Poloidal Component Of Magnetic Field Research Articles

Poloidal magnetic field in the dense plasma focus

Existence of an axial (poloidal) component of magnetic field in the dense plasma focus has been inferred using multiple diagnostics in many laboratories since 1979. It has not received much attention because its origin as well as role in plasma focus physics was unclear till recently. Recent discovery of long-lasting neutron emission perpendicular to the axis in PF-1000 and neutron fluence ratio (end/side) less than unity in Gemini shows that azimuthally accelerated and radially confined deuterons play an observable role in fusion reactions. A spontaneously generated poloidal magnetic field can provide both the azimuthal electric field necessary for acceleration and radial confinement of the ions being accelerated in the acceleration zone. A comprehensive survey of plasma focus research also confirms the role of spontaneously self-organized plasma objects in the fusion reaction process where their three-dimensional magnetic field structure provides a mechanism for accelerating and trapping ions making them repeatedly pass through a dense plasma target. With emerging appreciation of the likely role of the axial magnetic field in plasma focus neutron emission, it becomes imperative to consider models for its origin. This Letter proposes a partial theory of growth of the axial (poloidal) magnetic field via a simple dynamo, with the geomagnetic field as the seed, which converts the kinetic energy of the plasma into energy of the poloidal magnetic field. This theory leads to an experimentally testable proposition.

Linking the interiors and surfaces of magnetic stars

ABSTRACTStrong magnetic fields are observed in a substantial fraction of upper main sequence stars and white dwarfs. Many such stars are observed to exhibit photometric modulations as the magnetic poles rotate in and out of view, which could be a consequence of magnetic perturbations to the star’s thermal structure. The magnetic pressure is typically larger than the gas pressure at the star’s photosphere, but much smaller than the gas pressure in the star’s interior, so the expected surface flux perturbations are not clear. We compute magnetically perturbed stellar structures of young $3 \, \mathrm{M}_\odot$ stars that are in both hydrostatic and thermal equilibrium, and which contain both poloidal and toroidal components of a dipolar magnetic field as expected for stable fossil fields. This provides semi-analytical models of such fields in baroclinic stably stratified regions. The star’s internal pressure, temperature, and flux perturbations can have a range of magnitudes, though we argue the most likely configurations exhibit flux perturbations much smaller than the ratio of surface magnetic pressure to surface gas pressure, but much larger than the ratio of surface magnetic pressure to central gas pressure. The magnetic pole is hotter than the equator in our models, but a cooler magnetic pole is possible depending on the magnetic field configuration. The expected flux variations for observed field strengths are δL/L ≲ 10−6, much smaller than those observed in magnetic stars, suggesting that observed perturbations stem from changes to the emergent spectrum rather than changes to the bolometric flux.

Emission of poloidal magnetic flux from a dense plasma focus

A recent comprehensive review paper has summarized the current experimental state of knowledge on the question of existence of a poloidal component of magnetic field in the dense plasma focus and indirect evidence indicating its prominent role in neuron emission. This Letter looks at the role of the plasma as a curved, azimuthally continuous conductor moving in a small but nonzero geomagnetic field with a component along the device axis. The motional electric field then has an azimuthal component arising from the radial velocity, outward-directed near the cathode throughout the discharge, and inward directed in the radial implosion phase near the anode. This would drive an azimuthal current in the plasma. The electromechanical work done against the associated magnetic force density converts the kinetic energy into the associated magnetic energy density. This would act as a source of poloidal magnetic flux emission from a dense plasma focus, which has been experimentally demonstrated recently.

Writhed Analytical Magnetic Flux Rope Model.

Observations of magnetic clouds, within interplanetary coronal mass ejections (ICMEs), are often well described by flux rope models. Most of these assume either a cylindrical or toroidal geometry. In some cases, these models are also capable of accounting for non-axisymmetric cross-sections but they generally all assume axial invariance. It can be expected that any ICME, and its flux rope, will be deformed along its axis due to influences such as the solar wind. In this work, we aim to develop a writhed analytical magnetic flux rope model which would allow us to analytically describe a flux rope structure with varying curvature and torsion so that we are no longer constrained to a cylindrical or toroidal geometry. In this first iteration of our model we will solely focus on a circular cross-section of constant size. We describe our flux rope geometry in terms of a parametrized flux rope axis and a parallel transport frame. We derive expressions for the axial and poloidal magnetic field components under the assumption that the total axial magnetic flux is conserved. We find an entire class of possible solutions, which differ by the choice of integration constants, and present the results for a specific example. In general, we find that the twist of the magnetic field locally changes when the geometry deviates from a cylinder or torus. This new approach also allows us to generate completely new types of in situ magnetic field profiles which strongly deviate from those generated by cylindrical or toroidal models.

