The lunar dynamo

Science ◽  
2014 ◽  
Vol 346 (6214) ◽  
pp. 1246753 ◽  
Author(s):  
Benjamin P. Weiss ◽  
Sonia M. Tikoo
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
The Moon ◽  
Planetary Interiors ◽  
Mechanical Stirring ◽  
Lunar Rocks ◽  
Order Of Magnitude ◽  
Global Magnetic Field ◽  
Dynamo Process ◽  
Inductive Generation

The inductive generation of magnetic fields in fluid planetary interiors is known as the dynamo process. Although the Moon today has no global magnetic field, it has been known since the Apollo era that the lunar rocks and crust are magnetized. Until recently, it was unclear whether this magnetization was the product of a core dynamo or fields generated externally to the Moon. New laboratory and spacecraft measurements strongly indicate that much of this magnetization is the product of an ancient core dynamo. The dynamo field persisted from at least 4.25 to 3.56 billion years ago (Ga), with an intensity reaching that of the present Earth. The field then declined by at least an order of magnitude by ∼3.3 Ga. The mechanisms for sustaining such an intense and long-lived dynamo are uncertain but may include mechanical stirring by the mantle and core crystallization.

Introductory remarks

1994 ◽  
Vol 349 (1690) ◽  
pp. 169-170
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Royal Society ◽  
Lunar Regolith ◽  
The Moon ◽  
Global Magnetic Field ◽  
The Subject ◽  
Planetary Missions ◽  
Speculative Hypothesis ◽  
First Time

This year marks not only the twenty-fifth anniversary of the first manned landing on the Moon ( Apollo 11 ) but also the thirty-fifth anniversary of the first planetary missions. The latter was the Soviet Luna 1 and 2 carrying magnetometers to test whether the Moon possessed a global magnetic field. Luna 1 passed the Moon but Luna 2 crash landed, both showed that the Moon had no magnetic field as large as 50 or 100 y (1 y = 10 -5 G = 10 -9 T). Such an experiment had been proposed by S. Chapman ( Nature 160, 395 (1947)) to test a speculative hypothesis concerning magnetic fields of cosmic bodies by P. M. S. Blackett ( Nature 159, 658 (1947)). Chapman’s suggestion was greeted by general amusement: 12 years later it was accomplished. Also two years after the launch of Sputnik 1 in 1957, Luna 3 was launched and for the first time viewed the far side of the Moon on 9 October, 1959. Laboratories from many countries were invited by NASA to take part in the analysis of rocks returned from the Apollo missions and later from the Soviet automated return of cores from the lunar regolith. British laboratories were very active in this work, and a review of the results of the new understanding of the Moon as a result of space missions formed the subject of a Royal Society Discussion Meeting in 1975 (published in Phil. Trans. R. Soc. Lond . A 285). British laboratories received samples from the automated Soviet missions that took cores from the regolith and returned them to Earth. Work on Luna 16 and 20 samples were published in Phil. Trans. R. Soc. Lond . A 284 131-177 (1977) and on Luna 24 in Phil. Trans. R. Soc. Lond . A 297 1-50 (1979).

scholarly journals Observations of magnetic fields in Herbig Ae/Be stars

2018 ◽  
Vol 14 (A30) ◽  
pp. 123-123
Author(s):  
Markus Schöller ◽  
Swetlana Hubrig
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
T Tauri Stars ◽  
Be Stars ◽  
T Tauri ◽  
Weak Magnetic Fields ◽  
Order Of Magnitude ◽  
Field Geometry ◽  
Herbig Ae Stars ◽  
Magnetospheric Accretion

AbstractModels of magnetically driven accretion reproduce many observational properties of T Tauri stars. For the more massive Herbig Ae/Be stars, the corresponding picture has been questioned lately, in part driven by the fact that their magnetic fields are typically one order of magnitude weaker. Indeed, the search for magnetic fields in Herbig Ae/Be stars has been quite time consuming, with a detection rate of about 10% (e.g. Alecian et al. 2008), also limited by the current potential to detect weak magnetic fields. Over the last two decades, magnetic fields were found in about twenty objects (Hubrig et al. 2015) and for only two Herbig Ae/Be stars was the magnetic field geometry constrained. Ababakr, Oudmaijer & Vink (2017) studied magnetospheric accretion in 56 Herbig Ae/Be stars and found that the behavior of Herbig Ae stars is similar to T Tauri stars, while Herbig Be stars earlier than B7/B8 are clearly different. The origin of the magnetic fields in Herbig Ae/Be stars is still under debate. Potential scenarios include the concentration of the interstellar magnetic field under magnetic flux conservation, pre-main-sequence dynamos during convective phases, mergers, or common envelope developments. The next step in this line of research will be a dedicated observing campaign to monitor about two dozen HAeBes over their rotation cycle.

