Polar Field Reversal Research Articles

H-AMR FORGE’d in FIRE. I. Magnetic State Transitions, Jet Launching, and Radiative Emission in Super-Eddington, Highly Magnetized Quasar Disks Formed from Cosmological Initial Conditions

Abstract Quasars are powered by supermassive black hole (SMBH) accretion disks, yet standard thin disk models are inconsistent with many observations. Recently, P. F. Hopkins et al. simulated the formation of a quasar disk feeding an SMBH of mass M = 1.3 × 107 M ⊙ in a galaxy. The disk had surprisingly strong toroidal magnetic fields that supported it vertically from gravity and powered rapid accretion. What feedback can such a system produce? To answer this, we must follow the gas to the event horizon. For this, we interpolated the quasar into the general-relativistic radiation magnetohydrodynamics code H-AMR and performed 3D simulations with BH spins a = 0 and a = 0.9375. This remapping generates magnetic monopoles, which we erase using a novel divergence cleaning approach. Despite the toroidal magnetic field's dominance at large radii, vertical magnetic flux builds up near the event horizon, leading to a magnetic state transition within the inner 200 gravitational radii of the disk. This powers strong winds and, for spinning BHs, relativistic jets that can spin down the BH within 5−10 Myr. Sometimes, vertical magnetic fields of opposite polarity reach the BH, causing a polarity inversion event that briefly destroys the jets and, possibly, the X-ray corona. These strong fields power accretion at rates 5× the Eddington limit, which can double the BH mass in 5–10 Myr. When a = 0.9375 (a = 0), the energy in mechanical outflows and radiation equals about 60% (10%) and 100% (3%) of the accreted rest mass energy, respectively. Much of the light escapes in cool, ≳1300 au photospheres, consistent with quasar microlensing and spectral energy distributions.

THE INTERSUBBAND OPTICAL ABSORPTION COEFFICIENT OF THE QD WITH ACCEPTOR IMPURITY UNDER APPLIED ELECTRIC FIELD

In this work, a spherical quantum dot (QD) under the influence of an external electric field is investigated. A multi-band model of the valence band is applied. The influence of off-center acceptor impurity, electric field and QD size dispersion on the absorption coefficient during intersubband transitions between hole states has been analyzed. The results show that an electric field combined with an off-center impurity induces the appearance of two distinct absorption bands corresponding to different magnetic quantum numbers. The intensity of absorption bands depends on the direction and strength of the electric field, and significant differences are observed between fields of opposite polarity. It is important to note that there is a critical field strength that restores the degeneracy of the energy levels, narrowing the broadband absorption tail for systems with small or large dispersion sizes. This research aims to improve the understanding and optimization of the optical properties of nanomaterials.

A multi-instrument study of UV bursts and associated surges in AR 12957

The relationship between UV bursts and solar surges is complex, with these events sometimes being observed together and sometimes being observed independently. Why this sporadic association exists is unknown; however, it likely relates to the physical conditions at the site of the energy release that drives these events. Here, we aim to better understand the relationship between UV bursts and solar surges through a multi-instrument analysis of several associated events that occurred around the trailing sunspot in AR $12957$. We used data from Solar Orbiter, the Solar Dynamics Observatory (SDO), and the Interface Region Imaging Spectrograph (IRIS) to achieve our aims. These data were sampled on 3 March 2022 between $09$:$30$:$30$ UT and $11$:$00$:$00$ UT, during which time a coordinated observing campaign associated with the Slow Solar Wind Connection Solar Orbiter Observing Plan (SOOP) took place. Numerous small-scale negative polarity magnetic magnetic features (MMFs) were observed to move quickly (potentially up to $3.3$ km s$^ $) away from a sunspot until they collided with a more stable positive polarity plage region around $7$ Mm away. Several UV bursts were identified in IRIS slit-jaw imager (SJI) $1400$ data co-spatial to where these opposite polarity fields interacted, with spatial scales ($2$ Mm$<$) and lifetimes ($20<$ minutes) larger than typical values for such events. Two surges were also observed to occur at these locations, with one being short ($5$ Mm) and hot (bright in the IRIS SJI images), whilst the other was a cooler (dark in coronal imaging channels), longer surge that appeared to fill an active region loop. Magnetic reconnection between the negative polarity MMFs around the sunspot and the positive polarity plage region appears to be the driver of these events. Both the speed of the MMFs and the locally open magnetic topology of the plage region could possibly have been important for forming the surges.