Solar-cycle variation of quiet-Sun magnetism and surface gravity oscillation mode

Context. The origins of quiet-Sun magnetism (QS) is still under debate and investigating the solar cycle variation observationally in greater detail can provide clues on how to resolve the ensuing controversies. Aims. We investigate the solar cycle variation of the most magnetically quiet regions and their surface gravity oscillation (f-) mode-integrated energy, Ef. Methods. We used 12 years of Helioseismic and Magnetic Imager (HMI) data and applied a stringent selection criteria based on spatial and temporal quietness to avoid any influence from active regions (ARs). We developed an automated high-throughput pipeline to go through all available magnetogram data and to compute the value of Ef for the selected quiet regions. Results. We observed a clear solar cycle dependence of the magnetic field strength in the most quiet regions containing several supergranular cells. For patch sizes smaller than a supergranular cell, no significant cycle dependence was detected. The Ef at the supergranular scale is not constant over time. During the late ascending phase of Cycle 24 (SC24, 2011-2012), it is roughly constant, but starts diminishing in 2013, as the maximum of SC24 is approached. This trend continues until mid-2017, when hints of strengthening at higher southern latitudes are seen. Slow strengthening continues, stronger at higher latitudes than at the equatorial regions, but Ef never returns to the values seen in 2011-2012. In addition, the strengthening trend continues past the solar minimum, to the years when SC25 is already clearly ascending. Hence, the Ef behavior is not in phase with the solar cycle. Conclusions. The dependence of Ef on the solar cycle at supergranular scales is indicative of the fluctuating magnetic field being replenished by tangling from the large-scale magnetic field – and not solely due to the action of a fluctuation dynamo process in the surface regions. The absence of variations on smaller scales might be an effect of the limited spatial resolution and magnetic sensitivity of HMI. The anticorrelation of Ef with the solar cycle in gross terms is expected, but the phase shift of several years indicates a connection to the large-scale poloidal magnetic field component rather than the toroidal one. Calibrating AR signals with the QS Ef does not reveal significant enhancement of the f-mode prior to AR emergence.

Turbulent magnetic field amplification in binary neutron star mergers

Magnetic fields are expected to play a key role in the dynamics and the ejection mechanisms that accompany the merger of two neutron stars. General relativistic magnetohydrodynamic (MHD) simulations offer a unique opportunity to unravel the details of the ongoing physical processes. Nevertheless, current numerical studies are severely limited by the fact that any affordable resolution remains insufficient to fully capture the small-scale dynamo, initially triggered by the Kelvin-Helmholtz instability, and later sourced by several MHD processes involving differential rotation. Here, we alleviate this limitation by using explicit large-eddy simulations, a technique where the unresolved dynamics occurring at the subgrid scales (SGS) is modeled by extra terms, which are functions of the resolved fields and their derivatives. The combination of high-order numerical schemes, high resolutions, and the gradient SGS model allow us to capture the small-scale dynamos produced during the binary neutron star mergers, as shown in previous works. Here, we follow the first 50 milliseconds after the merger and, for the first time, we find numerical convergence on the magnetic field amplification, in terms of integrated energy and spectral distribution over spatial scales. Among other results, we find that the average intensity of the magnetic field in the remnant saturates at $\ensuremath{\sim}{10}^{16}\text{ }\text{ }\mathrm{G}$ around 5 ms after the merger. After 20--30 ms, both toroidal and poloidal magnetic field components grow continuously, fed by the winding mechanism that provides a slow inverse cascade, i.e., gradually transferring kinetic into magnetic energy. We find no clear hints for magnetorotational instabilities and no significant impact of the magnetic field on the redistribution of angular momentum in the remnant in our simulations, probably due to the very turbulent and dynamical topology of the magnetic field at all stages, with small-scale components largely dominating over the large-scale ones. Although the magnetic field grows near the rotation axis of the remnant, longer large-eddy simulations are necessary to further investigate the formation of large-scale, helical structures close to the rotational axis, which could be associated to jet formation.

