ΦHED. We reveal that ΦHED is a measure for the GOES flux upper limit of the flares in a given region. For a given ΦHED, there exists the lower limit of F SXR for eruptive flares. This means that only the relatively strong flares with the large fraction of energy release compared to the total free energy are likely to generate a CME. We also find that the combinations of E free–L PIL and E free–h c present a good ability to distinguish between C-class and M/X-class flaring ARs. Using determined critical values of E free and L PIL, one predicts correctly 93 out of 106 M/X-class flaring ARs and 159 out of 217 C-class flaring ARs. The large L PIL or h c for M/X-class flaring ARs probably implies the presence of a compact current with twisted magnetic fields winding about it.

Double-decker Pair of Flux Ropes Formed by Two Successive Tether-cutting Eruptions

Double-decker filaments and their eruptions have been widely observed in recent years, but their physical formation mechanism is still unclear. Using high spatiotemporal resolution, multi-wavelength observations taken by the New Vacuum Solar Telescope and the Solar Dynamics Observatory, we show the formation of a double-decker pair of flux rope system by two successive tether-cutting eruptions in a bipolar active region. Due to the combined effect of photospheric shearing and convergence motions around the active region’s polarity inversion line (PIL), the arms of two overlapping inverse-S-shaped short filaments reconnected at their intersection, which created a simultaneous upward-moving magnetic flux rope (MFR) and a downward-moving post-flare-loop (PFL) system striding the PIL. Meanwhile, four bright flare ribbons appeared at the footpoints of the newly formed MFR and the PFL. As the MFR rose, two elongated flare ribbons connected by a relatively larger PFL appeared on either side of the PIL. After a few minutes, another MFR formed in the same way at the same location and then erupted in the same direction as the first one. Detailed observational results suggest that the eruption of the first MFR might experienced a short pause before its successful eruption, while the second MFR was a failed eruption. This implies that the two newly formed MFRs might reach a new equilibrium at relatively higher heights for a while, which can be regarded as a transient double-decker flux rope system. The observations can well be explained by the tether-cutting model, and we propose that two successive confined tether-cutting eruptions can naturally produce a double-decker flux rope system, especially when the background coronal magnetic field has a saddle-like distribution of magnetic decay index profile in height.

Light Bridges and Solar Active Region Evolution Processes

The formation mechanism of light bridges (LBs) is strongly related to the dynamic evolution of solar active regions (ARs). To study the relationship between LB formation and AR evolution phases, we employ 109 LB samples from 69 ARs in 2014 using observational data from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. LBs are well matched with the weak field lanes (WFLs), except those aligned on the polarity inversion line of δ sunspots. For penumbral intrusion (type-A) and umbral-dot emergence (type-C) LBs, the WFLs represent the splitting of magnetic flux systems. The sunspots tend to decay and split into several parts after type-A and type-C LBs are formed. For sunspot/umbra-merging (type-B) LBs, the declining WFLs are caused by collisions of flux systems. The sunspots merged and remained stable after type-B LBs formed. We conclude that type-B LBs are formed by collisions of flux systems, while type-A and type-C LBs are generated by splits. The time differences (δ T) between LBs appearing and ARs peaking have an average value of 1.06, −1.60, and 1.82 days for type-A, B, and C LBs, with the standard deviations of 3.27, 2.17, and 1.89, respectively. A positive value of δ T means that the LB appears after the AR peaks, whereas a negative δ T means it appears before the peak. Type-A LBs tend to form in the decaying phase or around the peak time. Type-B LBs are more likely to be formed in the developing phase. Type-C LBs mostly take shape in the decaying phase of ARs.

Using relative field line helicity as an indicator for solar eruptivity

Context. Relative field line helicity (RFLH) is a recently developed quantity that can approximate the density of relative magnetic helicity. Aims. This paper aims to determine whether RFLH can be used as an indicator of solar eruptivity. Methods. Starting from magnetographic observations from the Helioseismic and Magnetic Imager instrument on board the Solar Dynamic Observatory of a sample of seven solar active regions (ARs), that comprises over 2000 individual snapshots, we reconstruct the AR’s coronal magnetic field with a widely used non-linear force-free method. This enables us to compute the RFLH using two independent gauge conditions for the vector potentials. We focus our study on the times of strong flares in the ARs, above the M class, and in regions around the polarity inversion lines (PILs) of the magnetic field, and of RFLH. Results. We find that the temporal profiles of the relative helicity that is contained in the magnetic PIL follow those of the relative helicity that is computed by the accurate volume method for the whole AR. Additionally, the PIL relative helicity can be used to define a parameter similar to the well-known parameter R, whose high values are related with increased flaring probability. This helicity-based R-parameter correlates closely with the original parameter, showing in some cases even higher values. Additionally, it experiences more pronounced decreases during flares. This means that there exists at least one parameter deduced from RFLH that is important as a solar eruptivity indicator.