Hemispheric analysis of the magnetic flux in regular and irregular solar active regions

ABSTRACT Studying the hemispheric distribution of active regions (ARs) with different magnetic morphologies may clarify the features of the dynamo process that is hidden under the photospheric level. The magnetic flux data for 3047 ARs from the CrAO catalogue (https://sun.crao.ru/databases/catalog-mmc-ars), between May 1996 and December 2021 (cycles 23 and 24) were used to study ARs cyclic variations and perform correlation analysis. According to the magneto-morphological classification (MMC) of ARs proposed earlier, subsets of the regular (obeying empirical rules for sunspots) and irregular (violating these rules) ARs were considered separately. Our analysis shows the following: For ARs of each MMC type, in each of the hemispheres, time profiles demonstrate a multipeak structure. The double-peak structure of a cycle is formed by ARs of both MMC types in both hemispheres. For the irregular ARs, the pronounced peaks occur in the second maxima (close to the polar field reversal). Their significant hemispheric imbalance might be caused by a weakening of the toroidal field in one of the hemispheres due to the interaction between the dipolar and quadrupolar components of the global field, which facilitates the manifestation of the turbulent component of the dynamo. The similarity of the irregular ARs activity that was found in adjacent cycles in different hemispheres also hints at realization of the mix-parity dynamo solution. For the quadrupolar-like component of the flux (compiled in the simple axisymmetric approximation), signs of oscillations with a period of about 15 years are found, and they are pronounced specifically for the irregular groups. This MMC type ARs might also contribute in $\alpha$-quenching.

Analysis of BMR Tilt from AutoTAB Catalog: Hinting toward the Thin Flux Tube Model?

One of the intriguing mechanisms of the Sun is the formation of bipolar magnetic regions (BMRs) in the solar convection zone (CZ), which are observed as regions of concentrated magnetic fields of opposite polarity on the photosphere. These BMRs are tilted with respect to the equatorial line, which statistically increases with latitude. The thin flux tube model, employing the rise of magnetically buoyant flux loops and their twist by Coriolis force, is a popular paradigm for explaining the formation of tilted BMRs. In this study, we assess the validity of the thin flux tube model by analyzing the tracked BMR data obtained through the Automatic Tracking Algorithm for BMRs. Our observations reveal that the tracked BMRs exhibit the expected collective behaviors. We find that the polarity separation of BMRs increases over their normalized lifetime, supporting the assumption of a rising flux tube from the CZ. Moreover, we observe an increasing trend of the tilt with the flux of the BMR, suggesting that rising flux tubes associated with lower flux regions are primarily influenced by drag force and Coriolis force, while in higher flux regions, magnetic buoyancy dominates. Furthermore, we observe Joy’s law dependence for emerging BMRs from their first detection, indicating that at least a portion of the tilt observed in BMRs can be attributed to the Coriolis force. Notably, lower flux regions exhibit a higher amount of fluctuations associated with their tilt measurement compared to stronger flux regions, suggesting that lower flux regions are more susceptible to turbulent convection.

The Polar Field Reversal Process over Five Solar Cycles

The Polar Field Reversal Process over Five Solar Cycles

Direct In Situ Measurements of a Fast Coronal Mass Ejection and Associated Structures in the Corona

We report on the first direct in situ measurements of a fast coronal mass ejection (CME) and shock in the corona, which occurred on 2022 September 5. In situ measurements from the Parker Solar Probe spacecraft near perihelion suggest two shocks, with the second one decayed, which is consistent with more than one eruption in coronagraph images. Despite a flank crossing, the measurements indicate unique features of the young ejecta: a plasma much hotter than the ambient medium, suggestive of a hot solar source, and a large plasma β implying a highly non-force-free state and the importance of thermal pressure gradient for CME acceleration and expansion. Reconstruction of the global coronal magnetic fields shows a long-duration change in the heliospheric current sheet (HCS), and the observed field polarity reversals agree with a more warped HCS configuration. Reconnection signatures are observed inside an HCS crossing as deep as the sonic critical point. As the reconnection occurs in the sub-Alfvénic wind, the reconnected flux sunward of the reconnection site can close back to the Sun, which helps balance magnetic flux in the heliosphere. The nature of the sub-Alfvénic wind after the HCS crossing as a low Mach-number boundary layer (LMBL) leads to in situ measurements of the near subsonic plasma at a surprisingly large distance. Specifically, an LMBL may provide favorable conditions for the crossings of the sonic critical point in addition to the Alfvén surface.