Synthetic solar cycle for active regions violating the Hale’s polarity law

ABSTRACTLong observational series for bipolar active regions (ARs) provide significant information about the mutual transformation of the poloidal and toroidal components of the global solar magnetic field. The direction of the toroidal field determines the polarity of leading sunspots in ARs in accordance with the Hale’s polarity law. The vast majority of bipolar ARs obey this regularity, whereas a few per cent of ARs have the opposite sense of polarity (anti-Hale ARs). However, the study of these ARs is hampered by their poor statistics. The data for five 11-yr cycles (16–18 and 23, 24) were combined here to compile a synthetic cycle of unique time length and latitudinal width. The synthetic cycle comprises data for 14838 ARs and 367 of them are the anti-Hale ARs. A specific routine to compile the synthetic cycle was demonstrated. We found that, in general, anti-Hale ARs follow the solar cycle and are spread throughout the time-latitude diagram evenly, which implies their fundamental connection with the global dynamo mechanism and the toroidal flux system. The increase in their number and percentage occurs in the second part of the cycle, which is in favour of their contribution to the polar field reversal. The excess in the anti-Hale ARs percentage at the edges of the butterfly diagram and near an oncoming solar minimum (where the toroidal field weakens) might be associated with the strengthening of the influence of turbulent convection and magnetic field fluctuations on the arising flux tubes. The evidence of the misalignment between the magnetic and heliographic equators is also found.

Multiscale magnetic fields in the central molecular zone: inference from the gradient technique

ABSTRACTThe central molecular zone (CMZ) plays an essential role in regulating the nuclear ecosystem of our Galaxy. To get an insight into magnetic fields of the CMZ, we employ the gradient technique (GT), which is rooted in the anisotropy of magnetohydrodynamic turbulence. Our analysis is based on the data of multiple wavelengths, including molecular emission lines, radio 1.4 GHz continuum image, and Herschel $70\, {\mu }{\rm m}$ image, as well as ionized [Ne ii] and Paschen-alpha emissions. The results are compared with the observations of Planck 353 GHz and High-resolution Airborne Wideband Camera Plus (HWAC+) $53\, {\mu }{\rm m}$ polarized dust emissions. We map the magnetic fields orientation at multiple wavelength across the central molecular zone, including close-ups of the Radio Arc and Sagittarius A West regions, on multiscales from ∼0.1 pc to 10 pc. The magnetic fields towards the central molecular zone traced by the GT are globally compatible with the polarization measurements, accounting for the contribution from the galactic foreground and background. This correspondence suggests that the magnetic field and turbulence are dynamically crucial in the galactic center. We find that the magnetic fields associated with the Arched filaments and the thermal components of the Radio Arc are in good agree with the HAWC+ polarization. Our measurement towards the non-thermal Radio Arc reveals the poloidal magnetic field components in the galactic center. For Sagittarius A West region, we find a great agreement between the GT measurement using [Ne ii] emission and HWAC+ $53\, {\mu }{\rm m}$ observation. We use the GT to predict the magnetic fields associated with ionized Paschen-alpha gas down to scales of 0.1 pc.

Application of Synoptic Magnetograms to Global Solar Activity Forecast

Abstract Synoptic magnetograms provide us with knowledge about the evolution of magnetic fields on the solar surface and present important information for forecasting future solar activity. In this work, poloidal and toroidal magnetic field components derived from synoptic magnetograms are assimilated, using the Ensemble Kalman Filter method, into a mean-field dynamo model based on Parker’s migratory dynamo theory complemented by magnetic helicity conservation. It was found that the predicted toroidal field is in good agreement with observations for almost the entire following solar cycle. However, poloidal field predictions agree with observations only for the first 2–3 yr of the predicted cycle. The results indicate that the upcoming Solar Maximum of Cycle 25 (SC25) is expected to be weaker than the current Cycle 24. The model results show that a deep extended solar activity minimum is expected during 2019–2021, and that the next solar maximum will occur in 2024–2025. The sunspot number at the maximum will be about 50 with an error estimate of 15%–30%. The maximum will likely have a double peak or show extended periods (for 2–2.5 yr) of high activity. According to the hemispheric prediction results, SC25 will start in 2020 in the southern hemisphere, and will have a maximum in 2024 with a sunspot number of about 28. In the northern hemisphere the cycle will be delayed for about 1 yr (with an error of ±0.5 yr), and reach a maximum in 2025 with a sunspot number of about 23.