Insights on the pulsed-DC powder-pack boriding process: The role of the electric charge on the growth of the boride layer and the semiconductor behavior of the boriding media

In pursuit of innovating the conventional powder-pack boriding (CPB) process by introducing an electrical driving force, novel findings regarding the kinetics growth of boride layers produced on an AISI 8620 steel through the pulsed-DC powder-pack boriding process (PDCPB) were obtained. The PDCPB accelerates B diffusion on the surface of the specimen, resulting in an augmentation of the growth kinetics around 12 % and 15 % for FeB and Fe2B, respectively, compared to those observed in the CPB process. Additionally, the effect of polarity inversion half-cycles on the growth of the FeB-Fe2B layer during the PDCPB was studied. The experiments were conducted at temperatures of 850 °C, 900 °C, and 950 °C, with different exposure times, and polarity inversion half-cycles of 30 s and 50 s, maintaining a constant current density of around 177 mA cm−2. An estimation of the electric charge (q) using a transfer function considering minimal and maximum temperatures was achieved. It was confirmed that, with a symmetrical inversion, q was similar for both 30 s and 50 s polarity inversion half-cycles, indicating no influence on the growth kinetics of the boride layers.

A model of solar magnetic flux rope eruption initiated primarily by magnetic reconnection

ABSTRACT There is a heated debate regarding the specific roles played by ideal magnetohydrodynamic (MHD) instability and magnetic reconnection in triggering solar eruptions. In the context of a pre-existing magnetic flux rope (MFR) before an eruption, it is widely believed that an ideal MHD instability, in particular, the torus instability, is responsible for triggering and driving the eruption, while reconnection, as invoked in the wake of the erupting MFR, plays a secondary role. Here, we present a new numerical MHD model in which the eruption of a pre-existing MFR is primarily triggered and driven by reconnection. In this model, a stable MFR embedded in a strapping field is set as the initial condition. A surface converging flow is then applied at the lower boundary, pushing magnetic flux towards the main polarity inversion line. It drives a quasi-static evolution of the system, during which a current layer is built up below the MFR with decreasing thickness. Once reconnection starts in the current sheet, the eruption commences, which indicates that the reconnection plays a determining role in triggering the eruption. By further analysing the works done by the magnetic flux of the pre-existing MFR and the newly reconnected flux during the acceleration stage of the eruption, we find that the latter plays a major role in driving the eruption. Such a model may explain observed eruptions in which the pre-eruption MFR has not reached the conditions for ideal instability.

Demonstration and STEM Analysis of Ferroelectric Switching in MOCVD‐Grown Single Crystalline Al0.85Sc0.15N

AbstractWurtzite‐type Al1−xScxN solid solutions grown by metal organic chemical vapor deposition are for the first time confirmed to be ferroelectric. The film with 230 nm thickness and x = 0.15 exhibits a coercive field of 5.5 MV cm−1 at a measurement frequency of 1.5 kHz. The single crystal quality and homogeneous chemical composition of the film are confirmed by X‐ray diffraction and spectroscopic methods such as time of flight secondary ion mass spectrometry. Annular bright field scanning transmission electron microscopy serves to prove the ferroelectric polarization inversion at the unit cell level. The single crystal quality further allows to image the large‐scale domain pattern of a wurtzite‐type ferroelectric for the first time, revealing a predominantly cone‐like domain shape along the c‐axis of the material. As in previous work, this again implies the presence of strong polarization discontinuities along this crystallographic axis, which can be suitable for current transport. The domains are separated by narrow domain walls, for which an upper thickness limit of 3 nm is deduced but which can potentially be atomically sharp. The authors are confident that these results will advance the commencement of the integration of wurtzite‐type ferroelectrics to GaN as well as generally III‐N‐based heterostructures and devices.

