Dynamo Generation Research Articles

The direction of core solidification in asteroids: Implications for dynamo generation

Paleomagnetic studies of meteorites over the past two decades have revealed that the cores of multiple meteorite parent bodies, including those of certain chondritic groups, generated dynamo fields as they crystallised. However, uncertainties in the direction and mode of core solidification in asteroid-sized bodies have meant using the timings and durations of these fields to constrain parent body properties, such as size, is challenging. Here, we use updated equations of state and liquidus relationships for Fe-FeS liquids at low pressures to calculate the locations at which solids form in these cores. We perform these calculations for core-mantle boundary (CMB) pressures from 0–2 GPa, and Fe-FeS liquid concentrations on the iron-rich side of the eutectic, as well as two values of iron thermal expansivity that cover the measured uncertainties in this parameter, and adiabatic and conductive cooling of these cores. We predict inward core crystallisation from the CMB in asteroids due to their low < 0.5 GPa pressures regardless of the uncertainties in other key core parameters. However, due to low internal pressures in these cores, remelting of any iron snow, as proposed to generate Ganymede’s present-day field, may be unlikely as the cores are approximately isothermal. Therefore a different mode of inward core solidification is possibly required to explain compositionally-driven dynamo action in asteroids. Additionally, we identify possible regimes at higher > 0.6 − 2 GPa pressures in which crystallisation can occur concurrently at the CMB and the centre.

Unlocking planetesimal magnetic field histories: A refined, versatile model for thermal evolution and dynamo generation

The thermal and magnetic histories of planetesimals provide unique insights into the formation and evolution of Earth’s building blocks. These histories can be gleaned from meteorites by using numerical models to translate measured properties into planetesimal behaviour. In this paper, we present a new 1D planetesimal thermal evolution and dynamo generation model. This magnetic field generation model is the first of a differentiated, mantled planetesimal that includes both mantle convection and sub-eutectic core solidification. We have improved fundamental aspects of mantle heat transport by including a more detailed viscosity model and stagnant lid convection parametrisations consistent with internal heating. We have also added radiogenic heating from 60Fe in the metallic Fe-FeS core. Additionally, we implement a combined thermal and compositional buoyancy flux, as well as the latest magnetic field scaling laws to predict magnetic field strengths during the planetesimal’s thermal evolution until core solidification is complete. We illustrate the consequences of our model changes with an example run for a 500km radius planetesimal. These effects include more rapid erosion of core thermal stratification and longer duration of mantle convection compared to previous studies. The additional buoyancy from core solidification has a marginal effect on dynamo strength, but for some initial core sulfur contents it can prevent cessation of the dynamo when mantle convection ends. Our model can be used to investigate the effects of individual parameters on dynamo generation and constrain properties of specific meteorite parent bodies. Combined, these updates mean this model can predict the most reliable and complete magnetic field history for a planetesimal to date, so is a valuable tool for deciphering planetesimal behaviour from meteorite properties.

Small-scale Kazantsev-Kraichnan dynamo in a MHD shell approach

The small-scale magnetic energy generation in a turbulent velocity field is studied by two different approaches. One of them is based on the Kazantsev-Kraichnan model developed for turbulence with short-time velocity correlations, and the other uses the shell model of magnetohydrodynamic turbulence, describing the turbulent energy cascade on a finite number of spectral shells. We have found that the injection of weak magnetic field at the initial moment in both models leads to an exponential growth of magnetic energy and tried to determine whether these effects are of the same or different nature. The investigations have shown that the rates of growths and magnetic energy spectra in two approaches can be very much different, which can be attributed to the contradictions of the model assumptions and unknown correlation time. The discussion of these contradictions allows us to formulate a possible explanation, which is likely related to the fact that the small-scale magnetic field generation is under the influence of some spectral subrange, rather than the entire kinetic spectrum. Varying the correlation time of the velocity field and considering the spectral regions, we have determined the range of kinetic energy spectrum responsible for the small-scale dynamo generation.