Measuring stellar magnetic helicity density

ABSTRACT Helicity is a fundamental property of a magnetic field but to date it has only been possible to observe its evolution in one star – the Sun. In this paper, we provide a simple technique for mapping the large-scale helicity density across the surface of any star using only observable quantities: the poloidal and toroidal magnetic field components (which can be determined from Zeeman–Doppler imaging) and the stellar radius. We use a sample of 51 stars across a mass range of 0.1–1.34 M⊙ to show how the helicity density relates to stellar mass, Rossby number, magnetic energy, and age. We find that the large-scale helicity density increases with decreasing Rossby number Ro, peaking at Ro ≃ 0.1, with a saturation or decrease below that. For both fully and partially convective stars, we find that the mean absolute helicity density scales with the mean squared toroidal magnetic flux density according to the power law: $|\langle {h\, }\rangle |$ ∝ $\langle {\rm {{\it B}_{tor}}^2_{}\, \rangle }^{0.86\, \pm \, 0.04}$. The scatter in this relation is consistent with the variation across a solar cycle, which we compute using simulations and observations across solar cycles 23 and 24, respectively. We find a significant decrease in helicity density with age.

Acceleration and trapping of fast ions in self-organized magneto-plasma structures in the dense plasma focus

Recent research at the PF-1000 Dense Plasma Focus facility strongly suggests that the early part of neutron emission is caused by fast deuterons with energy on the order of ∼100 keV, having approximately equal axial and radial velocity, temporally coinciding with the occurrence of self-organized, bounded magneto-plasma structures, which remain trapped within the reaction zone for tens of transit times. The experimental evidence, predominantly qualitative in nature, does not clarify the nature and origin of the accelerating electric field responsible for high ion energy and of the magnetic field that might be confining the ions to the reaction zone except for the suggestion that they have toroidal and poloidal magnetic field components whose presence is revealed by magnetic probes. Current theories, conjectures, and models of plasma focus find it difficult to accommodate three-dimensional features of ion motion and magnetic field revealed by multiple experiments within their scope. This paper revisits the relevant experimental evidence and introduces a model that is deliberately non-quantitative in order to accommodate the qualitative nature of the available experimental evidence. The model leads to a functional form for the 3-dimensional distribution of magnetic field associated with the spontaneously self-organized magneto-plasma structures. This enables the discussion of properties of 3-dimensional trajectories of ions accelerated by electric fields induced during their growth. Many qualitative observations about the nature of neutron emission in Dense Plasma Focus and the observed phenomenology of plasma evolution can be understood in terms of this model in a unified manner. The model also helps conceive a new generation of diagnostic schemes targeted at getting quantitative information that is out of reach of currently available diagnostics.

Electron skin depth in low density plasmas

We rescale the generalized Ohm’s law and consider the limits that imply the electric field which moves with an ideal (inviscid and perfectly conducting) plasma be proportional to the time rate of change of the current density. Therefore, we show that those limits are satisfied by a sufficiently low electron number density, $n_{\text{e}}$ . We also show that the electron–ion collision frequency, $\unicode[STIX]{x1D708}_{\text{ei}}$ , is much smaller than the ion cyclotron frequency, $\unicode[STIX]{x1D714}_{\text{ci}}$ . The combination of that condition with the Lawson criterion for a typical deuterium–tritium fusion in ITER reveals a lower bound for the geometric mean of the confinement time, $\unicode[STIX]{x1D70F}_{\text{C}}$ , and collision interval, $\sqrt{\unicode[STIX]{x1D70F}_{\text{C}}/\unicode[STIX]{x1D708}_{\text{ei}}}\gg 10^{-5}~\text{s}$ . For that reaction, we estimate that $n_{\text{e}}\sim 10^{19}~\text{m}^{-3}$ , and contrast typical parameters of our fully ionized gas with those of warm, hot and thermonuclear plasmas. When, in addition, $n_{\text{e}}$ varies slowly in time and weakly in space, we generalize Alfvén’s theorem, by showing that the frozen-in condition holds true for an effective magnetic field, which depends on a finite electron skin depth. We perform a (divergenceless) helical perturbation on an axisymmetric equilibrium, to derive a dispersion relation in the cylindrical tokamak limit, and, subsequently, apply our analytical formulation to the peaked model, which assumes a logarithmic derivative profile for the poloidal component of the equilibrium magnetic field. In that formulation, the definition of the safety factor in terms of the effective field yields a shift in the magnetic surfaces. We find that the instability peak may triple that predicted on neglect of a finite electron mass. We also find that inertial effects may centuple the radius of the stable cylindrical column.