Generation of a super-oscillation magnetization hotspot by reversing the electric dipole array radiation

We propose an all-optical approach to realize a super-oscillatory longitudinal magnetization hotspot with about 0.1λ in a 4π focusing system using time-reversed dipole radiation and vector diffraction theory calculations. Such a magnetization behavior is induced by focal fields of opposite polarities in the central and peripheral regions based on the inverse Faraday effect. By regulating the destructive interference in the central and peripheral regions of the focal field, a super-oscillating magnetization field is generated. Moreover, two important parameters affect the size of the magnetization field are also explored. Superoscillating magnetization hotspots in the focal region can be achieved by either increasing δ (the intensity of electric field) or reducing dn (the distance of electric field). The achieved ultrasmall magnetization spot shows great prospects in high-density magnetic recording, magnetic resonance imaging, and magnetic sensors.

Piezoelectric PVDF‐TrFE nanocomposite mats filled with BaTiO3 nanofibers: The effect of poling conditions

AbstractBaTiO3 nanofibers (BT NFs), prepared by electrospinning, were used as a filler for electrospun poly(vinylidene fluoride‐co‐trifluoroethylene) (PVDF‐TrFE) nanocomposite mats. The phase structure and the effect of poling conditions on the piezoelectric properties of PVDF‐TrFE/BT nanocomposites were investigated. The results showed an improved degree of crystallinity (78.6%) and a high β‐crystal phase (up to 98.3%) in all electrospun samples, independent of the nanofiber content. The two‐step poling method, applying electric fields of opposite polarity, led to significantly improved piezoelectric constants d33 (−31.7 pC N−1), strongly dependent on the added BaTiO3 nanofibers. The inclusion of piezoelectric ceramic nanofibers into a polymer matrix, easily carried out by means of electrospinning, followed by an ad hoc optimized poling treatment, allowed to develop flexible materials with enhanced piezoelectric properties, potentially exploitable in innovative conversion systems used in wearable and sensing devices.

Predicting the Timing of the Solar Cycle 25 Polar Field Reversal

The process of the Sun’s polar field cancellation reversal commences with the emergence of new cycle Hale’s polarity active regions. Once the Sun undergoes polarity reversal, typically occurring near the peak of solar activity, it begins the process of accumulating the seed field for the forthcoming solar cycle. In recent years, the advective flux transport (AFT) model has proven highly effective in forecasting the progression of polar fields by leveraging observations of surface flows and magnetic flux emergence. In this study, we make use of the predictive capability of the AFT model to simulate the evolution of the polar fields and estimate the timing of the Solar Cycle 25 polarity reversal in both hemispheres of the Sun. We use the statistical properties of active regions along with Solar Cycle 13, which closely resembles the current solar cycle (Cycle 25), to generate synthetic active regions in order to simulate future magnetic flux emergence in AFT to predict the evolution of the polar field. Based on our simulations, we anticipate that the northern hemisphere of the Sun will undergo a polarity reversal between 2024 June and November, with the center of our distribution at 2024 August. In the southern hemisphere, we anticipate a polarity reversal between 2024 November and 2025 July, centered around 2025 February. Additionally, assuming that the reversal of the axial dipole moment coincides with the peak of the solar cycle, our findings indicate that Cycle 25 is expected to peak in 2024 (likely between 2024 April and August).