A planetary resonant effect in Parker stellar dynamo

The effect of periodic pumping on dynamo generation in the simplest Parker model is studied in this work. Pumping is understood in the sense that the periodic parameters oscillations in the dynamo system leads to a change in the rate of the exponential growth of the mean magnetic field. And since the Parker model simultaneously describes its time oscillations as the field grows, this phenomenon is very similar to parametric resonance in the classical model of a harmonic oscillator. With the help of asymptotic analysis and numerical simulation, we demonstrate both pump regions similar to parametric resonance, as well as different amplification regions at high driving force frequencies, and suppression regions at low frequencies, find the gain maximum and investigate the behavior of the critical pump frequency separating the regions of generation and suppression.

The Elbert range of magnetostrophic convection. I. Linear theory

In magnetostrophic rotating magnetoconvection, a fluid layer heated from below and cooled from above is equidominantly influenced by the Lorentz and the Coriolis forces. Strong rotation and magnetism each act separately to suppress thermal convective instability. However, when they act in concert and are near in strength, convective onset occurs at less extreme Rayleigh numbers (, thermal forcing) in the form of a stationary, large-scale, inertia-less, inviscid magnetostrophic mode. Estimates suggest that planetary interiors are in magnetostrophic balance, fostering the idea that magnetostrophic flow optimizes dynamo generation. However, it is unclear if such a mono-modal theory is realistic in turbulent geophysical settings. Donna Elbert first discovered that there is a range of Ekman (, rotation) and Chandrasekhar (, magnetism) numbers, in which stationary large-scale magnetostrophic and small-scale geostrophic modes coexist. We extend her work by differentiating five regimes of linear stationary rotating magnetoconvection and by deriving asymptotic solutions for the critical wavenumbers and Rayleigh numbers. Coexistence is permitted if and . The most geophysically relevant regime, the Elbert range, is bounded by the Elsasser numbers . Laboratory and Earth’s core predictions both exhibit stationary, oscillatory, and wall-attached multi-modality within the Elbert range.

2022 year experiments at the Riga Dynamo facility

The Riga Dynamo facility was built for laboratory reproduction of the continuously running natural process which creates the entire magnetic field of the environment on the Earth, great planets, the Sun, stars and even galaxies. In all of them the field is created by an intense helical movement in large volumes of an electrically conducting fluid. We use a swirling flow of 2 m3 molten sodium to clarify the principal points of the generation process: scaling rules for both dynamo start and saturated generation, magnetic field profiles in both vertical and horizontal directions and its temporary spectrum, deformation of the environmental field during the start of dynamo. Figs 5, Refs 2.

Energy Generation from Footsteps Using Piezo-Electric Tiles

Piezoelectric generation and Dynamo generation can be used as means of converting waste energy into electrical energy, which can then be processed and used to run various gadgets. An Electric Harvesting Tile has been designed and constructed in this research. Mechanical stress has been applied on a Piezoelectric plate via walking/running. The different configurations of the Piezo-plates determine the electrical output. The configurations used in this study require twenty-four Piezo-plates, configured with four banks in parallel to each other. Each bank consists of six Piezo-plates connected in series. This configuration allows maximum energy to be generated since placing the four banks in parallel increases the current produced. The six Piezo-plates in each bank are configured in series since when piezoelectric plates are connected in series, the internal resistance will increase, and the voltage will be more stable. Using this configuration, a comparison has been made between the weight applied at the center and the external boundaries of the tile. It has been discovered that the voltage, the energy, the power, and the current have been higher for the weight applied at the center of the Tile. This increase can be attributed to the fact that applying force at the center ensures that an even force is applied to all the crystals, thereby producing higher voltage. Force applied to other sections indicates a lower voltage reading because the pressure is not applied to all the Piezo-plates, since their configuration includes four banks in parallel and each one consists of 6 Piezo-plates in series. Therefore, if pressure is not applied to one of Piezo-plates in series then the entire bank is open and no voltage is produced. It has been also discovered that the total voltage stored in the capacitor is the highest with the largest force applied since increasing the force increases the displacement of atoms, resulting in a higher voltage produced. The energy generated has been stored in a rechargeable battery, which has been used to power a mobile device.