Structure of Magnetic Plasma Fluctuations in the Uragan-3M Torsatron at Rare Collision Frequencies

Fluctuations of the poloidal component of the plasma magnetic field in the frequency range of 0.5–50 kHz are studied in the Uragan-3M (U-3M) torsatron. Hydrogen plasma is produced and heated by RF fields at frequencies close to that of the ion cyclotron. The studies are carried out using a set of 15 magnetic sensors installed in one of the torus cross sections. RF heating provided the plasma with rare collision frequencies and the presence of the bootstrap current. The study is carried out when the maximum amplitude of magnetic fluctuations is observed and their connection with the plasma energy content is noticeable. Two types of vibrations are observed. In the first type, the current structure rotates with a certain frequency mainly in the direction of the rotation of electrons in the magnetic field, and the amplitude varies slowly with time (the rotating structure). For the second type, the spatial structure does not rotate, but its amplitude changes with a certain frequency (the standing structure). The frequencies of fluctuations and rotations are close for structures with a given poloidal wave number. The standing vibration structures with different poloidal wave numbers in this frequency range are correlated. The maximum amplitude of the rotating structures is observed with m = 2, and for the standing structures with m = 3 and reaches the values of $$\tilde {B}$$ ≤ 0.3 G in the confinement region. The vibration frequency does not depend on poloidal wave number m for the studied cases; m = 0, 1, 2, 3.

Diffusion of stochastic magnetic field lines with average poloidal magnetic component

We study the diffusion of stochastic isotropic and anisotropic magnetic field lines in turbulence with average poloidal magnetic field component. In the current work we employ the so-called DeCorrelation Trajectory method in order to compute the running and asymptotic diffusion coefficients of stochastic magnetic field lines with average poloidal magnetic field component. A modification of the diffusion coefficients is caused by the average magnetic field component parameter in the presence of trajectory trapping. We calculate the field line diffusion coefficients Dxx and Dyy for different values of the magnetic Kubo number K, the average magnetic field component parameter Kav and the anisotropy parameter Λ. Furthermore we compare our new findings with those already obtained in papers dealing with the calculus of the diffusion coefficients for a stochastic sheared magnetic field.

Структура магнитных флуктуаций плазмы в торсатроне Ураган-3М при редких частотах соударений

AbstractFluctuations of the poloidal component of the plasma magnetic field in the frequency range of 0.5–50 kHz are studied in the Uragan-3M (U-3M) torsatron. Hydrogen plasma is produced and heated by RF fields at frequencies close to that of the ion cyclotron. The studies are carried out using a set of 15 magnetic sensors installed in one of the torus cross sections. RF heating provided the plasma with rare collision frequencies and the presence of the bootstrap current. The study is carried out when the maximum amplitude of magnetic fluctuations is observed and their connection with the plasma energy content is noticeable. Two types of vibrations are observed. In the first type, the current structure rotates with a certain frequency mainly in the direction of the rotation of electrons in the magnetic field, and the amplitude varies slowly with time (the rotating structure). For the second type, the spatial structure does not rotate, but its amplitude changes with a certain frequency (the standing structure). The frequencies of fluctuations and rotations are close for structures with a given poloidal wave number. The standing vibration structures with different poloidal wave numbers in this frequency range are correlated. The maximum amplitude of the rotating structures is observed with m = 2, and for the standing structures with m = 3 and reaches the values of $$\tilde {B}$$ ≤ 0.3 G in the confinement region. The vibration frequency does not depend on poloidal wave number m for the studied cases; m = 0, 1, 2, 3.

