Large-scale Toroidal Magnetic Field Research Articles

Modified models of radiation pressure instability applied to 10, 105, and 107 M⊙ accreting black holes

Context. Some accreting black holes exhibit much stronger variability patterns than the usual stochastic variations. Radiation pressure instability is one of the proposed mechanisms that might account for this effect. Aims. We model luminosity changes for objects with a black hole mass of 10, 105, and 107 solar masses, using the time-dependent evolution of an accretion disk that is unstable as a result of the dominant radiation pressure. We concentrate on the outburst timescales. We explore the influence of the hot coronal flow above the cold disk, the inner purely hot flow, and the effect of the magnetic field on the time evolution of the disk-corona system. For intermediate-mass black holes and active galactic nuclei, we also explore the role of the disk outer radius because a disk that is fed by tidal disruption events (TDE) can be quite small. Methods. We used a 1D vertically integrated time-dependent numerical scheme that models the simultaneous evolution of the disk and corona, which is coupled by the vertical mass exchange. We parameterized the strength of the large-scale toroidal magnetic fields according to a local accretion rate. We also discuss a possible inner optically thin flow, the advection-dominated accretion flow (ADAF). This flow would require modification of the inner boundary condition of the cold disk flow. For the set of the global parameters, we calculated the variability timescales and outburst amplitudes of the disk and the corona. Results. We found that the role of the inner ADAF and the accreting corona are relatively unimportant, but the outburst character strongly depends on the magnetic field and on the outer radius of the disk if this radius is smaller (due to the TDE phenomenon) than the size of the instability zone in a stationary disk with infinite radius. For microquasars, the dependence on the magnetic field is monotonic, and the period decreases with the field strength. For higher black hole masses, the dependence is nonmonotonic, and an initial rise of the period is later replaced with a relatively rapid decrease as the magnetic field continues to rise. A still stronger magnetic field stabilizes the disk. When we assumed a smaller disk outer radiusfor 105 and 107 M⊙, the outbursts were shorter and led to complex multiscale outbursts for some parameters, thus approaching the behavior of deterministic chaos. Conclusions. Our computations confirm that the radiation pressure instability model can account for heartbeat states in microquasars. The rapid variability detected in intermediate-mass black holes in the form of quasi-periodic eruptions can be consistent with the model, but only when it is combined with the TDE phenomenon. The yearly repeating variability in changing-look active galactic nuclei in our model also requires a small outer radius either due to the recent TDE or due to the gap in the disk that is related to a secondary black hole.

Peering into the Milky Way by FAST: III. Magnetic fields in the Galactic halo and farther spiral arms revealed by the Faraday effect of faint pulsars

The Five-hundred-meter Aperture Spherical radio Telescope (FAST) is the most sensitive radio telescope for pulsar observations. We make polarimetric measurements of a large number of faint and distant pulsars using the FAST. We present the new measurements of Faraday rotation for 134 faint pulsars in the Galactic halo. Significant improvements are also made for some basic pulsar parameters for 15 of them. We analyse the newly determined rotation measures (RMs) for the Galactic magnetic fields by using these 134 halo pulsars, together with previously available RMs for pulsars and extragalactic radio sources and also the newly determined RMs for another 311 faint pulsars which are either newly discovered in the project of the Galactic Plane Pulsar Snapshot (GPPS) survey or previously known pulsars without RMs. The RM tomographic analysis in the first Galactic quadrant gives roughly the same field strength of around 2~$\mu$G for the large-scale toroidal halo magnetic fields. The scale height of the halo magnetic fields is found to be at least 2.7$\pm$0.3~kpc. The RM differentiation of a large number of pulsars in the Galactic disk in the Galactic longitude range of $26^{\circ}<l<90^{\circ}$ gives evidence for the clockwise magnetic fields (viewed from the north Galactic pole) in two interarm regions inside the Scutum arm and between the Scutum and Sagittarius arm, and the clockwise fields in the Local-Perseus interarm region and field reversals in the Perseus arm and beyond.