A Study of the Magnetic Field of the Moon

1962 ◽  
Vol 14 ◽  
pp. 45-52 ◽  
Author(s):  
S. S. Dolginov ◽  
E. G. Eroshenko ◽  
L. I. Zhuzgov ◽  
N. V. Pushkov
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
The Moon ◽  
Earth’S Magnetic Field ◽  
Earth's Magnetic Field ◽  
The Magnetic Field

The question as to whether the planets and their satellites possess magnetic fields unavoidably arose in connection with the question as to the origin of the Earth's mágnetic field and the nature of a number of geophysical effects.

scholarly journals 17. Some considerations on thermal conduction and magnetic fields in prominences

1958 ◽  
Vol 6 ◽  
pp. 150-157 ◽  
Author(s):  
S. Rosseland ◽  
E. Jensen ◽  
E. Tandberg-Hanssen
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Temperature Region ◽  
Thermal Conduction ◽  
Order Of Magnitude ◽  
Long Life

Prominences which extend into the million degree temperature region of the corona will, in the absence of magnetic fields, be heated up to temperatures of the same order of magnitude in the course of at most a few hours. A magnetic field of reasonable magnitude inside the prominence, will, however, be sufficient to cut down thermal conduction and turbulence to such an extent that the long life of some prominences seems understandable.

scholarly journals Study of magnetism cyclicity of the Sun in the framework of the macroscopic magnetohydrodynamics theory

2019 ◽  
pp. 22-32
Author(s):  
V. Krivodubskij
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Lower Half ◽  
Dynamo Model ◽  
The Sun ◽  
Solar Plasma ◽  
Radial Gradient ◽  
Magnetic Buoyancy ◽  
Global Magnetic Field

Since the mid-70s of the last century, a new direction in theoretical studies of the evolution of the global magnetism of the Sun in the framework of macroscopic MHD has been launched at the Astronomical Observatory of the Taras Shevchenko National University of Kyiv. The paper presents the results of a study of the processes of generation and restructuring of a large-scale (global) magnetic field based on the αΩ-dynamo model, taking into account new turbulent effects discovered in the theory of macroscopic MHD and data of helioseismological experiments on the internal rotation of the Sun. It was established that a sharp radial gradient of turbulent velocity in the lower half of the solar convective zone (SCZ) leads to a change in the sign of the azimuthal component of the helicity parameter α, resulting in the formation of a relatively thin layer of negative α-effect near the bottom of the SCZ. It was found that the layer of negative α-effect, together with the sign of the radial gradient of the angular velocity, detected in helioseismological experiments, makes it possible to explain the direction of migration of dynamo-waves on the solar surface. The magnetic saturation of the α-effect (alpha-quenching) in the deep layers of the SCZ was calculated. An explanation of the protracted duration of the 23rd solar cycle of about 13 years is proposed. For this, we used the observed data on a significant increase of the annual module of the magnetic fields of sunspots in the 23rd cycle. The calculated north-south asymmetry of the structure of the global magnetic field provides an opportunity to explain the phenomenon of the seeming magnetic “monopole”, which is observed during reversal of polar magnetism. It was found that the values of turbulent electrical conductivity and turbulent magnetic permeability of the solar plasma are significantly less than the corresponding gas-kinetic parameters. Therefore, the turbulent dissipation of solar magnetic fields is enhanced by 4–9 orders of magnitude compared with classical ohmic dissipation. Macroscopic turbulent diamagnetism of solar plasma was investigated. It has been found that in the lower part of the SCZ, turbulent diamagnetism acts against magnetic buoyancy, thus fulfilling the role of “negative magnetic buoyancy”. As a result of the balance of the effects of magnetic buoyancy and turbulent diamagnetism, a layer of blocked magnetic field of magnitude ≈ 3000 G is formed in the depths of the SCZ. The turbulent advection of a magnetic field in an inhomogeneous plasma density of the SCZ was studied. It was found that in the lower half of the SCZ of the equatorial domain, turbulent advection is directed upwards. As a result of the combined action of magnetic buoyancy and turbulent advection, deep strong toroidal fields are carried to the surface of the Sun in the latitudinal “royal zone” of sunspots. The role of horizontal turbulent diamagnetism in ensuring the long-term stability of sunspots was noted. To explain the observed phenomenon of double maxima of the solar spot cycle, a scenario was developed containing the generation of a magnetic field in the tachocline at the bottom of the SCZ and subsequent removal of this magnetic field from the depth layers to the surface in the latitudinal “royal zone”. The role of the radial omega-effect in the radiant zone in explaining the observed asymmetry in the amplitude of two neighbouring 11-years sunspot cycles was noted.