Scattering of radio frequency waves by cylindrical filaments with general orientation relative to the magnetic field

Radio frequency (RF) waves are routinely used in tokamak fusion plasmas for plasma heating, current control, as well as in diagnostics. These waves are excited by antenna structures placed near the tokamak’s wall and they have to propagate through a turbulent layer known as the scrape-off layer, before reaching the core plasma (which is their target). This layer exhibits coherent density fluctuations in the form of filaments and blobs. The scattering processes of RF plane waves by a single filament is studied with the assumption that the filament has a cylindrical shape and infinite length. Furthermore, besides the major toroidal component of the externally imposed magnetic field, there is also a small poloidal magnetic field component. Considering also that the cylindrical filament’s axis is not necessarily aligned with the toroidal direction, the total magnetic field is in general neither aligned with the axis of the cylinder nor with the toroidal direction. The investigation concerns the case of electron cyclotron (EC) waves (of frequency $f_{0}=170~\text{GHz}$) for tokamak applications. The study covers a variety of density contrasts between the filament and the ambient plasma, different magnetic field inclinations with respect to the cylinder axis (for the same magnitude of magnetic induction $B=4.5T$) and a wide range of filament radii.

High cadence, linear, and circular polarization monitoring of OJ 287

Aims. We present a multifrequency, dense radio monitoring program of the blazar OJ 287 using the 100-m Effelsberg radio telescope. The program aims to test different binary supermassive black hole (SMBH) scenarios and studying the physical conditions in the central region of this bright blazar. Here, we analyze the evolution in total flux density, linear and circular polarization as a means to study the OJ 287 jet structure and its magnetic field geometry. Methods. We used a recently developed, high-precision data analysis methodology to recover all four Stokes parameters. We measured the total flux density of OJ 287 at nine bands from 2.64 GHz to 43 GHz, the linear polarization parameters at four bands between 2.64 GHz and 10.45 GHz, and the circular polarization at two bands, 4.85 GHz and 8.35 GHz. The mean cadence of our measurements is ten days. Results. Between December 2015 and January 2017 (MJD 57370–57785), OJ 287 showed flaring activity and complex linear and circular polarization behavior. The radio electric vector position angle (EVPA) showed a large clockwise (CW) rotation by ∼340° with a mean rate of −1.04°/day. Based on concurrent very long baseline interferometric (VLBI) polarization data at 15 GHz and 43 GHz, the rotation seems to originate within the jet core at 43 GHz (projected angular size ≤0.15 mas or 0.67 pc at the redshift of the source). Moreover, optical polarization data show a similar monotonic CW rotation with a rate of about −1.1°/day which is superposed with shorter and faster rotations that exhibit rates of about 7.8°/day, mainly in the CW sense. Conclusions. The flux density and polarization variability of the single dish, VLBI and optical data is consistent with a polarized emission component propagating on a helical trajectory within a bent jet. We constrained the helix arc length to 0.26 pc and radius to ≤0.04 pc as well as the jet bending arc length projected on the plane of the sky to ≤1.9–7.6 pc. A similar bending has been observed also in high angular resolution VLBI images of the OJ 287 jet at its innermost regions. The helical trajectory covers only a part of the jet width, possibly its spine. In addition, our results indicate the presence of a stable polarized emission component. Its EVPA (−10°) is oriented perpendicular to the large scale jet, suggesting dominance of the poloidal magnetic field component. Finally, the EVPA rotation begins simultaneously with an optical flare and hence the two might be physically connected. That optical flare has been suggested to be linked to the interaction of a secondary SMBH with the inner accretion disk or originating in the jet of the primary.

Long Term Evolution of Solar Meridional Circulation and Phase Synchronization Viewed Through a Symmetrical Kuramoto Model

The solar-cycle oscillations of the toroidal and poloidal components of the solar magnetic field in the northern solar hemisphere have a persistent phase difference of about $\pi $ . We propose a symmetrical Kuramoto model with three coupled oscillators as a simple way to understand this anti-synchronization. We solve an inverse problem and reconstruct natural frequencies of the top and bottom oscillators under the conditions of a constant coupling strength and a non-delayed coupling. These natural frequencies are associated with angular velocities of the meridional flow circulation near the solar surface and in the deep layer of the solar convection zone. A relationship between our reconstructions of the shallow and the deep meridional flow speed during recent Solar Cycles 21 – 23 is in agreement with estimates obtained in helioseismology and flux-transport dynamo modeling. The reconstructed top oscillator speed presents significant solar-cycle like variations that agree with recent helioseismical reconstructions. The evolution of reconstructed natural frequencies strongly depends on the coupling strength. We find two stable regimes in the case of strong coupling with a change of regime during anomalous solar cycles. We see the onset of a new transition in Solar Cycle 24. We estimate the admitted range of coupling values and find evidence of cross-equatorial coupling between solar hemispheres not accounted for by the model.

