Magnetic Field Generation Process Research Articles

A conclusive non-detection of magnetic field in the Am star o Peg with high-precision near-infrared spectroscopy

The A-type metallic-line (Am) stars are typically considered to be non-magnetic or to possess very weak sub-G magnetic fields. This view has been repeatedly challenged in the literature; most commonly for the bright hot Am star Several studies claim to have detected 1--2 kG field of unknown topology in this object, possibly indicating a new process of magnetic-field generation in intermediate-mass stars. In this study, we revisit the evidence of a strong magnetic field in using new high-resolution spectropolarimetric observations and advanced spectral fitting techniques. We estimated the mean magnetic field strength in from the high-precision CRyogenic InfraRed Echelle Spectrograph $) measurement of near-infrared (NIR) sulphur lines. We modelled this observation with a polarised radiative transfer code, including treatment of the departures from local thermodynamic equilibrium. In addition, we used the least-squares deconvolution multi-line technique to derive longitudinal field measurements from archival optical spectropolarimetric observations of this star. Our analysis of the NIR S i lines reveals no evidence of Zeeman broadening, ruling out magnetic field with a strength exceeding 260 G. This null result is compatible with the relative intensification of Fe ii lines in the optical spectrum, taking into account blending and uncertain atomic parameters of the relevant diagnostic transitions. Longitudinal field measurements on three different nights also yield null results with a precision of 2 G. This study refutes the claims of kG-strength dipolar or tangled magnetic field in This star therefore appears to be non-magnetic, with surface magnetic field characteristics no different from those of other Am stars.

Multimode Representation of the Magnetic Field for the Analysis of the Nonlinear Behavior of Solar Activity as a Driver of Space Weather

Estimating and predicting space weather is important to the space industry and space missions. The driver of space weather, especially near the Earth, is solar activity, the study of which is an important task. In particular, there is a direction of problems based on models of solar magnetic field generation that require research. In our work, we build a nonlinear dynamic system of equations that describes the behavior of the solar magnetic field harmonics based on the alpha-omega dynamo model. We found that, at the beginning of the magnetic field generation process, when the dynamo number significantly exceeds the threshold, the most rapidly growing waves are in the lead. Then, over time, these waves stop growing quite quickly. In this case, the initially slowly increasing harmonics of the magnetic field become the leaders, which then make the main contribution to the process of magnetic field generation.

Renormalization group analysis of the turbulent hydromagnetic dynamo: Effect of anisotropy

The turbulent hydromagnetic dynamo is a process of magnetic field generation by chaotic flow of an electrically conducting fluid (plasma, liquid iron etc.). It is responsible for generation of large-scale magnetic fields of astrophysical objects such as planets, stars, accretion discs, galaxies, galaxy clusters etc. In particular, the dynamical process of induction of large-scale fields is strongly nonlinear and the effect of the Lorentz force is crucial. The aim of the analysis presented here is to provide analytic expressions for the large-scale electromotive force (EMF) in the limit of rapid rotation, with inclusion of both types of nonlinearities - the effect of the Lorentz force (hitherto scarcely considered) and the nonlinear evolution of turbulent fluctuations. The renormalization group technique is applied to extract the final expression for the mean EMF from the full nonlinear dynamical equations (Navier-Stokes, induction equation) and the mean-field equations are studied in order to demonstrate amplification and saturation of the mean magnetic field in two simplifying limits, that is the limit of a thin disc and a limit of a narrow spherical gap.