Dynamos in the Inner Solar System

2021 ◽  
Vol 50 (1) ◽  
Author(s):  
Sonia M. Tikoo ◽  
Alexander J. Evans
Keyword(s):  
Solar System ◽  
Magnetic Fields ◽  
Thermal Evolution ◽  
Annual Review ◽  
Publication Date ◽  
The Moon ◽  
Conductive Liquid ◽  
Mechanical Stirring ◽  
Planetary Cores ◽  
Thermochemical Convection

Dynamo magnetic fields are primarily generated by thermochemical convection of electrically conductive liquid metal within planetary cores. Convection can be sustained by secular cooling and may be bolstered by compositional buoyancy associated with core solidification. Additionally, mechanical stirring of core fluids and external perturbations by large impact events, tidal effects, and orbital precession can also contribute to sustaining dynamo fields. Convective dynamos cease when the core-mantle heat flux becomes subadiabatic or if specific crystallization regimes inhibit core fluid flows. Therefore, exploring the histories of magnetic fields across the Solar System provides a window into the thermal and chemical evolution of planetary interiors. Here we review how recent spacecraft-based studies of remanent crustal magnetism, paleomagnetic studies of rock samples, and planetary interior models have revealed the magnetic and evolutionary histories of Mercury, Earth, Mars, the Moon, and several planetesimals, as well as discuss avenues for future exploration and discovery. ▪ Paleomagnetism and remanent crustal magnetism studies elucidate the magnetic histories of rocky planetary bodies. ▪ Records of ancient dynamo fields have been obtained from 3 out of 4 terrestrial planets, the Moon, and several planetesimals. ▪ The geometries, intensities, and longevities of dynamo fields can provide information on core processes and planetary thermal evolution. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 50 is May 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

scholarly journals Misalignment of Magnetic Fields, Outflows and Discs in Star-forming Clouds

2019 ◽  
Author(s):  
Masahiro N Machida ◽  
Shingo Hirano ◽  
Hideyuki Kitta
Keyword(s):  
Magnetic Field ◽  
Star Formation ◽  
Magnetic Fields ◽  
Rotation Axis ◽  
Local Magnetic Field ◽  
Initial State ◽  
Star Forming ◽  
Significant Difference ◽  
Protostellar Jets ◽  
Global Magnetic Field

Abstract Using resistive magnetohydrodynamics simulations, the propagation of protostellar jets, the formation of circumstellar discs and the configuration of magnetic fields are investigated from the prestellar cloud phase until ∼500 yr after protostar formation. As the initial state, we prepare magnetized rotating clouds, in which the rotation axis is misaligned with the global magnetic field by an angle θ0. We calculate the cloud evolution for nine models with different θ0( = 0, 5, 10, 30, 45, 60, 80, 85, 90○). Our simulations show that there is no significant difference in the physical quantities of the protostellar jet, such as the mass and momentum, among the models except for the model with θ0 = 90○. On the other hand, the directions of the jet, disc normal and magnetic field are never aligned with each other during the early phase of star formation except for the model with θ0 = 0○. Even when the rotation axis of the prestellar cloud is slightly inclined to the global magnetic field, the directions of the jet, disc normal and local magnetic field differ considerably, and they randomly change over time. Our results indicate that it is very difficult to extract any information from the observations of the directions of the outflow, disc and magnetic field at the scale of $\lesssim$1000  au. Thus, we cannot use such observations to derive any restrictions on the star formation process.

scholarly journals Global Evolution of Photospheric Magnetic Fields

1990 ◽  
Vol 138 ◽  
pp. 281-295
Author(s):  
V. I. Makarov ◽  
K. R. Sivaraman
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Active Regions ◽  
Magnetic Activity ◽  
Field Pattern ◽  
The Sun ◽  
Coronal Emission ◽  
Global Magnetic Field ◽  
Global Modes

The main features concerning the evolution of the large scale photospheric magnetic fields derived from synoptic maps as well as from H-alpha synoptic charts are reviewed. The significance of a variety of observations that indicate the presence of a high latitude component as a counterpart to the sunspot phenomenon at lower latitudes is reviewed. It is argued that these two components describe the global magnetic field on the sun. It is demonstrated that this scenario is able to link many phenomena observed on the sun (coronal emission, ephemeral active regions, geomagnetic activity, torsional oscillations, polar faculae and global modes in the magnetic field pattern) with the global magnetic activity.

scholarly journals The Sun's global magnetic field

2012 ◽  
Vol 370 (1970) ◽  
pp. 3151-3168 ◽  
Author(s):  
Duncan H. Mackay
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Large Scale ◽  
Future Research ◽  
Flux Transport ◽  
Solar Filaments ◽  
Recent Developments ◽  
Global Magnetic Field ◽  
Open Flux ◽  
Stellar Magnetic Fields

Our present-day understanding of solar and stellar magnetic fields is discussed from both an observational and theoretical viewpoint. To begin with, observations of the Sun's large-scale magnetic field are described, along with recent advances in measuring the spatial distribution of magnetic fields on other stars. Following this, magnetic flux transport models used to simulate photospheric magnetic fields and the wide variety of techniques used to deduce global coronal magnetic fields are considered. The application and comparison of these models to the Sun's open flux, hemispheric pattern of solar filaments and coronal mass ejections are then discussed. Finally, recent developments in the construction of steady-state global magnetohydrodynamic models are considered, along with key areas of future research.

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