Evolution of localized magnetic field perturbations and the nature of turbulent dynamo

Kinematic dynamo in incompressible isotropic turbulent flows with high magnetic Prandtl number is considered. The approach interpreting an arbitrary magnetic field distribution as a superposition of localized perturbations (blobs) is developed. We derive a general relation between stochastic properties of an isolated blob and a stochastically homogenous distribution of magnetic field advected by the same stochastic flow. This relation allows us to investigate the evolution of a localized blob at a late stage when its size exceeds the viscous scale. It is shown that in three-dimensional flows, the average magnetic field of the blob increases exponentially in the inertial range of turbulence, as opposed to the late-batchelor stage when it decreases. Our approach reveals the mechanism of dynamo generation in the inertial range both for blobs and homogenous contributions. It explains the absence of dynamo in the two-dimensional case and its efficiency in three dimensions. We propose a way to observe the mechanism in numerical simulations.

The Thermal Evolution of Planetesimals During Accretion and Differentiation: Consequences for Dynamo Generation by Thermally‐Driven Convection

AbstractThe meteorite paleomagnetic record indicates that differentiated (and potentially, partially differentiated) planetesimals generated dynamo fields in the first 5–40 Myr after the formation of calcium‐aluminum‐rich inclusions (CAIs). This early period of dynamo activity has been attributed to thermal convection in the liquid cores of these planetesimals during an early period of magma ocean convection. To better understand the controls on thermal dynamo generation in planetesimals, we have developed a one dimensional model of the thermal evolution of planetesimals from accretion through to the shutdown of convection in their silicate magma oceans for a variety of accretionary scenarios. The heat source of these bodies is the short‐lived radiogenic isotope 26Al. During differentiation, 26Al partitions into the silicate portion of these bodies, causing their magma oceans to heat up and introducing stable thermal stratification to the top of their cores, which inhibits dynamo generation. In “instantaneously” accreting bodies, this effect causes a delay on the order of &gt;10 Myr to whole core convection and dynamo generation while this stratification is eroded. However, gradual core formation in bodies that accrete over &gt;0.1 Myr can minimize the development of this stratification, allowing dynamo generation from ∼4 Myr after CAI formation. Our model also predicts partially differentiated planetesimals with a core and mantle overlain by a chondritic crust for accretion timescales &gt;1.2 Myr, although none of these bodies generate a thermal dynamo field. We compare our results from thousands of model runs to the meteorite paleomagnetic record to constrain the physical properties of their parent bodies.

Constraining the Decline of the Lunar Dynamo Field at ≈3.1 Ga Through Paleomagnetic Analyses of Apollo 12 Mare Basalts

AbstractRecent paleomagnetic studies of lunar rocks have suggested that the magnetic field of the Moon reached peak intensities on the order of ≈77 μT between 3.85 billion and 3.56 billion years ago (Ga) and subsequently declined to surface intensities of ≈4 μT by 3.19 Ga. However, this decline in the intensity of the lunar field has only been shown in a small number of samples, presenting challenges for constraint of its timing and thus the dynamo generation mechanisms that could be responsible. We present microscopic and magnetic analyses of Apollo samples 12008, 12009, and 12015, three fine‐grained mare vitrophyre basalts with high magnetic fidelity, indicating that these samples were not magnetized in conditions consistent with a planetary magnetic field exceeding 4–7 μT during their formation. We further report updated radiometric ages for samples 12008 and 12009 and the first‐ever radiometric age for sample 12015, dating this lunar field intensity constraint to ≈3.1 Ga. These data are consistent with results of previous work on the initial decline of the Moon's magnetic field and confirm that the mechanism of lunar dynamo generation changed dramatically between 3.6 and 3.1 Ga.