Magnetotherapy for group treatment

The advantage of complex magnetic therapy devices (MTD) is the possibility of forming magnetic fields that are complexly distributed in space and time, which provides focused treatment of a specific disease, taking into account the individual characteristics of the patient. To increase the throughput of the physiotherapy room, a solution is proposed for organizing the simultaneous treatment of several patients using one device. Within the framework of the study, an approach based on the use of a reconfigurable system was considered, in accordance with which the MTD is built in the form of a set of the same type of intelligent magnetotherapy cells (MTC) and a central control module. This structure is the basis of the hardware component of the MTC, designed for group magnetotherapy. The conceptual model of the MTC includes the following elements: a local control unit, a local interface module, a memory unit, an identification unit, a power interface unit, an inductor unit, and a control unit. The specified composition of MTC components, on the one hand, provides the possibility of autonomous operation of the cell, and on the other hand, it supports centralized control, diagnostics, and simultaneous start and stop of the magnetic field generation process. The main idea of the method for conducting group magnetotherapy is the generation by the central control module of a unified magnetotherapy technique formed on the basis of magnetotherapy techniques, each of which is selected taking into account the required individual treatment of a particular patient. Conventionally, the aggregate of MTC is divided into groups – field-forming systems, each of which is used to treat an individual patient. Each MFT operates in an autonomous mode, which is achieved due to the presence of a local control unit in the cell, which, through the local interface module, receives the data necessary for carrying out a magnetotherapy procedure, and after a group launch independently generates and controls the acting magnetic field. In addition, the analysis of the proposed solution is carried out in the work, as well as issues of possible technical implementation are considered. Thus, the advantage, determined by the autonomous mode of operation of the MTC, allows you to set the duration of the operation of each cell within the framework of the current magnetotherapy procedure, individually. As a result, at the end of a separate magnetotherapy procedure, the MTCs that were used for this procedure can be used to create a field-forming system for treating the next patient, without the need to wait for the end of all active magnetotherapy procedures. In the practical construction of an MTD intended for group magnetotherapy, it is recommended to use microcontrollers as control units, motor drivers as a power interface unit, and specialized DS2411 type microcircuits as an identification unit. One of the possible implementations can be a solution based on an Internet server, then it is recommended to choose Ethernet as the interface, and it is preferable to use the IP address of the MTC as the MTC identifier. The ideas and solutions considered in the framework of the task of implementing group magnetotherapy can be generalized and used for other physical fields used in physiotherapy.

Manifestations in the Solar Corona of the Process of Magnetic-Field Generation

The situation with the reliability of calculations of the magnetic field of the solar corona remains a topic of discussion. There is, in particular, the problem of significantly lower calculated values of the magnetic field than those recorded near the Earth. The authors propose an explanation in which the magnetic fields are underestimated due to the existence of magnetic sources at different distances from the photosphere. Sources that are farther away from the photosphere create local zones of weaker photospheric fields, which are not accurately measured against strong fields from sources of another level. Model estimates of the contributions of sources showed that sources located closer to the photosphere give only 30% of the total contribution of sources of both levels. This value corresponds to the observed ratio of the calculated values of the magnetic field and recorded in the solar wind. This fact confirms the assumption that the magnetic fields of sources farther from the photosphere are not really taken into account in the calculations of the magnetic field of the solar corona.

Renormalization Group Analysis of the Turbulent Hydromagnetic Dynamo: The Effect of Nonstationarity

Abstract The turbulent hydromagnetic dynamo is a process of magnetic field generation by chaotic flow of an electrically conducting fluid (plasma, liquid iron, etc.). It is responsible for generation of large-scale magnetic fields of astrophysical objects such as planets, stars, accretion disks, galaxies, galaxy clusters, etc. In particular, the dynamical process of induction of large-scale fields by highly conducting plasma has been very difficult to understand, as very low resistivity is not capable of creating a phase shift between magnetic and kinetic components of waves, making their interaction ineffective for generation of a large-scale electromotive force (EMF). The aim of the analysis presented here is to demonstrate that when the typically invoked statistical stationarity of turbulence is relaxed, large-scale magnetic fields can be very effectively generated by low-resistivity plasma. The renormalization group technique is applied to extract the final expression for the mean EMF from the fully nonlinear dynamical equations (Navier–Stokes, induction equation), and the mean field equations are solved for a force-free mode. Nonstationarity is shown to strongly enhance the process of large-scale EMF generation via wave interactions, and the dynamo effect induced by nonstationarity is proved to be effective. The results are also used to demonstrate the influence of magnetic fields and nonstationarity on energy and helicity spectra of turbulent flows.

Defying Inertia: How Rotating Superconductors Generate Magnetic Fields

AbstractI discuss the process of magnetic field generation in rotating superconductors in simply connected and multiply connected geometries. In cooling a normal metal into the superconducting state while it is rotating, electrons slow down or speed up depending on the geometry and their location in the sample, apparently defying inertia. I argue that the conventional theory of superconductivity does not explain these processes. Instead, the theory of hole superconductivity does. Its predictions agree with experimental observations of Hendricks, King and Rohrschach for solid and hollow cylinders.