Global solar photospheric and coronal magnetic field over activity cycles 21–25

The evolution of the global solar magnetic field from the beginning of cycle 21 (mid-1970s) until the currently-ascending cycle 25 is described using photospheric full-disk and synoptic magnetograms from NSO Kitt Peak Vacuum Telescope (KPVT) 512-channel and Spectromagnetograph (SPMG) and the Synoptic Optical Long-term Investigation of the Sun (SOLIS) Vector Spectro-Magnetograph (VSM) and Global Oscillations Network Group (GONG), and Stanford University’s Wilcox Solar Observatory (WSO). The evolving strength and symmetry of the global coronal field are described by potential-field source-surface models decomposed into axisymmetric and non-axisymmetric, and even- and odd-ordered magnetic multipoles. The overall weakness of the global solar magnetic field since cycle 23 splits the 50-year observing window into the stronger, simpler, more hemispherically symmetric cycles 21 and 22 and the weaker, more complex cycles 23 and 24. An anomalously large decrease in the global solar field strength occurred during cycles 23, and an anomalously weak axial/polar field resulted from that cycle, accompanied by an anomalously weak radial interplanetary magnetic field (IMF) during cycle 23 activity minimum and a weakened radial IMF overall since cycle 23. The general long-term decline in solar field strength and the development during cycle 24 of strong swings of hemispheric and polar asymmetry are analyzed in detail, including their transfer through global coronal structural changes to dominate mean in situ interplanetary field measurements for several years. Although more symmetric than cycle 24, the rise phase of cycle 25 began with the southern leading the northern hemisphere, but the north has recovered to lead this cycle’s polar field reversal. The mean polar flux (poleward of ±60°) has reversed at each pole, so far more symmetrically than the cycle 23 and 24 polar reversals.

Dynamics of small-scale magnetic fields before small and large solar flares

We have examined the dynamics of the longitudinal magnetic field of active region NOAA 12673, using data from the Solar Dynamics Observatory (SDO). During the passage of the active region (AR) across the solar disk, its spots and background fields showed complex motion trajectories, and numerous small-scale short-lived local polarity inversion lines (LPILs) were formed when new magnetic fluxes appeared in the AR and came closer to fields of opposite polarity. The length of LPILs was less than 15.000 km (~20 arcsec); their lifetime was several hours. Study of the flare activity of NOAA 12673 has shown that low-power flares (optical class S, area ˂2 sq. degrees) generally occur near LPILs. Before small flares and the September 06, 2017 large flare (optical importance 3B, X-ray class X9.3), in limited sites of local and main polarity inversion lines there were shear stresses and an increase in the magnetic field gradient: in the region of low-power flares, to 1.3–1.5 G/km; in the region of the large flare, 3–3.5 G/km. The results obtained suggest that the longitudinal magnetic field behaves similarly before both small and large flares.

Динамика мелкомасштабных магнитных полей перед малыми и крупными солнечными вспышками

We have examined the dynamics of the longitudinal magnetic field of active region NOAA 12673, using data from the Solar Dynamics Observatory (SDO). During the passage of the active region (AR) across the solar disk, its spots and background fields showed complex motion trajectories, and numerous small-scale short-lived local polarity inversion lines (LPILs) were formed when new magnetic fluxes appeared in the AR and came closer to fields of opposite polarity. The length of LPILs was less than 15.000 km (~20 arcsec); their lifetime was several hours. Study of the flare activity of NOAA 12673 has shown that low-power flares (optical class S, area ˂2 sq. degrees) generally occur near LPILs. Before small flares and the September 06, 2017 large flare (optical importance 3B, X-ray class X9.3), in limited sites of local and main polarity inversion lines there were shear stresses and an increase in the magnetic field gradient: in the region of low-power flares, to 1.3–1.5 G/km; in the region of the large flare, 3–3.5 G/km. The results obtained suggest that the longitudinal magnetic field behaves similarly before both small and large flares.

