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.

The Present and Future Geomagnetic Field

Abstract: The Earth's magnetic field is the result of the superposition of a main field produced by the geodynamo within the core and of many other fields produced by a variety of secondary sources. Unfortunately, magnetometers can only measure the resulting field, and a single measurement cannot distinguish the contributions from each source. However, provided enough ground and satellite measurements are being acquired and processed in appropriate ways, a separation of the contributions from each type of field can be achieved and global models of the Earth's magnetic field constructed. The main purpose of this chapter is to provide an up-to-date review of such global models. We introduce the relevant types of magnetic data, review the models, and focus on the current behavior of the main field. We also report on the emerging field of geomagnetic data assimilation, which aims at both improving our understanding of the geodynamo and forecasting the main field evolution.

Magnetic cloud models with bent and oblate cross-section boundaries

Abstract: Context. Magnetic clouds (MCs) are formed by magnetic flux ropes that are ejected from the Sun as coronal mass ejections. These structures generally have low plasma beta and travel through the interplanetary medium interacting with the surrounding solar wind. Thus, the dynamical evolution of the internal magnetic structure of a MC is a consequence of both the conditions of its environment and of its own dynamical laws, which are mainly dominated by magnetic forces. Aims. With in-situ observations the magnetic field is only measured along the trajectory of the spacecraft across the MC. Therefore, a magnetic model is needed to reconstruct the magnetic configuration of the encountered MC. The main aim of the present work is to extend the widely used cylindrical model to arbitrary cross-section shapes. Methods. The flux rope boundary is parametrized to account for a broad range of shapes. Then, the internal structure of the flux rope is computed by expressing the magnetic field as a series of modes of a linear force-free field. Results. We analyze the magnetic field profile along straight cuts through the flux rope, in order to simulate the spacecraft crossing through a MC. We find that the magnetic field orientation is only weakly affected by the shape of the MC boundary. Therefore, the MC axis can approximately be found by the typical methods previously used (e.g., minimum variance). The boundary shape affects the magnetic field strength most. The measurement of how much the field strength peaks along the crossing provides an estimation of the aspect ratio of the flux-rope cross-section. The asymmetry of the field strength between the front and the back of the MC, after correcting for the time evolution (i.e., its aging during the observation of the MC), provides an estimation of the cross-section global bending. A flat or/and bent cross-section requires a large anisotropy of the total pressure imposed at the MC boundary by the surrounding medium. Conclusions. The new theoretical model developed here relaxes the cylindrical symmetry hypothesis. It is designed to estimate the cross-section shape of the flux rope using the in-situ data of one spacecraft. This allows a more accurate determination of the global quantities, such as magnetic fluxes and helicity. These quantities are especially important for both linking an observed MC to its solar source and for understanding the corresponding evolution.

Magnetohydrodynamic Simulations of Hot Jupiter Upper Atmospheres

Abstract: Two-dimensional simulations of hot Jupiter upper atmospheres including the planet's magnetic field are presented. The goal is to explore magnetic effects on the layer of the atmosphere that is ionized and heated by stellar EUV radiation, and the imprint of these effects on the Ly$\alpha$ transmission spectrum. The simulations are axisymmetric, isothermal, and include both rotation and azimuth-averaged stellar tides. Mass density is converted to atomic hydrogen density through the assumption of ionization equilibrium. The three-zone structure -- polar dead zone, mid-latitude wind zone, and equatorial dead zone -- found in previous analytic calculations is confirmed. For a magnetic field comparable to that of Jupiter, the equatorial dead zone, which is confined by the magnetic field and corotates with the planet, contributes at least half of the transit signal. For even stronger fields, the gas escaping in the mid-latitude wind zone is found to have a smaller contribution to the transit depth than the equatorial dead zone. Transmission spectra computed from the simulations are compared to HST STIS and ACS data for HD 209458b and HD 189733b, and the range of model parameters consistent with the data is found. The central result of this paper is that the transit depth increases strongly with magnetic field strength when the hydrogen ionization layer is magnetically dominated, for dipole magnetic field $B_0 > 10\ {\rm G}$. Hence transit depth is sensitive to magnetic field strength, in addition to standard quantities such as the ratio of thermal to gravitational binding energies.

Quantitative estimates of magnetic field reconnection properties from electric and magnetic field measurements

Abstract: Reconnection occurs in a reconnection magnetic field geometry when there are positive electric field components tangential to the magnetopause and a magnetic field component normal to it. Because these three components are the smallest of the six electric and magnetic fields, their magnitudes are difficult to determine because of errors in, or oscillations of, the assumed constant direction normal to the current sheet. A method is described for minimizing these errors by appropriate selection of the normal direction and by analyzing the correlations between the large normal electric field and the large tangential magnetic field. The correlation coefficients are equal to ratios of the small fields, which are combined with the less accurate measurements of the averages of the small fields to produce best estimates of the small fields. For more than 120 magnetopause crossings, about 40% had such correlations that signify static conditions during those crossings. This method is applied to 22 polar subsolar magnetopause crossings to show that most were located in the ion diffusion region, as defined by the change of the total magnetic field, and that 14 had a large and steady reconnection rate with a zero parallel electric field. In these events the reconnection rate decreased with increasing guide magnetic field.