Active Dynamo Research Articles

Experimental observations of detached bow shock formation in the interaction of a laser-produced plasma with a magnetized obstacle

The magnetic field produced by planets with active dynamos, like the Earth, can exert sufficient pressure to oppose supersonic stellar wind plasmas, leading to the formation of a standing bow shock upstream of the magnetopause, or pressure-balance surface. Scaled laboratory experiments studying the interaction of an inflowing solar wind analog with a strong, external magnetic field are a promising new way to study magnetospheric physics and to complement existing models, although reaching regimes favorable for magnetized shock formation is experimentally challenging. This paper presents experimental evidence of the formation of a magnetized bow shock in the interaction of a supersonic, super-Alfvénic plasma with a strongly magnetized obstacle at the OMEGA laser facility. The solar wind analog is generated by the collision and subsequent expansion of two counter-propagating, laser-driven plasma plumes. The magnetized obstacle is a thin wire, driven with strong electrical currents. Hydrodynamic simulations using the FLASH code predict that the colliding plasma source meets the criteria for bow shock formation. Spatially resolved, optical Thomson scattering measures the electron number density, and optical emission lines provide a measurement of the plasma temperature, from which we infer the presence of a fast magnetosonic shock far upstream of the obstacle. Proton images provide a measure of large-scale features in the magnetic field topology, and reconstructed path-integrated magnetic field maps from these images suggest the formation of a bow shock upstream of the wire and as a transient magnetopause. We compare features in the reconstructed fields to two-dimensional MHD simulations of the system.

Depth of Martian Magnetization From Localized Power Spectrum Analysis

AbstractMagnetic field measurements show that Mars possesses strong crustal magnetic anomalies that formed when the planet had an active dynamo. To investigate the origin of this magnetization, we used localized power spectrum analyses to constrain the equivalent depths of thin magnetic layers in the crust. Using a new martian magnetic field model that incorporates data from both the Mars Atmosphere and Volatile EvolutioN and Mars Global Surveyor missions, we found that the equivalent magnetization depths on Mars vary from the surface to 72 km. In the northern hemisphere the magnetization depths are found to be on average 9 km, whereas in the southern hemisphere the depths are on average 32 km. If these depths are interpreted in terms of a thick magnetic layer, magnetization in the northern lowlands could extend from the surface to about 18 km depth, whereas for the southern highlands the magnetic layer could extend from about 20 km depth to the base of the crust. The strongest magnetic anomalies are, in general, associated with deep source depths. Our results are consistent with the hypothesis where strong primordial magnetic materials of the northern hemisphere were excavated from the Borealis impact basin, leaving only strong deep remanent magnetization in the southern hemisphere.

Paleointensity and Rock Magnetism of Martian Nakhlite Meteorite Miller Range 03346: Evidence for Intense Small‐Scale Crustal Magnetization on Mars

AbstractThe martian dynamo’s strength and duration are essential for understanding Mars' habitability and deep interior dynamics. Although most northern volcanic terranes were likely emplaced after the martian dynamo ceased, recent data from the InSight mission show stronger than predicted crustal fields. Studying young volcanic martian meteorites offers a precise, complementary method to characterize the strength of the martian crustal field and examine its implications for past dynamo activity. We present the first rock and paleomagnetic study of nine mutually oriented samples from the martian Nakhlite meteorite Miller Range (MIL) 03346, which is well‐suited for paleomagnetic analysis due to its well‐known age (1,368 ± 83 Ma) and lack of significant aqueous, thermal, and shock overprinting. Rock magnetic analysis, including quantum diamond microscope imaging, showed that the natural remanent magnetization (NRM) is carried by Ti‐magnetite crystals containing µm‐scale ilmenite exsolution lamellae, which can accurately record ancient magnetic fields. Demagnetization of the NRM revealed a high coercivity magnetization interpreted to date from the age of eruption based on its intensity, unidirectionality, and a passing fusion crust baked contact test. Paleointensities of four samples reveal a 5.1 ± 1.5 µT paleofield, representing the most reliable martian paleointensity estimates to‐date and stronger than the 2 µT surface fields measured by InSight. Modeling shows that the observed fields can be explained by an older subsurface magnetized layer without a late, active dynamo and support a deeply buried, highly magnetized crust in the northern hemisphere of Mars. These results provide corroborating evidence for strong, small‐scale crustal fields on Mars.