The influence of small bipolar magnetic regions on basic solar quantities

Understanding the evolution of the solar magnetic field is of great importance for heliosphere, dynamo, and irradiance studies, for example. While the contribution of the field in active regions (ARs) hosting sunspots to the Sun's large-scale field has been extensively modelled, we still lack a realistic model of the contribution of smaller-scale magnetic regions such as ephemeral regions which do not contain any sunspots. For this work, we studied the effect of small and large bipolar magnetic regions (BMRs) on the large-scale solar magnetic field. The evolution of the total and open magnetic flux, the polar fields, and the toroidal flux loss since 1874 has been simulated with a surface flux transport model (SFTM) and the results were compared to analytical considerations and observational data. For this purpose, we constructed semi-synthetic BMR records using the international sunspot number as a proxy. We calculated the emergence rate of all BMRs from a single power-law size distribution, whose exponent varies with solar activity. The spatial distribution of the BMRs was calculated from statistical relationships derived from various solar observations. We included BMRs with a magnetic flux as low as $2 $ Mx in our SFTM, corresponding to regions with lifetimes down to one day. We found a good agreement between the computed total magnetic flux and observations, even though we do not have a free parameter to adjust the simulated total flux to observations, as in earlier versions of the employed SFTM. The open flux, the polar fields, and the toroidal flux loss are also consistent with observations and independent reconstructions. In our model, small BMRs contribute about one-third of the total and open flux at activity maximum, while their contribution increases to roughly half at activity minimum. An even greater impact is found on the polar fields and the toroidal flux loss, for which the contribution of small BMRs is comparable to that of spot-containing ARs at all activity levels. Even so, smaller regions, not included in our simulations, do not seem to play a significant role due to their high tilt angle scatter. Our simulation results suggest that most of the statistical noise is caused by large ARs, while small BMRs have a stabilising effect on the magnetic flux evolution, especially for the polar field reversals. We conclude that small BMRs (here, with magnetic fluxes between $2 $ Mx and $3 $ Mx) may also play an important role in the evolution of the solar magnetic field at large spatial scales. Their impact is largest at low solar activity, but it is also substantial during activity maxima, although the actual relative contributions by small and large regions depend on the steepness of their emergence rate distribution. The inclusion of small BMRs in SFTM simulations will allow the secular variability in solar irradiance to be better constrained and the generation of the poloidal field in the Babcock-Leighton dynamo to be better understood.

Probing the variations in the timing of the Sun’s polar magnetic field reversals through observations and surface flux transport simulations

ABSTRACT The polar field reversal is a crucial process in the cyclic evolution of the large-scale magnetic field of the Sun. Various important characteristics of a solar cycle, such as its duration and strength, and also the cycle predictability, are determined by the polar field reversal time. While the regular measurements of solar magnetic field have been accumulated for more than half a century, there is no consensus in the heliophysics community concerning the interpretation of the Sun’s polar field measurements and especially the determination of polar field reversal time. There exists a severe problem of non-reproducibility in the reported results even from studies of the same observational data set, and this causes an obstacle to make more accurate forecasts of solar cycle. Here, we analyze the solar magnetograms from four instruments for the last four cycles, to provide a more correct interpretation of the polar field observations and to find more accurate time of the reversals. We show the absence of triple (multiple) reversals in Cycles 21–24, significant variations in the time interval between reversals in the hemispheres and in the time interval between a reversal and a cycle beginning. In order to understand the origin of the reversal time variation, we perform Surface Flux Transport (SFT) simulations and find out that the presence of the ‘anomalous’ bipolar magnetic regions (BMRs) in different phases of a cycle can cause cycle-to-cycle variations of the reversal time within the similar range found in observations.

Enhanced magnetic modulation at a border of magnetic ordering in La1−xSrxMnO3/BaTiO3(100) heterostructure

In La1−xSrxMnO3 (LSMO)/BaTiO3 (BTO) heterostructures with a multiferroic interface, an artificial modulation of the magnetic structure is observed. The saturation magnetization of La1−xSrxMnO3 changes discontinuously due to in-plane distortions caused by a structural phase transition of a BaTiO3 substrate. Polarity reversal of the external electric field also causes a reversible switching in the magnetization. The magnitude of both magnetic modulations, due to the magnetoelastic and electric field effects, is concomitantly enhanced at a critical composition xc∼0.55, locating at a border of the magnetic phase transition. The polarity-dependent change in magnetization is possibly attributed to a change in the concentration of oxygen ions at the LSMO/BTO interface, indicating that the exchange interaction is reciprocally driven from being ferromagnetic to antiferromagnetic by the electric field polarity.

Multicomponent Activity Cycles Using Hilbert–Huang Analysis

The temporal analysis of stellar activity evolution is usually dominated by a complex trade-off between model complexity and interpretability, often by neglecting the nonstationary nature of the process. Recent studies appear to indicate that the presence of multiple coexisting cycles in a single star is more common than previously thought. The correct identification of physically meaningful cyclic components in spectroscopic time series is therefore a crucial task, which cannot overlook local behaviors. Here we propose a decomposition technique that adaptively recovers amplitude- and frequency-varying components. We present our results for the solar activity as measured both by the sunspot number and the K-line emission index, and we consistently recover the Schwabe and Gleissberg cycles as well as the Gnevyshev–Ohl pattern probably related to the Hale cycle. We also recover the known 8 yr cycle for 61 Cygni A, in addition to evidence of a three-cycles-long pattern reminiscent of the Gnevyshev–Ohl rule. This is particularly interesting as we cannot discard the possibility of a relationship between the measured field polarity reversals and this Hale-like periodicity.