On the effect of surface bipolar magnetic regions on the convection zone dynamo

ABSTRACTWe investigate the effect of surface bipolar magnetic regions (BMRs) on the large-scale dynamo distributed in the bulk of the convection zone. The study employs the non-linear three-dimensional mean-field dynamo model. We model the emergence of the BMRs on the surface through the non-axisymmetric magnetic buoyancy effect, which acts on the large-scale toroidal magnetic field in the upper half of the convection zone. The non-axisymmetric magnetic field that results from this mechanism is shallow. On the surface, the effect of the BMRs on the magnetic field generation is dominant. However, because of the shallow distribution of BMRs, its effect on the global dynamo is less compared with the effect on the convective zone dynamo. We find that the mean-field α-effect, which acts on the non-axisymmetric magnetic field of the BMRs, provides the greater contribution to the dynamo process than the tilt of the BMRs. Even so, the fluctuations of the tilt of the BMRs lead to parity braking in the global dynamo. At the surface, the non-axisymmetric magnetic fields, which are generated because of the activity of the BMRs, show a tendency for the bihelical spectrum with positive sign for the low ℓ modes during the maximum of the magnetic activity cycle.

Possible formation of ring galaxies by torus-shaped magnetic wormholes

We present the hypothesis that some of ring galaxies were formed by relic magnetic torus-shaped wormholes. In the primordial plasma before the recombination magnetic fields of wormholes trap baryons whose energy is smaller than a threshold energy. They work as the Maxwell’s demons collecting baryons from the nearest (horizon size) region and thus forming clumps of baryonic matter which have the same torus-like shapes as wormhole throats. Such clumps may serve as seeds for the formation of ring galaxies and smaller objects having the ring form. Upon the recombination torus-like clumps may decay and merge. Unlike galaxies, such objects may contain less or even no dark matter in halos. However the most stringent feature of such objects is the presence of a large-scale toroidal magnetic field. We show that there are threshold values of magnetic fields which give the upper and lower boundary values for the baryon clumps in such protogalaxies.

Flux Tubes Forming Instability Near the Base of the Rotating Convection Zone: A Possible Explanation for the Low Latitudes of Sunspots

Abstract The rise of flux tubes with intense magnetic fields from the base of the convection zone to the solar surface has been substantiated as a probable mechanism for sunspot formation. The origin of flux tubes of a sufficient strength (∼105 G) is, however, uncertain. This paper considers the instability of a large-scale toroidal magnetic field caused by the magnetic suppression of convective heat transport as a candidate for the flux-tube formation mechanism. The consideration employs the analytical dependence of eddy diffusion on the magnetic field supplied by mean-field magnetohydrodynamics. The instability tends to produce regions of increased field strength with spatial scales of the order of 100 Mm at the base of the convection zone. Characteristic growth times of the instability are short compared to the 11 yr cycle. The threshold field strength for the onset of the instability increases from several hundred Gauss in the vicinity of the equator to some kilo-Gauss at middle latitudes. Growth rates of unstable disturbances decrease with latitude. These latitudinal trends can be the reason for the observed confinement of sunspot activity to a near-equatorial belt.

The Nature of Variations in Anomalies of the Chemical Composition of the Solar Corona with the 11-Year Cycle

Evidence that the distribution of the abundances of admixtures with low first-ionization potentials (FIP 10 eV) in active regions and closed magnetic configurations in the lower corona. Observations with the ULYSSES spacecraft and at the Stanford Solar Observatory have revealed strong correlations between the manifestation of the FIP effect in the solar wind, the strength of the open magnetic flux (without regard to sign), and the ratio of the large-scale toroidal and poloidal magnetic fields at the solar surface. Analyses of observations of the Sun as a star show that the enhancement of the abundances of admixtures with low FIPs in the corona compared to their abundances in the photosphere (the FIP effect) is closely related to the solar-activity cycle and also with variations in the topology of the large-scale magnetic field. A possible mechanism for the relationship between the FIP effect and the spectral type of a star is discussed in the framework of solar–stellar analogies.