Thermal conductivities of solid and molten silicates: Implications for dynamos in mercury-like proto-planets

Remanent magnetization and active magnetic fields have been detected for several telluric planetary bodies in the solar system (Earth, Mercury, Moon, Mars) suggesting the presence of core dynamos active at the early stages of the planet formation and variable lifetimes. Among the factors controlling the possibility of core dynamos generation, the dynamics of the surrounding silicate mantle and its associated thermal properties are crucial. The mantle governs the heat evacuation from the core and as a consequence the likeliness of an early thermally driven dynamo. In the case of planets with a thick mantle (associated with supercritical Rayleigh numbers), the core heat is efficiently removed by mantle convection and early thermally-driven dynamos are likely. At the opposite, planets with a thin mantle (associated with subcritical Rayleigh numbers) might evacuate their inner heat by diffusion only, making early thermally-driven dynamos difficult. Within the Solar System, Mercury is a potential example of such a regime. Its small mantle thickness over the planet radius ratio might be inherent to its small orbital semi-axis and hence, might be ubiquitous among the terrestrial objects formed close to their star.To constrain the likeliness of a thermally driven dynamo on “Mercury-like” planets (i.e. with large Rc/R), we present new thermal diffusivity measurements of various solid, glassy and molten samples. We applied the Angstrom method on cylindrical samples during multi-anvil apparatus experiments at pressures of 2 GPa and temperatures up to 1700 K. Thermal diffusivities and conductivities were estimated for solid and partially molten peridotites, with various melt fractions, and for basaltic and rhyolitic glasses and melts. Our study demonstrates that melts have similar thermal properties despite a broad range of composition investigated. The melts reveal much lower thermal conductivities than the solids with almost an order of magnitude of decrease: 1.70 (±0.19) to 2.29 (±0.26) W/m/K against 0.18 (±0.01) to 0.41 (±0.03) W/m/K for peridotites at high temperatures and various melts respectively. Partially molten samples lie in between and several predictive laws are proposed as a function of the melt fraction and solid/melt texture.Using our results into forward calculations of heat fluxes for dynamo generation for Mercury-like planets, we quantify the effect of mantle melting on the occurrence of thermally driven dynamos. The presence of a mushy mantle and partial melting could significantly reduce the ability of the mantle to evacuate the heat from the core and can prevent, shut or affect the presence of a planetary magnetic field. The buoyancy and fate of molten material in such bodies can thus influence the magnetic history of the planet. Future observations of Mercury-like planets accreted near their star and the detections of their magnetic signatures could provide constraints on their inner state and partial melting histories.

Oscillatory thermal–inertial flows in liquid metal rotating convection

Abstract

No-z model: results and perspectives for accretion discs

Accretion discs surround different compact astrophysical objects such as black holes, neutron stars and white dwarfs. Also they are situated in systems of variable stars and near the galaxy center. Magnetic fields play an important role in evolution and hydrodynamics of the accretion discs: for example, they can describe such processes as the transition of the angular momentum. There are different approaches to explain the magnetic fields, but most interesting of them are connected with dynamo generation. As for disc, it is quite useful to take no-$z$ approximation which has been developed for galactic discs to solve the dynamo equations. It takes into account that the disc is quite thin, and we can solve the equations only for two plane components of the field. Here we describe the time dependence of the magnetic field for different distances from the center of the disc. Also we compare the results with another approaches which take into account more complicated field structure.