End of magmatism in the upper crust of asteroid 4 Vesta

AbstractAsteroid 4 Vesta is the only largely preserved differentiated asteroid and thus is an excellent proxy to study early magmatism occurring on planets and moons. In this study, we focus on eucrite Pecora Escarpment (PCA) 82502, a medium‐ to fine‐grained eucrite which chemical analyses suggest belongs to the main howardite–eucrite–diogenite clan, albeit with some peculiarities. We carried out backscattered electron and electron backscattered diffraction microscopy analyses of the meteorite along with step‐heating 40Ar/39Ar dating analyses of various types of groundmass aliquots. We show that Pecora Escarpment 82502 is composed of medium‐grained igneous crystalline clasts and smaller fractured satellite clasts surrounded by approximately 50 µm wide impact melt veins of the same composition. Our results show that the large crystalline clasts and fine‐grained veins both display little evidence of shock processes. Six groundmass aliquots from large crystalline clasts returned concordant plateau (>70% of 39Ar) or mini‐plateau (50–70% of 39Ar) 40Ar/39Ar ages with a weighted mean of 4531 ± 6 Ma (P = 0.67). Thermodynamic cooling and 40Ar diffusion models suggest that the K/Ar system recorded and preserved the igneous age despite subsequent infiltration of hot and quickly quenched melt veins. Our new igneous age, combined with evidence for four other young volcanic and plutonic eucrites of similar age, shows that Vesta was still magmatically active around 4531 Ma. The lack of younger ages suggests that this age might well represent the end of the magmatic activity in the upper crust of Vesta. When combined with existing paleomagnetic constraints, our data suggest that 4 Vesta had an active dynamo that was still active ~35 Ma after accretion.

Thermal Conductivity of FeS and Its Implications for Mercury's Long‐Sustaining Magnetic Field

AbstractThe MESSENGER mission revealed that Mercury's magnetic field might have operated since 3.7–3.9 Ga. While the intrinsic magnetism suggests an active dynamo within Mercury's core, the mechanism that is responsible for sustaining the dynamo for prolonged period of time remains unknown. Here we investigated the electrical conductivity of Fe‐S alloys at pressure of 8 GPa and temperatures up to 1,700 K. We show that the electrical conductivity of Fe‐S alloys at 1,500 K is about 103 S/m, 2 orders of magnitude lower than the previously assumed value for dynamo calculations. The thermal conductivity was estimated using the Wiedemann‐Franz law. The total thermal conductivity of FeS is estimated to be ~4 Wm/K at the Mercurian core‐mantle boundary conditions. The low thermal conductivity suggests that a thermally driven dynamo operating on Mercury is more likely than expected. If coupled with chemical buoyancy sources, it is possible to sustain an intrinsic dynamo during time scales compatible with the MESSENGER observations.

Time variation of Jupiter’s internal magnetic field consistent with zonal wind advection

Determination of the time dependency (secular variation) of a planet’s magnetic field provides a window into understanding the dynamo responsible for generating its field. However, of the six Solar System planets with active dynamos, secular variation has been firmly established only for Earth. Here, we compare magnetic field observations of Jupiter from the Pioneer 10 and 11, Voyager 1 and Ulysses spacecraft (acquired 1973–1992) with a new Juno reference model (JRM09). We find a consistent, systematic change in Jupiter’s field over this 45-year time span, which cannot be explained by changes in the magnetospheric field or by changing the assumed rotation rate of Jupiter. Through a simplified forward model, we find that the inferred change in the field is consistent with advection of the field by Jupiter’s zonal winds, projected down to 93–95% of Jupiter’s radius (where the electrical conductivity of the hydrogen envelope becomes sufficient to advect the field). This result demonstrates that zonal wind interactions with Jupiter’s magnetic field are important and lends independent support to atmospheric and gravitational-field determinations of the profile of Jupiter’s deep winds.