Changes in Photospheric Lorentz Force in Eruptive and Confined Solar Flares

Solar flares are known to leave imprints on the magnetic field in the photosphere, often manifested as an abrupt and permanent change in the downward-directed Lorentz force in localized areas inside the active region. Our study aims to differentiate eruptive and confined solar flares based on the variations in vertical Lorentz force. We select 26 eruptive and 11 confined major solar flares (stronger than the GOES M5 class) observed during 2011–2017. We analyze these flaring regions using SHARP vector magnetograms obtained from NASA’s Helioseismic and Magnetic Imager. We also compare data corresponding to two synthetic flares from a δ-sunspot simulation reported by Chatterjee et al. We estimate the change in the horizontal magnetic field and the total Lorentz force integrated over an area around the polarity inversion line (PIL) that encompasses the location of the flare. Our results indicate a rapid increase in the horizontal magnetic field along the flaring PIL, accompanied by a significant change in the downward-directed Lorentz force in the same vicinity. Notably, we find that all the confined events under study exhibit a total change in Lorentz force of <1.8 × 1022 dyn. This threshold plays an important role in effectively distinguishing eruptive and confined flares. Further, our analysis suggests that the change in total Lorentz force also depends on the reconnection height in the solar corona at the associated flare onset. The results provide significant implications for understanding the flare-related upward impulse transmission for the associated coronal mass ejection.

Atroposelective Synthesis of 2‐Arylindoles via Chiral Phosphoric Acid‐Catalyzed Direct Amination of Indoles†

Comprehensive SummaryIndole‐based atropisomers are a very important class of axially chiral compounds. However, the atroposelective synthesis of axially chiral 2‐arylindole remains largely unexplored. In this study, we report the successful synthesis of atropisomeric 2‐arylindoles using direct amination of indoles with p‐quinonediimines in the presence of chiral phosphoric acid as a catalyst. Quinonediimine acts as an aminating reagent through formal polarity inversion of imine. The malonate group on the 2‐aryl of 2‐indoles was found to be essential for high enantioselectivity of the products. This could be due to the additional interaction between the ester group and the catalyst, as well as the intramolecular hydrogen bonding. Our findings provide a new strategy for the asymmetric construction of 2‐arylindole atropisomers.

Sunspots Identification Through Mathematical Morphology

The implementation of automated methods for sunspot detection is essential to obtain better objectivity, efficiency, and accuracy in identifying sunspots and analysing their morphological properties. A desired application is the contouring of sunspots. In this work, we construct sunspot contours from Solar Dynamics Observatory (SDO)/ Helioseismic and Magnetic Imager intensity images by means of an automated method based on development and application of mathematical morphology. The method is validated qualitatively – the resulting contours accurately delimit sunspots. Here, it is applied to high-resolution data (SDO intensitygrams) and validated quantitatively by illustrating a good agreement between the measured sunspot areas and the ones provided by two standard reference catalogues. The method appears to be robust for sunspot identification, and our analysis suggests its application to more complex and irregular-shaped solar structures, such as polarity inversion lines inside delta-sunspots.

4]Helicenium Ion as Bipolar Redox Material for Symmetrical Fully Organic Pole‐less Redox Flow Battery

AbstractLong duration storage batteries such as Redox Flow Batteries (RFBs) are promising storage system to address the energy storage requirement that our society will require in the years to come. Recent effort has been focused on the development of metal free and high energy density system such as all‐organic non‐aqueous redox flow batteries (NAORFBs). However high‐voltage NAORFBs currently use distinct anolytes and catholytes, which are separated by a membrane sensitive to osmotic pressure, resulting in rapid capacity and energy density degradation over time. To address this issue, symmetric organic redox flow batteries (SORFBs) have been proposed as an elegant solution. We have introduced dimethoxyquinacridiniums (DMQA+) ions as efficient bipolar redox molecules (BRMs) in static H‐cell conditions. In this study, we present the first application of DMQA+ ions in a complete flow RFB prototype, showcasing their ability to operate with polarity reversal. Key kinetic properties were evaluated through cyclic voltammetry and DFT calculations. While coulombic and energy efficiency metrics were moderate, pegylated DMQA+ demonstrated impressive capacity retention of over 99.99 % and the capability to operate under polarity inversion, making it a highly attractive choice for grid‐scale, long‐lifespan energy storage applications.