The Direct Effect of Toroidal Magnetic Fields on Stellar Oscillations: An Analytical Expression for the General Matrix Element

Abstract Where is the solar dynamo located and what is its modus operandi? These are still open questions in solar physics. Helio- and asteroseismology can help answer them by enabling us to study solar and stellar internal structures through global oscillations. The properties of solar and stellar acoustic modes are changing with the level of magnetic activity. However, until now, the inference on subsurface magnetic fields with seismic measures has been very limited. The aim of this paper is to develop a formalism to calculate the effect of large-scale toroidal magnetic fields on solar and stellar global oscillation eigenfunctions and eigenfrequencies. If the Lorentz force is added to the equilibrium equation of motion, stellar eigenmodes can couple. In quasi-degenerate perturbation theory, this coupling, also known as the direct effect, can be quantified by the general matrix element. We present the analytical expression of the matrix element for a superposition of subsurface zonal toroidal magnetic field configurations. The matrix element is important for forward calculations of perturbed solar and stellar eigenfunctions and frequency perturbations. The results presented here will help to ascertain solar and stellar large-scale subsurface magnetic fields, and their geometric configuration, strength, and change over the course of activity cycles.

Dual behavior of the toroidal magnetic field versus the Rossby wave instability

The Rossby wave instability (RWI) theory has been considered as one of the top topics in astrophysics due to the fact that it deals with some ambiguous questions, such as the angular momentum transport in the poorly ionized regions of the protoplanetary discs. Based on the theoretical and simulation works, two important factors in the study of the RWI are the viscosity and magnetic field, which are directly connected to each other because the large-scale toroidal magnetic fields are produced by the magnetohydrodynamic (MHD) turbulence. Therefore, it is essential to consider more details about the toroidal magnetic field both in the steady state and perturbation state. In this paper, the role of the strength and gradient of the toroidal magnetic field is investigated on the RWI at the dead zone in the regions of bump. The obtained results show that the gradient of the toroidal magnetic field or its strength have a major role in the RWI occurrence, which have received relatively less attention in previous works. Also, the role of the gradient of the toroidal magnetic field in the RWI is important even in the weak magnetic fields. Hence, the obtained results are very different from what we previously expected, and it seems crucial to research and develop this issue in the theoretical and simulation works. This paper can be helpful on the study of the angular momentum transport in the cold accretion discs, such as accretion discs in quiescent dwarf novae or around the white-dwarf primary.

ACCRETION DISK DYNAMO AS THE TRIGGER FOR X-RAY BINARY STATE TRANSITIONS

Magnetohydrodynamic accretion disk simulations suggest that much of the energy liberated by the magnetorotational instability (MRI) can be channeled into large-scale toroidal magnetic fields through dynamo action. Under certain conditions, this field can dominate over gas and radiation pressure in providing vertical support against gravity, even close to the midplane. Using a simple model for the creation of this field, its buoyant rise, and its coupling to the gas, we show how disks could be driven into this magnetically dominated state and deduce the resulting vertical pressure and density profiles. Applying an established criterion for MRI to operate in the presence of a toroidal field, we show that magnetically supported disks can have two distinct MRI-active regions, separated by a "dead zone" where local MRI is suppressed, but where magnetic energy continues to flow upward from the dynamo region below. We suggest that the relative strengths of the MRI zones, and the local poloidal flux, determine the spectral states of X-ray binaries. Specifically, "intermediate" and "hard" accretion states occur when MRI is triggered in the hot, upper zone of the corona, while disks in "soft" states do not develop the upper MRI zone. We discuss the conditions under which various transitions should take place and speculate on the relationship of dynamo activity to the various types of quasi-periodic oscillations that sometimes appear in the hard spectral components. The model also explains why luminous accretion disks in the "soft" state show no signs of the thermal/viscous instability predicted by standard alpha models.

Activity of the sun in the age of 1–2 Gyr

We discuss basic properties of the solar activity in the modern epoch and the further development of ideas about estimate the age of stars by the level of activity (gyrochronology). From comparison of properties of solar activity and processes on other G stars we have determined activity levels of the chromosphere and corona of the young Sun in its age of 1–2 Gyr. So, activity of the Sun in such an age is closest to the processes in the G stars, HD 152391 (V2292 Oph) and HD 1835 (BE Cet) with the rotational periods of 11 and 8 days correspondingly. The total spot area is of 2–3 orders of magnitude higher than the current value at the maximum of the solar cycle, the activity levels of the chromosphere and the corona are quite high and similar to the Hyades, but it did not reach the saturation level. We have analyzed new measurements of longitudinal magnetic fields and distribution of the toroidal and poloidal fields on the G main sequence stars. The mean value of the longitudinal fields for active G stars is of one order of magnitudes stronger than the mean daily magnetic field of the Sun as a star, for instance those during the 21th cycle maximum. Besides, the detection of large-scale toroidal magnetic field in such stars is important. Taking into account the activity levels and magnetic fields, we analyzed data on superflares on G stars registered with the Kepler mission. We have defined the upper limit of energy of flares that can occur in the magnetic fields of active G stars.