Could giant basin-forming impacts have killed Martian dynamo?

The observed strong remanent crustal magnetization at the surface of Mars suggests an active dynamo in the past and ceased to exist around early to middle Noachian era, estimated by examining remagnetization strengths in extant and buried impact basins. We investigate whether the Martian dynamo could have been killed by these large basin-forming impacts, via numerical simulation of subcritical dynamos with impact-induced thermal heterogeneity across the core-mantle boundary. We find that subcritical dynamos are prone to the impacts centered on locations within 30° of the equator but can easily survive those at higher latitudes. Our results further suggest that magnetic timing places a strong constraint on postimpact polar reorientation, e.g., a minimum 16° polar reorientation is needed if Utopia is the dynamo killer.

Planetary magnetic fields: Observations and models

The state of knowledge and understanding of planetary magnetic fields is reviewed. All of the planets, with the possible exception of Venus, have had active dynamos at some time in their evolution. The properties and characteristics of the dynamos are as diverse as the planets themselves. Even within the subclasses of terrestrial and giant planets, the contrasting compositions, sizes, and internal pressures and temperatures of the planets result in strikingly different dynamos. As an example, the dynamos in Mercury and Ganymede are likely driven by compositional buoyancy distributions different from that in the Earth’s core. Dynamo models operate far from the parameter regimes appropriate to the real planets, yet provide insight into the dynamics of their interiors. While Boussinesq models are generally adequate for simulating terrestrial planet dynamos, anelastic models that also account for large density and electrical conductivity variations are needed to simulate the dynamos in giant planets. Future spacecraft missions to planets with active dynamos are needed to learn about the character and temporal variability of the planetary magnetic fields.

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.

Size and compositional constraints of Ganymede's metallic core for driving an active dynamo

Ganymede has an intrinsic magnetic field which is generally considered to originate from a self-excited dynamo in the metallic core. Driving of the dynamo depends critically on the satellite's thermal state and internal structure. However, the inferred structure based on gravity data alone has a large uncertainty, and this makes the possibility of dynamo activity unclear; variations in core size and composition significantly change the heat capacity and alter the cooling history of the core. The main objectives of this study is to explore the structural conditions for a currently active dynamo in Ganymede using numerical simulations of the thermal history, and to evaluate under which conditions Ganymede can maintain the dynamo activity at present. We have investigated the satellite's thermal history using various core sizes and compositions satisfying the mean density and moment of inertia of Ganymede, and evaluate the temperature and heat flux at the core–mantle boundary (CMB). Based on the following two conditions, we evaluate the possibility of dynamo activity, thereby reducing the uncertainty of the previously inferred interior structure. The first condition is that the temperature at the CMB must exceed the melting point of a metallic core, and the second is that the heat flux through the CMB must exceed the adiabatic temperature gradient. The mantle temperature starts to increase because of the decay of long-lived radiogenic elements in the rocky mantle. After a few Gyr, radiogenic elements are exhausted and temperature starts to decrease. As the rocky mantle cools, the heat flux at the CMB steadily increases. If the temperature and heat flux at the CMB satisfy these conditions simultaneously, we consider the case as capable of driving a dynamo. Finally, we identify the Dynamo Regime, which is the specific range of internal structures capable of driving the dynamo, based on the results of simulations with various structures. If Ganymede's self-sustained magnetic field were maintained by thermal convection, the satellite's metallic core would be relatively large and, in comparison to other terrestrial-type planetary cores, strongly enriched in sulfur. The dynamo activity and the generation of the magnetic field of Ganymede should start from a much later stage, possibly close to the present.