Large H2O solubility in dense silica and its implications for the interiors of water-rich planets

Sub-Neptunes are common among the discovered exoplanets. However, lack of knowledge on the state of matter in [Formula: see text]O-rich setting at high pressures and temperatures ([Formula: see text]) places important limitations on our understanding of this planet type. We have conducted experiments for reactions between [Formula: see text] and [Formula: see text]O as archetypal materials for rock and ice, respectively, at high [Formula: see text] We found anomalously expanded volumes of dense silica (up to 4%) recovered from hydrothermal synthesis above ∼24 GPa where the [Formula: see text]-type (Ct) structure appears at lower pressures than in the anhydrous system. Infrared spectroscopy identified strong OH modes from the dense silica samples. Both previous experiments and our density functional theory calculations support up to 0.48 hydrogen atoms per formula unit of ([Formula: see text])[Formula: see text] At pressures above 60 GPa, [Formula: see text]O further changes the structural behavior of silica, stabilizing a niccolite-type structure, which is unquenchable. From unit-cell volume and phase equilibrium considerations, we infer that the niccolite-type phase may contain H with an amount at least comparable with or higher than that of the Ct phase. Our results suggest that the phases containing both hydrogen and lithophile elements could be the dominant materials in the interiors of water-rich planets. Even for fully layered cases, the large mutual solubility could make the boundary between rock and ice layers fuzzy. Therefore, the physical properties of the new phases that we report here would be important for understanding dynamics, geochemical cycle, and dynamo generation in water-rich planets.

High pressure melt curve of iron from atom-in-jellium calculations

Although usually considered as a technique for predicting electron states in dense plasmas, atom-in-jellium calculations can be used to predict the mean displacement of the ion from its equilibrium position in colder matter, as a function of compression and temperature. The Lindemann criterion of a critical displacement for melting can then be employed to predict the melt locus, normalizing for instance to the observed melt temperature or to more direct simulations such as molecular dynamics (MD). This approach reproduces the high pressure melting behavior of Al as calculated using the Lindemann model and thermal vibrations in the solid. Applied to Fe, we find that it reproduces the limited-range melt locus of a multiphase equation of state (EOS) and the results of ab initio MD simulations, and agrees less well with a Lindemann construction using an older EOS. The resulting melt locus lies significantly above the older melt locus for pressures above 1.5\,TPa, but is closer to recent ab initio MD results and extrapolations of an analytic fit to them. This study confirms the importance of core freezing in massive exoplanets, predicting that a slightly smaller range of exoplanets than previously assessed would be likely to exhibit dynamo generation of magnetic fields by convection in the liquid portion of the core.

Study of MHD dynamo effect at the outlet from plasma accelerator in the presence of longitudinal magnetic field

AbstractProperties of compressible flows in the quasi‐stationary plasma accelerator have been studied in the presence of an additional longitudinal magnetic field and the arising rotation of plasma flow. Numerical study was carried out within the framework of two‐dimensional magnetic hydrodynamics (MHD) model of the axisymmetric plasma flows taking into account the finite conductivity of medium and radiation transport. Dynamics of compressible plasma flows is accompanied by the MHD dynamo effect or generation of magnetic field on a conical shock wave forming at the outlet from the accelerator.

Identification of the mechanisms responsible for anomalies in the tropical lower thermosphere/ionosphere caused by the January 2009 sudden stratospheric warming