Axisymmetric and non-axisymmetric magnetostrophic MRI modes

The magnetorotational-instability is central to the understanding of many astrophysical magnetohydrodynamic flows (in particular accretion discs). We have recently shown that a modified version of this instability, the magnetostrophic MRI (MS-MRI), is relevant to the dynamics of the Earth liquid core (Petitdemange et al., 2008). Our previous study used a purely axial imposed magnetic field and considered only axisymmetric instabilities. We investigate here the effects of a large scale toroidal magnetic field on the development and saturation of the MS-MRI both in an axisymmetric setup and in fully three-dimensional configurations. We use direct numerical modeling of the full MHD equations (both in the linear and non-linear regimes) in a spherical geometry. We interpret our results using WKB expansions and shearing coordinates. We find that three-dimensional MS-MRI modes exhibit a strong helical structure for parameters relevant to planetary interiors. Three-dimensional MS-MRI modes share some similarities with their axisymmetric counterparts. They are amplified on the same short timescale. When non-linear effects become significant, they act to decrease the shear. During saturation, the magnetic structure expands spatially, while drifting at a constant rate along the direction of the rotation axis. A striking result is that an m=1 mode can dominate in the non-linear regime when a sufficiently large toroidal field is present. Three-dimensional MS-MRI modes could play an important role in planetary interior dynamics. They can cause rapid magnetic field variations and also act to limit the shear in the conducting liquid core.

THE SUBSURFACE-SHEAR-SHAPED SOLAR αΩ DYNAMO

We propose a solar dynamo model distributed in the bulk of the convection zone with the toroidal magnetic field the flux concentrated in the near-surface layer. We show that if the boundary conditions at the top of the dynamo region allow the large-scale toroidal magnetic fields to penetrate closer to the surface, then the pattern of the modeled butterfly diagram for the toroidal magnetic fields in the upper part of the convection zone is formed by the surface rotational shear layer. The model is in agreement with observed properties of the magnetic solar cycle.

Some developments of hot accretion flow theory in the past ten years

The hot accretion flow model was re-discovered in 1994 by Narayan and collaborators. Intensive theoretical works have been conducted and significant progresses have been achieved. In this paper, we review several developments in the past ten years. This mainly includes the finding of outflow and convection and its dynamical effect on inflow; the direct electron heating by viscous dissipation; the effect of large scale toroidal magnetic fields in the inner region of the accretion flow; and the effect of global Compton scattering. Their observational applications are also introduced very briefly.

Observation of Toroidal Magnetic Fields on 100 pc Scales in the Galactic Center

We present new submillimeter polarimetric observations of the Galactic center region, made using the SPARO polarimeter that operates at the South Pole. Compared with previous submillimeter polarimetry of this region, our measurements cover much more sky area, and they imply that the molecular gas in the central few hundred pc is threaded by a large scale toroidal magnetic field. We consider this result together with radio observations that show evidence for poloidal fields in the Galactic center, and with Faraday rotation observations. We compare all of these observations with a magnetodynamic model for the Galactic center.

First Results from the Submillimeter Polarimeter for Antarctic Remote Observations: Evidence of Large-Scale Toroidal Magnetic Fields in the Galactic Center

We have observed the linear polarization of 450 µm continuum emission from the Galactic center, using a new polarimetric detector system that is operated on a 2 m telescope at the South Pole. The resulting polarization map extends � 170 pc along the Galactic plane and � 30 pc in Galactic latitude, and thus covers a significant fraction of the central molecular zone. Our map shows that this region is permeated by large-scale toroidal magnetic fields. We consider our results together with radio observations that show evidence for poloidal fields in the Galactic center, and with Faraday rotation observations. We compare all of these observations with the predictions of a magnetodynamic model for the Galactic center that was proposed in order to explain the Galactic Center Radio Lobe as a magnetically driven gas outflow. We conclude that the observations are basically consistent with the model.

