Dipole model of the Earth's magnetic field

About: Dipole model of the Earth's magnetic field is a research topic. Over the lifetime, 2756 publications have been published within this topic receiving 83021 citations.

Videomagnetograph studies of solar magnetic fields. I - Magnetic field diffusion in weak plage regions

Abstract: Observations of magnetic field diffusion in weak plage regions have been made using the analog videomagnetograph at the California Institute of Technology. Points of magnetic flux, usually described as ‘vertex points’ of the magnetic network, were found to have a mean lifetime of three to four days, and to disperse primarily by means of two mechanisms: a random walk with a step time short compared to 24 h, and a sudden transport of magnetic flux over distances of 5000 to 20000 km during a time span of one to three hours. The second mechanism is probably the predominant one. Similar observations have been made using K3 spectroheliograms.

On the internal structure of the magnetic field in magnetic clouds and interplanetary coronal mass ejections: writhe versus twist

Abstract: In this study, we test the flux rope paradigm by performing a blind reconstruction of the magnetic field structure of a simulated interplanetary coronal mass ejection (ICME). The ICME is the result of a magnetohydrodynamic numerical simulation and does not exhibit much magnetic twist, but appears to have some characteristics of a magnetic cloud, due to a writhe in the magnetic field lines. We use the Grad-Shafranov technique with simulated spacecraft measurements at two different distances and compare the reconstructed magnetic field with that of the ICME in the simulation. While the reconstructed magnetic field is similar to the simulated one as seen in two dimensions, it yields a helically twisted magnetic field in three dimensions. To further verify the results, we perform the reconstruction at three different position angles at every distance point, and all results are found to be in agreement. This work demonstrates that the current paradigm of associating magnetic clouds with flux ropes may have to be revised.

Frame Dependence of the Electric Field Spectrum of Solar Wind Turbulence

Abstract: We present the first survey of electric field data using the ARTEMIS spacecraft in the solar wind to study inertial range turbulence. It was found that the average perpendicular spectral index of the electric field depends on the frame of measurement. In the spacecraft frame it is -5/3, which matches the magnetic field due to the large solar wind speed in Lorentz transformation. In the mean solar wind frame, the electric field is primarily due to the perpendicular velocity fluctuations and has a spectral index slightly shallower than -3/2, which is close to the scaling of the velocity. These results are an independent confirmation of the difference in scaling between the velocity and magnetic field, which is not currently well understood. The spectral index of the compressive fluctuations was also measured and found to be close to -5/3, suggesting that they are not only passive to the velocity but may also interact nonlinearly with the magnetic field.

The magnetic field vector of the Sun-as-a-star

Abstract: Direct comparison between stellar and solar magnetic maps are hampered by their dramatic differences in resolution. Here, we present a method to filter out the small-scale component of vector fields, in such a way that comparison between solar and stellar (large-scale) magnetic field vector maps can be directly made. Our approach extends the technique widely used to decompose the radial component of the solar magnetic field to the azimuthal and meridional components as well. For that, we self-consistently decompose the three-components of the vector field using spherical harmonics of different $l$ degrees. By retaining the low $l$ degrees in the decomposition, we are able to calculate the large-scale magnetic field vector. Using a synoptic map of the solar vector field at Carrington Rotation CR2109, we derive the solar magnetic field vector at a similar resolution level as that from stellar magnetic images. We demonstrate that the large-scale field of the Sun is not purely radial, as often assumed -- at CR2109, $83\%$ of the magnetic energy is in the radial component, while $10\%$ is in the azimuthal and $7\%$ is in the meridional components. By separating the vector field into poloidal and toroidal components, we show that the solar magnetic energy at CR2109 is mainly ($>90\%$) poloidal. Our description is entirely consistent with the description adopted in several stellar studies. Our formalism can also be used to confront synoptic maps synthesised in numerical simulations of dynamo and magnetic flux transport studies to those derived from stellar observations.

Towards a rapidly rotating liquid sodium dynamo experiment

Abstract: The main characteristics of the Earth's dynamo are reviewed. The combined actions of Coriolis and Lorentz forces lead to the so-called \magnetostrophic" regime. We derive an estimate of the power needed to sustain the magnetic eld in this regime. We show that an experiment with liquid sodium can be designed to operate in the magnetostrophic regime. Such an experiment would bring most valuable information on the mechanisms of planetary dynamos. In order to prepare this large-scale experiment and explore the magnetostrophic balance, a smaller scale liquid sodium set-up has been designed and is being built. It consists of a rapidly rotating spherical shell lled with liquid sodium, in which motions are set by spinning at a dierent rotation rate an inner core permeated by a strong magnetic eld. We discuss the processes that can be explored with this new device. 1. The magnetic elds in the solar system. Most planets of the solar system have or have had an internal magnetic eld. Recent satellite missions have revealed that Mars probably had a magnetic eld in its early history (1), while two moons of Jupiter, Io and Ganymede (2), show evidence for present magnetic activity. Explaining the origin and behaviour of planetary magnetic elds is a fascinating challenge. As a starting point, one may attempt to build a model of the geodynamo since the Earth's magnetic eld is, by far, the best documented. 2. The Earth's magnetic eld. The characteristics of the magnetic eld of the Earth are known over a wide range of time scales. In historical times, observa- tory records permit to monitor the secular variation of magnetic structures, with typical drift velocities of 0.1 mm/s. Sudden variations of the internal magnetic eld, aecting the entire core surface, have also been discovered in these records, the so-called \jerks", while the total intensity of the eld displays large fluctua- tions and has dropped by more than 30% in the past 2000 years, as recorded in archaeological artefacts.The most striking property of the Earth's magnetic eld, as seen at present, but also in the paleomagnetic records over several hundred of million years, is that it is dominated by a dipole component, whose axis is aligned with the axis of rotation of the Earth. Just as fascinating is the evidence that this dipole has changed polarity many times over the Earth's history, the last inversion having taken place only 780,000 years ago (3). Models of the geodynamo aim at explaining this rich set of behaviours, starting with the dominant dipole character of the eld. 3. The mechanism. Since the pioneer work of Larmor, Bullard and Elsasser, it is widely accepted that the Earth's magnetic eld is created by self-induction in its core. In an electrically conducting liquid, fluid motions in a magnetic eld produce electrical currents, which in turn feed the magnetic eld. When the induc- tion of the magnetic eld overcomes its diusion (i.e., for a large enough magnetic