Differential two-wave mixing interferometer for crack detection in metallic structures based on laser-induced ultrasound

To meet the need of high-sensitivity crack detection in metallic structures with a contactless method, a differential two-wave mixing interferometer based on two nonlinear Bi12SiO20 photorefractive crystals is proposed for laser-induced ultrasound signal measurement. The two Bi12SiO20 crystals are applied with the electric fields of opposite polarities to obtain the dual-path differential signals corresponding to the laser-induced ultrasound under detection, in which the noise is not affected by the polarity of the applied electric field. The signal noise can be effectively suppressed and the sensitivity is doubled by the differential processing of the dual-path signals, by which the high-sensitivity detection of laser-induced ultrasound can be achieved to characterize the crack in metallic structures. The further optimization of system parameters is performed to obtain the optimal detection sensitivity. Both the numerical analysis and experiments have been carried out to demonstrate the feasibility of the proposed method. The proposed system provides a powerful and reliable tool for contactless detection of laser-induced ultrasound and characterization of metal cracks.

Numerical simulations of the laser-driven Petschek-type magnetic reconnection

This paper describes a numerical study of the magnetic reconnection between two magnetic fields of opposite polarity. The magnetic fields are created by an electric current in a coil connected to two metal disks. One of the disks is irradiated by a strong laser beam, whereby large amounts of free electrons flow toward the other disk, constituting a closed circuit for the electric current flowing through the coil. Two parallel coils are arranged to connect the two disks, and irradiation of the laser beam on one disk results in parallel electric currents in the two coils, inducing two magnetic fields of opposite polarity in the region between them. The magnetic reconnection that occurs in this region is three-dimensional. This three-dimensional magnetic reconnection is investigated via magnetohydrodynamic numerical simulations. The characteristics of the Petschek-type magnetic reconnection are observed for the first time in such numerical simulations of magnetic reconnection. Changes in the shape of the magnetic field lines form the boundary of the dissipation region and the outflow region. Moreover, the thermal plasma generated by reconnection is strongly confined to the region where the reconnecting current sheet and the slow-mode shock are located, and no leaks of thermal plasma are observed. Comparisons with existing laboratory experiment results confirm that our numerical simulations reproduce the experimental outcomes and provide reasonable explanations for the results observed in laboratories.

Deciphering Solar Magnetic Activity: The Solar Cycle Clock

The Sun’s variability is controlled by the progression and interaction of the magnetized systems that form the 22-year magnetic activity cycle (the “Hale Cycle”) as they march from their origin at ∼55° latitude to the equator, over ∼19 years. We will discuss the end point of that progression, dubbed “terminator” events, and our means of diagnosing them. In this paper we expand on the Extended Solar Cycle framework to construct a new solar activity “clock” which maps all solar magnetic activity onto a single normalized epoch based on the terminations of Hale Magnetic Cycles. Defining phase 0*2π on this clock as the Terminators, then solar polar field reversals occur at ∼ 0.2*2π, and the geomagnetically quiet intervals centered around solar minimum start at ∼ 0.6*2π and end at the terminator, thus lasting 40% of the cycle length. At this onset of quiescence, dubbed a “pre-terminator,” the Sun shows a radical reduction in active region complexity and, like the terminator events, is associated with the time when the solar radio flux crosses F10.7 = 90 sfu. We use the terminator-based clock to illustrate a range of phenomena that further emphasize the strong interaction of the global-scale magnetic systems of the Hale Cycle: the vast majority, 96%, of all X-flares happen between the Terminator and pre-Terminator. In addition to the X-rays from violent flares, rapid changes in the number of energetic photons—EUV spectral emission from a hot corona and the F10.7 solar radio flux—impinging on the atmosphere are predictable from the Terminator-normalized unit cycle, which has implications for improving the fidelity of atmospheric modelling.