Jet Collimation by Small‐Scale Magnetic Fields

A popular model for jet collimation is associated with the presence of a large-scale and predominantly toroidal magnetic field originating from the central engine (a star, a black hole, or an accretion disk). Besides the problem of how such a large-scale magnetic field is generated, in this model the jet suffers from the fatal long-wave mode kink magnetohydrodynamic instability. In this paper we explore an alternative model: jet collimation by small-scale magnetic fields. These magnetic fields are assumed to be local, chaotic, and tangled, but are dominated by toroidal components. Just as in the case of a large-scale toroidal magnetic field, we show that the "hoop stress" of the tangled toroidal magnetic fields exerts an inward force which confines and collimates the jet. The magnetic "hoop stress" is balanced either by the gas pressure of the jet or by centrifugal force if the jet is spinning. Since the length scale of the magnetic field is small (< the cross-sectional radius of the jet ≪ the length of the jet), in this model the jet does not suffer from the long-wave mode kink instability. Many other problems associated with the large-scale magnetic field are also eliminated or alleviated for small-scale magnetic fields. Though it remains an open question how to generate and maintain the required small-scale magnetic fields in a jet, the scenario of jet collimation by small-scale magnetic fields is favored by the current study on disk dynamo which indicates that small-scale magnetic fields are much easier to generate than large-scale magnetic fields.

Solar convection and sunspot formation mechanishm

Abstract The rising-flux-tube model of the formation of a bipolar sunspot group sharply disgrees with the normally observed features of the process. Meanwhile, these features can be successfully accounted for in terms of local magnetic-field amplification due to zellular convective motions of the solar plasma. Here, magnetoconvection in a plane horizontal fluid layer is simulated numberically magnetic field and a weak cellular flow are present. Convention can produce bipolar configurations of strongly amplified magnetic field. Indications are found for the nontrivial effect of flow freezing. The action of the convective mechanism may be controlled by the large-scale toroidal magnetic field of the Sun.

Convective mechanism for the formation of photospheric magnetic fields

The well-known model that attributes the formation of a bipolar sunspot group to the emergence of a flux tube disagrees sharply with the usual observed pattern of phenomena. At the same time, the observed patterns can be accounted for quite convincingly in terms of local magnetic-field amplification due to cellular convective motions of the solar plasma. In this study, magnetoconvection in a plane horizontal fluid layer is simulated numerically in the framework of the fully nonlinear, three-dimensional problem. A weak horizontal magnetic field and weak cellular flow are assumed to be present initially. Convection is shown to be capable of producing bipolar magnetic configurations of the strongly amplified magnetic field. Indications of magnetic freezing of the flow in the cell are found. The action of the amplification mechanism under study may be controlled by the large-scale toroidal magnetic field of the Sun.

Optical diagnostics of femtosecond laser plasmas

Optical diagnostics of evolution of plasmas produced by ultrashort laser pulses is carried out using a femtosecond probing beam. The time sequence of plasma shadowgrams and interferograms are obtained. The filamentation instability in high-density region induces the local density modification. Large-scale toroidal magnetic fields confine plasma expansion in the transverse direction, resulting in the formation of a plasma jet. The plasma expansion along the target normal direction is found to scale as t1/2.

Modeling Polarization Maps of External Galaxies.

We extend previous radiation transfer models of disk galaxies to include bulge plus disk systems. The Monte Carlo radiation transfer models include the polarizing effects of dust scattering and transmission through aligned grains. We assume that the dust grains are aligned by large scale toroidal magnetic fields and we model the interstellar polarization using empirical formulae derived for the Milky Way. Although our galaxy models do not include dust or star inhomogeneities arising from clumping or spiral arm structures, they do successfully reproduce the observed dust lanes and polarization patterns and null points. We apply our techniques to NGC 4565 and NGC 891 which are examples of nearly edge-on spiral galaxies with prominent dust lanes. We can successfully model the polarimetric structure of NGC 4565 with uniformly aligned grains and a smooth dust lane. However, our models fail to reproduce the structure within NGC 891's dust lane. This failure is attributed to the striking disk activity present in NGC 891 and the polarimetric evidence for vertical magnetic field structures. The dust lane activity and non-toroidal magnetic fields in NGC 891 appear to break the large scale grain alignment to such an extent that our smooth model is unable to reproduce the flux and polarization maps. Although NGC 891 is often assumed to be very similar to our own Galaxy, our modeling suggests that, polarimetrically at least, NGC 4565 behaves more like the Milky Way than does NGC 891.