Numerical simulation of Martian historical dynamo: Impact of the Rayleigh number on the dynamo state

The observed Mars remnant magnetism suggests that there was an active dynamo in the Martian core. We use the MoSST core dynamics model to simulate the Martian historical dynamo, focusing on the variation of the dynamo states with the Rayleigh number Ra (a non-dimensional parameter describing the buoyancy force in the core). Our numerical results show that the mean field length scale does not vary monotonically with the Rayleigh number, and the field morphology at the core mantle boundary changes with Rayleigh number. In particular, it drifts westward with a speed decreasing with Rayleigh number.

Magnetic helicity and the relaxation of fossil fields

In the absence of an active dynamo, purely poloidal magnetic field configurations are unstable to large-scale dynamical perturbations, and decay via reconnection on an Alfvenic time-scale. Nevertheless, a number of classes of dynamo-free stars do exhibit significant, long-lived, surface magnetic fields. Numerical simulations suggest that the large-scale poloidal field in these systems is stabilized by a toroidal component of the field in the stellar interior. Using the principle of conservation of total helicity, we develop a variational principle for computing the structure of the magnetic field inside a conducting sphere surrounded by an insulating vacuum. We show that, for a fixed total helicity, the minimum energy state corresponds to a force-free configuration. We find a simple class of axisymmetric solutions, parametrized by angular and radial quantum numbers. However, these solutions have a discontinuity in the toroidal magnetic field at the stellar surface which will exert a toroidal stress on the surface of the star. We then describe two other classes of solutions, the standard spheromak solutions and ones with fixed surface magnetic fields, the latter being relevant for neutron stars with rigid crusts. We discuss the implications of our results for the structure of neutron star magnetic fields, the decay of fields, and the origin of variability and outbursts in magnetars.

The effects of explosive-driven shocks on the natural remanent magnetization and the magnetic properties of rocks

The effects of shock waves on the natural remanent magnetization and the intrinsic magnetic properties of geological materials remain poorly known. Still, hypervelocity impact phenomena are of primary importance in the evolution of many extraterrestrial bodies and of Earth. We present new experiments in which four rocks with different lithology and magnetic mineralogy (titanomagnetite, magnetite, monoclinic pyrrhotite, titanohematite) were shocked using a high-order explosive (penthrite) that provided maximum pressure of about 30 GPa. The shock wave was modelled numerically and we studied the effects on the natural remanent magnetization as well as on the intrinsic magnetic properties of the shocked rocks as a function of the distance to the explosion. The intrinsic magnetic properties of the rock are permanently modified by the shock wave. The shock wave was able to superimpose a new fabric (with a minimum susceptibility axis parallel to the direction of shock) to the original magnetic fabric of the rock. Magnetite-, titanomagnetite- and pyrrhotite-bearing rocks show a noticeable increase of coercivity for pressure above 10 GPa. These changes are attributed to fracturing and/or dislocations of the ferromagnetic grains. These results show that the magnetic properties of meteorites, which are commonly shocked to pressures well above 10 GPa (e.g. Martian meteorites), may not be representative of the magnetic properties of their parent body. Natural remanent magnetization appears to be much more resistant to shock than isothermal remanent magnetization, probably because it is carried by grains with higher coercivity. For titanomagnetite-bearing rocks, we observe both a partial shock demagnetization of the original thermoremanent magnetization and the appearance of a shock magnetization. The demagnetizing effect of the shock wave depends closely on the coercivity spectrum of the grains carrying the original remanence, and the shock-remagnetizing effect depends on the presence of low-coercivity magnetic grains. With an impact occurring in an ambient magnetic field of similar intensity to the original magnetizing field, the post-shock magnetization may be higher or lower than the pre-shock magnetization depending on these two factors. The magnetic anomalies observed above impact basins are often used as a proxy to the presence or absence of an active dynamo at the time of impact. Our results show that the magnetic anomalies associated to shocked rocks can be misleading, so that the only decisive clue to the presence or absence of a dynamo at the time of impact is the possible thermoremanence carried by the volume of rock heated above blocking temperatures during the impact.