Magnetic field in a young circumbinary disk

Context. Polarized continuum emission at millimeter-to-submillimeter wavelengths is usually attributed to thermal emission from dust grains aligned through radiative torques with the magnetic field. However, recent theoretical work has shown that under specific conditions polarization may arise from self-scattering of thermal emission and by radiation fields from a nearby stellar object. Aims. We use multi-frequency polarization observations of a circumbinary disk to investigate how the polarization properties change at distinct frequency bands. Our goal is to discern the main mechanism responsible for the polarization through comparison between our observations and model predictions for each of the proposed mechanisms. Methods. We used the Atacama Large Millimeter/submillimeter Array to perform full polarization observations at 97.5 GHz (Band 3), 233 GHz (Band 6) and 343.5 GHz (Band 7). The ALMA data have a mean spatial resolution of 28 AU. The target is the Class I object BHB07-11, which is the youngest object in the Barnard 59 protocluster. Complementary Karl G. Jansky Very Large Array observations at 34.5 GHz were also performed and revealed a binary system at centimetric continuum emission within the disk. Results. We detect an extended and structured polarization pattern that is remarkably consistent between the three bands. The distribution of polarized intensity resembles a horseshoe shape with polarization angles following this morphology. From the spectral index between Bands 3 and 7, we derived a dust opacity index β ~ 1 consistent with maximum grain sizes larger than expected to produce self-scattering polarization in each band. The polarization morphology and the polarization levels do not match predictions from self-scattering. On the other hand, marginal correspondence is seen between our maps and predictions from a radiation field model assuming the brightest binary component as main radiation source. Previous molecular line data from BHB07-11 indicates disk rotation. We used the DustPol module of the ARTIST radiative transfer tool to produce synthetic polarization maps from a rotating magnetized disk model assuming combined poloidal and toroidal magnetic field components. The magnetic field vectors (i.e., the polarization vectors rotated by 90°) are better represented by a model with poloidal magnetic field strength about three times the toroidal one. Conclusions. The similarity of our polarization patterns among the three bands provides a strong evidence against self-scattering and radiation fields. On the other hand, our data are reasonably well reproduced by a model of disk with toroidal magnetic field components slightly smaller than poloidal ones. The residual is likely to be due to the internal twisting of the magnetic field due to the binary system dynamics, which is not considered in our model.

Determination of the Coronal and Interplanetary Magnetic Field Strength and Radial Profiles from Large-Scale Photospheric Magnetic Fields

A new model has been proposed for magnetic field determination at different distances from the Sun during different solar cycle phases. The model depends on the observed large-scale non-polar photospheric magnetic fields and that measured at polar regions from 55N to 90N and from 55S to 90S,which are the visible manifestations of cyclic changes in the toroidal and poloidal components of the global magnetic field of the Sun. The modeled magnetic field is determined as the superposition of the non-polar and the polar photospheric magnetic field cycle variations. The agreement between the model predictions and magnetic fields derived from direct, in-situ, measurements at different distances from the Sun, obtained by different methods, and at different solar activity phases is quite satisfactory. From a comparison of the magnetic fields as observed and as calculated from the model at 1 AU, it should be concluded that the model magnetic-field variations adequately explains the major features of the IMF Bx component cycle evolution at the Earth's orbit. The model CR-averaged magnetic fields correlate with CR-averaged IMF Bx component at the Earth's orbit with a coefficient of 0.688, and for seven CR-averaged data the correlation reaches 0.808. The model magnetic-field radial profiles were compared with that of the already existing models. In addition, the decrease in the non-polar and polar photospheric magnetic fields has been revealed. Both magnetic fields during solar cycle maxima and that during minima phases decreased from Cycle 21 to Cycle 24. It means that both the toroidal and poloidal components and therefore, the solar global magnetic field decreased from Cycle 21 to Cycle 24.