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 heliospheric magnetic field from 850 to 2000 AD inferred from 10Be records

Abstract: [1] 10Be found in ice cores is an indicator of cosmic ray intensity in the past. We use this isotope to study cosmic ray transport and the heliospheric magnetic field before the advent of instrumental cosmic-ray measurements in the modern space era. The galactic cosmic ray intensity is governed by scattering, convection, and drift of the charged particles in the heliospheric magnetic field, which leads to a modulation in their intensity. We model these cosmic ray intensity changes observed at Earth during the space era with solutions of the cosmic ray transport equation. This gives a set of diffusion mean free paths during the past few solar activity cycles. A relationship is then determined between these diffusion mean free paths and satellite observations of the heliospheric magnetic field during the same period, yielding a relationship between the observed cosmic ray intensity and the heliospheric magnetic field. We then calculate the diffusion mean free paths that explain the variations in the 10Be concentration during the last millennium and use the space-era calibration to infer heliospheric magnetic field since 850 AD. It is shown how this inversion of the 10Be data depends on the strength of the heliospheric magnetic field and variations in its turbulence, both of which are quite uncertain. Nevertheless, the results show that for a wide range of parameters, there was a significant heliospheric magnetic field with a strength of 2 to 5 nT at Earth during the so-called Grand Minima of solar activity. It is also shown that the strength of this field has attained six maxima in the past 1150 years, all approximating the present-day field strength, and we speculate that a limiting mechanism may be in operation. On several occasions the strength of the field has switched rapidly from ≈2 nT to ≈6 nT within 40 years. During the Grand Minima the total field derived from the 10Be data differs significantly from the open solar magnetic field calculated from the models of Solanki et al. [2002] and Schrijver et al. [2002].

Free Magnetic Energy in Solar Active Regions above the Minimum-Energy Relaxed State

Abstract: To understand the physics of solar flares, including the local reorganization of the magnetic field and the acceleration of energetic particles, we have first to estimate the free magnetic energy available for such phenomena, which can be converted into kinetic and thermal energy. The free magnetic energy is the excess energy of a magnetic configuration compared to the minimum-energy state, which is a linear force-free field if the magnetic helicity of the configuration is conserved. We investigate the values of the free magnetic energy estimated from either the excess energy in extrapolated fields or the magnetic virial theorem. For four different active regions, we have reconstructed the nonlinear force-free field and the linear force-free field corresponding to the minimum-energy state. The free magnetic energies are then computed. From the energy budget and the observed magnetic activity in the active region, we conclude that the free energy above the minimum-energy state gives a better estimate and more insights into the flare process than the free energy above the potential field state.

Goniopolarimetric study of the revolution 29 perikrone using the Cassini Radio and Plasma Wave Science instrument high‐frequency radio receiver

Abstract: [1] We present goniopolarimetric (also known as direction finding) results of the Saturn kilometric radiation (SKR), using the Cassini Radio and Plasma Wave Science instrument high-frequency radio receiver data. Tools to retrieve the characteristics of the SKR sources have been developed that allow us to measure their 3-D location and beaming angle relative to the magnetic field in the source and, thus, to deduce the location of the footprints of the active magnetic field lines. We present results from these analyses on SKR observed during the revolution 29 perikrone (25–26 September 2006) with a relatively high orbital inclination. These results provide for the first time the observed beaming angle, the invariant latitude, and the local time of the SKR sources. We provide evidence that the SKR is mainly emitted in the right-hand extraordinary (R-X) mode and marginally in the left-hand ordinary (L-O) mode. We observe the footprint of the active magnetic field lines in the ∼70° to ∼80° northern and southern latitudinal range and in the 0400 to 1600 local time range. The northern sources are observed at slightly higher latitude than southern sources. The location matches that of the UV and IR aurorae. Duskside and nightside sources are also detected.

The polar cusp location and its dependence on dipole tilt

Abstract: Polar cusp crossings at high altitudes are seen in the POLAR data as decreases in the magnetic field and increases in magnetosheath-like plasma. Close to 500 polar cusp crossings identified from the magnetic field, low-energy electron and ion data observed by POLAR, are used to determine the statistical location of the polar cusp. When compared with Tsyganenko's 1989 vacuum model with an ellipsoidal magnetopause [Tsyganenko, 1989], the medians of the cusp crossings are located between the magnetic field lines with invariant latitudes of 80° and 82°. Statistically the shape of the polar cusp in this region is consistent with this model although there is much scatter around the median value. The position of the cusp is significantly dependent on the dipole tilt angle. When dipole tilts more toward the Sun, the cusp moves more poleward to higher invariant latitude from 77.2° at −30° tilt, to 80.0° at 0° tilt, to 81.8° at 30° or roughly 1° for every 14° of tilt.