Numerical dynamo models of Uranus' and Neptune's magnetic fields

The non-axisymmetric, non-dipolar magnetic fields of Uranus and Neptune are markedly different from the axially-dipolar dominated fields of the other planets in our Solar System with active dynamos. Stanley and Bloxham [Stanley, S., Bloxham, J., 2004. Nature 428, 151–153] used numerical modeling to demonstrate that Uranus' and Neptune's unusual fields could be the result of a different convective region geometry in these planets. They found that a numerical dynamo operating in a thin shell surrounding a stably-stratified fluid interior produces magnetic field morphologies similar to those of Uranus and Neptune. This geometry for the convective region was initially proposed by Hubbard et al. [Hubbard, W.B., Podolak, M., Stevenson, D.J., 1995. In: Cruickshank, D. (Ed.), Neptune and Triton. Univ. of Arizona Press, Tucson, pp. 109–138] to explain both the magnetic field morphology as well as the low intrinsic heat flows from these planets. Here we examine the influence of varying the stable layer radius in numerical models and compare the results to thin shell models surrounding solid inner cores. We find that a limited range of stable-layer shell thicknesses exist in which Uranus/Neptune-like field morphologies result. This allows us to put constraints on the size of the convective layers in Uranus and Neptune.

Unusual magnetic signature of the Hadriaca Patera Volcano: Implications for early Mars

Typically, Martian volcanoes show either a total absence of crustal magnetism or a local magnetic mimimum. Hadriaca Patera is the only volcano on Mars with a clear positive magnetic anomaly directly over the volcanic edifice, as determined by electron reflection magnetometry. Hadriaca's topography, lava flow crater ages, gravity anomaly, position relative to the Hellas rim and past aqueous environment provide geological constraints on possible scenarios to explain this unusual magnetic signature. The two most probable scenarios are a) pre‐Hellas thermoremanent magnetization (TRM) of a deep‐seated magma body followed by post‐Hellas intrusive partial thermal demagnetization and b) post‐Hellas emplacement of the volcano and TRM in a global magnetic field. It is thus likely that Mars had at least one of the following: highland volcanism in pre‐Hellas times (older than any datable volcanic feature on the planet) or an active dynamo some time after the Hellas impact.

Mars Crustal Magnetism

Mars lacks a detectable magnetic field of global scale, but boasts a rich spectrum of magnetic fields at smaller spatial scales attributed to the spatial variation of remanent magnetism in the crust. On average the Mars crust is 10 times more intensely magnetized than that of the Earth. It appears likely that the Mars crust acquired its remanence in the first few hundred million years of evolution when an active dynamo sustained an intense global field. An early dynamo era, ending in the Noachian, or earliest period of Mars chronology, would likely be driven by thermal convection in an early, hot, fluid core. If crustal remanence was acquired later in Mars history, a dynamo driven by chemical convection associated with the solidification of an inner core is likely. Thermal evolution models cannot yet distinguish between these two possibilities. The magnetic record contains a wealth of information on the thermal evolution of Mars and the Mars dynamo, but we have just begun to decipher its message.

The dynamics of planetary magnetospheres

Mercury, Earth, Jupiter, Saturn, Uranus, Neptune, and the moon, Ganymede, have presently-active internal dynamos while Venus, Mars, at least two of the Galilean moons, the Earth's moon, comets and asteroids do not. These active dynamos produce magnetic fields that have sufficient strength to stand off the pressure of the exterior plasma environment. Because of changes in these exterior plasma environments these magnetospheres are very dynamic. The jovian magnetosphere includes a strong time-varying energy source that adds to the dynamics of its magnetosphere and produces a quite different circulation pattern than that found at Earth and, presumably, Mercury. Not only intrinsic planetary magnetic fields produce magnetospheres but also unmagnetized planets. Venus, Mars and comets have induced magnetospheres associated with the solar wind interaction with their atmospheres. Cometary magnetospheres, parts of which can be remotely sensed, exhibit spectacular disruptions called tail disconnections. Even the atmosphereless bodies with weak magnetic fields can interact with the solar wind. Small magnetic anomalies on the moon and possibly asteroids cause weak deflections of the solar wind. The dynamics of these various magnetospheres provide a rich spectrum of behavior which we review herein.