Iron snow, crystal floats, and inner-core growth: modes of core solidification and implications for dynamos in terrestrial planets and moons

Recent planetary space missions, new experimental data, and advanced numerical techniques have helped to improve our understanding of the deep interiors of the terrestrial planets and moons. In the present review, we summarize recent insights into the state and composition of their iron (Fe)-rich cores, as well as recent findings about the magnetic field evolution of Mercury, the Moon, Mars, and Ganymede. Crystallizing processes in iron-rich cores that differ from the classical Earth case (i.e., Fe snow and iron sulfide (FeS) crystallization) have been identified and found to be important in the cores of terrestrial bodies. The Fe snow regime occurs at pressures lower than that in the Earth’s core on the iron-rich side of the eutectic, where iron freezes first close to the core–mantle boundary rather than in the center. FeS crystallization, instead, occurs on the sulfur-rich side of the eutectic. Depending on the core temperature profile and the pressure range considered, FeS crystallizes either in the core center or close to the core–mantle boundary. The consequences of the various crystallizing mechanisms for core dynamics and magnetic field generation are discussed. For the Moon, revised paleomagnetic data obtained with advanced techniques suggest a peculiar history of its internal dynamo, with an early strong field persisting between 4.25 and 3.5 Ga, and subsequently a much weaker field. In addition, the long-lasting dynamo and the possible presence of an inner core, as inferred from a revised interpretation of Apollo seismic data, suggest core crystallization as a viable process of magnetic field generation for a substantial period during lunar evolution. The present-day magnetic fields of Mercury and Ganymede (if they occur on the iron-rich side of the Fe–FeS eutectic) and the related dynamo action are likely generated in the Fe snow regime and seem to be recent features. An earlier dynamo in Mercury would have been powered differently. For Mercury, MESSENGER data further suggest core formation under reducing conditions that may have resulted in an Fe–S–Si composition, further complicating the core crystallization process. Mars, with its early and strong paleo-field, likely has not yet started to freeze out an inner iron core.

Effect of width, amplitude, and position of a core mantle boundary hot spot on core convection and dynamo action

Within the fluid iron cores of terrestrial planets, convection and the resulting generation of global magnetic fields are controlled by the overlying rocky mantle. The thermal structure of the lower mantle determines how much heat is allowed to escape the core. Hot lower mantle features, such as the thermal footprint of a giant impact or hot mantle plumes, will locally reduce the heat flux through the core mantle boundary (CMB), thereby weakening core convection and affecting the magnetic field generation process. In this study, we numerically investigate how parametrised hot spots at the CMB with arbitrary sizes, amplitudes, and positions affect core convection and hence the dynamo. The effect of the heat flux anomaly is quantified by changes in global flow symmetry properties, such as the emergence of equatorial antisymmetric, axisymmetric (EAA) zonal flows. For purely hydrodynamic models, the EAA symmetry scales almost linearly with the CMB amplitude and size, whereas self-consistent dynamo simulations typically reveal either suppressed or drastically enhanced EAA symmetry depending mainly on the horizontal extent of the heat flux anomaly. Our results suggest that the length scale of the anomaly should be on the same order as the outer core radius to significantly affect flow and field symmetries. As an implication to Mars and in the range of our model, the study concludes that an ancient core field modified by a CMB heat flux anomaly is not able to heterogeneously magnetise the crust to the present-day level of north--south asymmetry on Mars. The resulting magnetic fields obtained using our model either are not asymmetric enough or, when they are asymmetric enough, show rapid polarity inversions, which are incompatible with thick unidirectional magnetisation.

Discriminating solar and antisolar differential rotation in high-precision light curves

Surface differential rotation (DR) is one major ingredient of the magnetic field generation process in the Sun and likely in other stars. The term solar-like differential rotation describes the observation that solar equatorial regions rotate faster than polar ones. The opposite effect of polar regions rotating faster than equatorial ones (termed as antisolar DR) has only been observed in a few stars, although there is evidence from theoretical dynamo models. We present a new method to detect the sign of DR (i.e. solar-like or antisolar DR) by analyzing long-term high-precision light curves with the Lomb-Scargle periodogram.We compute the Lomb-Scargle periodogram and identify a set of significant periods $P_k$, which we associate with active regions located at different latitudes on the the stellar surface. If detectable, the first harmonics ($P_k'$) of these periods were identified to compute their peak-height-ratios $r_k:=h(P_k')/h(P_k)$. Spots rotating at lower latitudes generate less sine-shaped light curves, which requires additional power in the harmonics, and results in larger ratios $r_k$. Comparing different ratios $r_k$ and the associated periods $P_k$ yields information about the spot latitudes, and reveals the sign of DR. We tested our method on different sets of synthetic light curves all exhibiting solar-like DR. The number of cases where our method detects antisolar DR is the false-positive rate of our method. Depending on the set of light curves, the noise level, the required minimum peak separation, and the presence or absence of spot evolution, our method fails to detect the correct sign in at most 20%. We applied our method to 50 Kepler G stars and found 21-34 stars with solar-like DR and 5-10 stars with antisolar DR, depending on the minimum peak separation.