We apply the Entire Atmosphere GLobal (EAGLE) model to investigate the upper atmosphere response to the January 2009 sudden stratospheric warming (SSW) event. The model successfully reproduces neutral temperature and total electron content (TEC) observations. Using both model and observational data, we identify a cooling in the tropical lower thermosphere caused by the SSW. This cooling affects the zonal electric field close to the equator, leading to an enhanced vertical plasma drift. We demonstrate that along with a SSW-related wind disturbance, which is the main source to form a dynamo electric field in the ionosphere, perturbations of the ionospheric conductivity also make a significant contribution to the formation of the electric field response to SSW. The post-sunset TEC enhancement and pre-sunrise electron content reduction are revealed as a response to the 2009 SSW. We show that at post-sunset hours the SSW affects low-latitude TEC via a disturbance of the meridional electric field. We also show that the phase change of the semidiurnal migrating solar tide (SW2) in the neutral wind caused by the 2009 SSW at the altitude of the dynamo electric field generation has a crucial importance for the SW2 phase change in the zonal electric field. Such changes lead to the appearance of anomalous diurnal variability of the equatorial electromagnetic plasma drift and subsequent low-latitudinal TEC disturbances in agreement with available observations.Plain Language Summary– Entire Atmosphere GLobal model (EAGLE) interactively calculates the troposphere, stratosphere, mesosphere, thermosphere, and plasmasphere–ionosphere system states and their response to various natural and anthropogenic forcing. In this paper, we study the upper atmosphere response to the major sudden stratospheric warming that occurred in January 2009. Our results agree well with the observed evolution of the neutral temperature in the upper atmosphere and with low-latitude ionospheric disturbances over America. For the first time, we identify an SSW-related cooling in the tropical lower thermosphere that, in turn, could provide additional information for understanding the mechanisms for the generation of electric field disturbances observed at low latitudes. We show that the SSW-related vertical electromagnetic drift due to electric field disturbances is a key mechanism for interpretation of an observed anomalous diurnal development of the equatorial ionization anomaly during the 2009 SSW event. We demonstrate that the link between thermospheric winds and the ionospheric dynamo electric field during the SSW is attained through the modulation of the semidiurnal migrating solar tide.

Dynamo generation of a magnetic field by decaying Lehnert waves in a highly conducting plasma

Random waves in a uniformly rotating plasma in the presence of a locally uniform seed magnetic field and subject to weak kinematic viscosity and resistivity are considered. These “Lehnert” waves may have either positive or negative helicity, and it is supposed that waves of a single sign of helicity are preferentially excited by a symmetry-breaking mechanism. A mean electromotive force proportional to is derived, demonstrating the conflicting effects of the two diffusive processes. Attention is then focussed on the situation , relevant to conditions in the universe before and during galaxy formation. An -effect, axisymmetric about the rotation vector, is derived, decaying on a time-scale proportional to ; this amplifies a large-scale seed magnetic field to a level independent of , this field being subsequently steady and having the character of a “fossil field”. Subsequent evolution of this fossil field is briefly discussed.

Long-term stellar magnetic field study at the Crimean Astrophysical Observatory

AbstractThe long-term monitoring of magnetic cycles is a key diagnostic in understanding how dynamo generation and amplification of magnetic fields occur in solar-like stars. One of the current key problems is the establishment of the magnetic field behavior during the activity cycles for stars of different ages and evolutionary statuses. We present the experience of using own long-term datasets for study of activity cycles in selected stars at the Crimean Astrophysical Observatory.

On the magnetic activity cycle and rotation of the yellow subgiant β Aql

The cycles of activity similar to the solar one are thought to exist in other stars with outer convection zones. The long‐term monitoring of magnetic cycles in stars similar in structure to the Sun is a main diagnostic method for understanding how dynamo generation and amplification of magnetic fields occur. We have performed a search for solar‐like magnetic activity on the yellow subgiant β Aql. Direct measurements of the longitudinal magnetic field of β Aql were performed by measuring the Zeeman splitting in spectral lines, using the circularly polarized spectra obtained at the Crimean Astrophysical Observatory over 51 nights during 1997–2015. The magnetic field on β Aql was detected with the confidence level above 3σ over 24 nights. The activity cycle of β Aql was found to be 969 ± 27 days. We assume that the activity of β Aql is similar to that of stars younger than the Sun. The most probable rotation period was found to be Prot = 5.08697 ± 0.00031 days. Assuming the global magnetic field of β Aql as a dipole, we estimated the polar field strength Bpol = 24 G, the angle between the rotation axis and the line of sight i = 25°, and the angle between the rotation and dipole axes β = 96°.