Mars — a planet in magnetic transition?

Planet Mars is currently an object of increasing scientific and public interest. Several spacecraft are on their way or planned to be sent to our neighbour planet to unravel its secrets. One of the most unknown properties of planet Mars is whether it possesses a planetary magnetic field of dynamo origin. Observed weak surface magnetic fields seem to argue against a currently operating planetary dynamo. However, weak surface fields are also expected if a polarity reversal occurs. We shall discuss arguments in favour and against the speculation that Mars still operates an active dynamo with the weak surface magnetic fields pointing towards an ongoing reversal. As the possible way to validate this speculation we discuss the importance of a spherical harmonic expansion of the Martian magnetic field. The planned NetLander project and its anticipated magnetic field measurements are a powerful tool to provide at least a low-order spherical harmonic expansion coefficient determination of the Martian magnetic field, thereby allowing to study characteristics of crustal magnetic field anomalies as well as giving clues to the question whether Mars is a planet in magnetic transition.

Mars: a magnetic field due to thermoremanence?

Presently available magnetic field data suggest a dipole moment of Mars of less than 10 −4 times the Earth's dipole moment. Presumably, Mars does not have an active dynamo at present which could give rise to a significant magnetic field. Nevertheless, the presently available data do not rule out a minor intrinsic field which may originate from a magnetized lithosphere. The objective of this study is to examine how much magnetization the Martian lithosphere could have acquired in the presence of an internal dynamo in early Martian history. Two models are presented: the first one assumes a laterally homogeneous lithosphere with a globally uniform thickness; the second model takes a possible dichotomy of the Martian lithosphere into account, which is assumed to be two to three times thicker in the southern hemisphere than in the northern hemisphere. The gradual cooling of the lithosphere below the Curie temperature is reproduced by a multilayer model. Curie isotherm motion with time and the dipole moment of the dynamo field are calculated from a thermal evolution model. For simplicity, the magnetic properties of the lithosphere are parameterized by its iron content. In the homogeneous case, the magnetostatic boundary value problem is solved exactly; in addition, an approximate solution is presented which is applicable with the dichotomy model. Using either method, the demagnetizing field in the separate layers mainly leads to the resulting remanent dipole moment. The remanent dipole moment is found to depend approximately linearly on the Curie isotherm depth and approximately quadratically on the iron content of the lithosphere. To reproduce the upper limit of the dipole moment derived from the Phobos mission (∼10 18 Am 2), a mean depth of 150 to 200 km for the Curie isotherm and a free iron content ranging from 5 to 6 vol.% are needed. The mean surface field may be less than 10 nT for both the homogeneous and the dichotomy model. The latter model yields a marked difference between the northern and southern hemisphere, with a pronounced anomaly at the dichotomy boundary.

On the internal structure and magnetic fields of

First, a sequence of four-zone models for the interior of Venus is constructed under the assumption of hydrostatic equilibrium. While the equation of state for each zone is taken to be the Bullen's relation with its coefficients consistent with the PREM Earth model (Dziewonski and Anderson, 1981), the position of core-mantle boundary is determined by matching solutions of the Emden's equation in different regions. The results of hydrostatic models indicate the presence of a reasonably large molten iron core in Venus, broadly similar to the Earth. It is also found that the position of the core-mantle interface is nearly model-independent. Second, we focus on the question why Venus does not possess a significant global magnetic field and on what we can learn from this fact. Solutions of magnetohydrodynamic equations appropriate for the molten core of Venus are discussed. It is argued that, because the Elsasser number Λ measuring the relative importance of Coriolis and Lorentz forces satisfies Λ ≪ 1, equations for the problem of thermal convection in the Venusian fluid core must be nearly uncoupled with the dynamo equation. The existence of a global magnetic field, though small, then suggests that the size of the magnetic Reynolds numberR m must beR m =O(10), sustaining a dynamo action near its marginal state but not an active dynamo in the Venusian molten core. On the basis of asymptotic relations for finite amplitude convection, a useful constraint on important physical parameters for the liquid core of Venus is derived and discussed.