Nonstationary contrasting structures in the vicinity of a critical point

Evolution equations of the internal transition layer (ITL) have been derived for the reaction-diffusion equation and for a pseudo-parabolic third-order equation with a small parameter in the highest derivatives, which describes different processes in physics, chemistry, biology, and, in particular, the process of magnetic field generation in the turbulent medium. We consider a case where there is a point with a zero velocity of the ITL drift (critical point), while the drift velocity both on the right and on the left of this point does not change its sign. It is shown that in the case of balanced cubic nonlinearity, which is fairly common in physical applications, the ITL drift velocity in the first-order asymptotic expansion in the power of a small parameter is also zero, but the second-order approximation makes it possible to obtain the drift velocity at the critical point. It is demonstrated that ITL crosses the critical point in finite time.

Formation of sheet plumes, current coils, and helical magnetic fields in a spherical magnetohydrodynamic dynamo

Aiming at understanding of magnetic field generation process in rapidly rotating stars and planets represented by the Earth, computer simulations of magnetohydrodynamic (MHD) dynamo were performed in a rotating spherical shell geometry. Thermal convection and dynamo process with Ekman number of the order of 10−7 were studied. New structures of convection motion, dynamo-generated electrical current, and magnetic field are found. The flow is organized as a set of thin, sheet-like plumes. The current is made of small-scale coil structure with magnetic flux tubes within each of the coil. These flux tubes are connected each other to form a large scale helical magnetic field structure.

Dynamo Models for Planets Other Than Earth

Observations from planetary spacecraft missions have demonstrated a spectrum of dynamo behaviour in planets. From currently active dynamos, to remanent crustal fields from past dynamo action, to no observed magnetization, the planets and moons in our solar system offer magnetic clues to their interior structure and evolution. Here we review numerical dynamo simulations for planets other than Earth. For the terrestrial planets and satellites, we discuss specific magnetic field oddities that dynamo models attempt to explain. For the giant planets, we discuss both non-magnetic and magnetic convection models and their ability to reproduce observations of surface zonal flows and magnetic field morphology. Future improvements to numerical models and new missions to collect planetary magnetic data will continue to improve our understanding of the magnetic field generation process inside planets.

Determining palaeointensity from the Gilbert Gauss Reversal recorded in the Pu’u Heleakala lava section, Wai’anae Volcano, Oahu, Hawaii

The ability of the Earth's magnetic field to reverse is one of its most intriguing characteristics. A full vector (directions and intensity) description during reversals is necessary to fully understand the magnetic field generation process in the Earth's liquid outer core. The reverse to normal Gilbert Gauss reversal (3.58 Ma) is recorded in a 470 m high sequence of lava flows at Pu’u Heleakala, Waianae Volcano, Oahu, Hawaii. 133 samples from 34 flows (sister samples from the directional study of Herrero-Bervera and Coe (J. Geophys. Res., 104 (1999) 29157–29173)) were subjected to microwave demagnetisation experiments. Unfortunately many samples have been overprinted with a lightning induced remanence so that only 65 samples (49%) were deemed suitable for microwave palaeointensity analysis. Both the 8.2 and 14 GHz systems were used with the perpendicular applied field and Coe analogue methods. Individually acceptable results were obtained for 63% of the studied samples however not all flows gave internally consistent results. In total, 10 flows yielded microwave palaeointensity results from 2 or more samples that met the 25% self-consistency criteria with flow mean palaeointensities ranging from 16 to 39 μT. It is pertinent to ascertain if there is any systematic behaviour seen between different reversals. From the sparse data set obtained in this study the behaviour of the field appears to be different to that of the nearby Pu’u Paheehee section that records the Lower Mammoth (3.33 Ma) transition. However, the two sections have different temporal resolution and the uncertainties in the palaeointensity estimates found in this study may account for some of the discrepancies.

Magnetic field filamentation and quasi-acoustic-gravity waves in Hall plasmas at finite gas-to-magnetic pressure ratios

The Hall effect in a plasma flow orthogonal to the magnetic field and invariable in the magnetic field direction can give rise to filamentation of the magnetic flux, splitting it into narrow flux tubes (filaments). This process is due to particular features of the magnetic field generation process in Hall plasmas. The analytical results obtained previously for sharply nonuniform perturbations of the smoothly varying flows of such plasmas are here extended to arbitrary gas-to-magnetic pressure ratios β. The revealed phenomena can be manifested both in laboratory plasmas and under natural conditions (on the Sun).