Topic
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.
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TL;DR: The magnetic field strength of Mercury at the planet's surface is approximately 1% that of Earth's surface field as discussed by the authors, which presents a number of challenges, both theoretically to understand how it is generated and observationally to distinguish the internal field from that due to the solar wind interaction.
Abstract: The magnetic field strength of Mercury at the planet’s surface is approximately 1% that of Earth’s surface field. This comparatively low field strength presents a number of challenges, both theoretically to understand how it is generated and observationally to distinguish the internal field from that due to the solar wind interaction. Conversely, the small field also means that Mercury offers an important opportunity to advance our understanding both of planetary magnetic field generation and magnetosphere-solar wind interactions. The observations from the Mariner 10 magnetometer in 1974 and 1975, and the MESSENGER Magnetometer and plasma instruments during the probe’s first two flybys of Mercury on 14 January and 6 October 2008, provide the basis for our current knowledge of the internal field. The external field arising from the interaction of the magnetosphere with the solar wind is more prominent near Mercury than for any other magnetized planet in the Solar System, and particular attention is therefore paid to indications in the observations of deficiencies in our understanding of the external field. The second MESSENGER flyby occurred over the opposite hemisphere from the other flybys, and these newest data constrain the tilt of the planetary moment from the planet’s spin axis to be less than 5°. Considered as a dipole field, the moment is in the range 240 to 270 nT-R
M
3
, where R
M is Mercury’s radius. Multipole solutions for the planetary field yield a smaller dipole term, 180 to 220 nT-R
M
3
, and higher-order terms that together yield an equatorial surface field from 250 to 290 nT. From the spatial distribution of the fit residuals, the equatorial data are seen to reflect a weaker northward field and a strongly radial field, neither of which can be explained by a centered-dipole matched to the field measured near the pole by Mariner 10. This disparity is a major factor controlling the higher-order terms in the multipole solutions. The residuals are not largest close to the planet, and when considered in magnetospheric coordinates the residuals indicate the presence of a cross-tail current extending to within 0.5R
M altitude on the nightside. A near-tail current with a density of 0.1 μA/m2 could account for the low field intensities recorded near the equator. In addition, the MESSENGER flybys include the first plasma observations from Mercury and demonstrate that solar wind plasma is present at low altitudes, below 500 km. Although we can be confident in the dipole-only moment estimates, the data in hand remain subject to ambiguities for distinguishing internal from external contributions. The anticipated observations from orbit at Mercury, first from MESSENGER beginning in March 2011 and later from the dual-spacecraft BepiColombo mission, will be essential to elucidate the higher-order structure in the magnetic field of Mercury that will reveal the telltale signatures of the physics responsible for its generation.
143 citations
TL;DR: In this paper, the authors present a review of techniques for computational modeling of magnetic fields in solar active regions, including current-free, constant-α and general force-free fields.
Abstract: The magnetic field plays an important role in various solar activities. This paper reviews techniques for computational modeling of magnetic fields in solar active regions. The input data are photospheric magnetic fields supplied by magnetograph observations. The field above the photosphere is computed by assuming an equation for the magnetic field. Three classes of magnetic fields, namely current-free fields, constant-α force-free fields, and general force-free fields are considered. Their physical/mathematical significances and computational procedures are systematically presented.
142 citations
TL;DR: In this paper, the authors presented the formalism and numerical results for torsional oscillations of relativistic stars endowed with a strong dipole magnetic field, assumed to be confined to the crust.
Abstract: We present the formalism and numerical results for torsional oscillations of relativistic stars endowed with a strong dipole magnetic field, assumed to be confined to the crust. In our approach, we focus on axisymmetric modes and neglect higher order couplings induced by the magnetic field. We do a systematic search of parameter space by computing torsional mode frequencies for various values of the harmonic index l and for various overtones, using an extended sample of models of compact stars, varying in mass, high-density equation of state (EOS) and crust model. We show that torsional mode frequencies are sensitive to the crust model if the high-density EOS is very stiff. In addition, torsional mode frequencies are drastically affected by a dipole magnetic field, if the latter has a strength exceeding roughly 10 15 G and we find that the magnetic field effects are sensitive to the adopted crust model. Using our extended numerical results we derive empirical relations for the effect of the magnetic field on torsional modes as well as for the crust thickness. We compare our numerical results to observed frequencies in soft gamma repeaters and find that certain high-density EOS and mass values are favoured over others in the non-magnetized limit. On the other hand, if the magnetic field is strong, then its effect has to be taken into account in attempts to formulate a theory of asteroseismology for magnetars.
141 citations
01 Jan 1979
TL;DR: In this article, a mechanism for the generation of field aligned currents during magnetospheric substorms is presented, where the energy which is injected to the geomagnetic tail is converted into plasma flow energy in solar and anti-solar directions.
Abstract: A mechanism for the generation of field aligned currents during magnetospheric substorms is presented. The energy which is injected to the geomagnetic tail is converted into plasma flow energy in solar and anti-solar directions. When the flow meets the inner magnetosphere, a viscous interaction occurs. This interaction creates time-increasing vorticities which produce field aligned currents upward in the pre-midnight sector and downward in the post-midnight sector.
140 citations
01 Jan 1990
TL;DR: In this article, a magnetic cloud associated with a 2N flare on January 1, 1978 was observed by IMP-8, Helios A and Voyager 2, and the times of the estimated boundaries of the magnetic cloud at each spacecraft, together with the speeds of the boundaries and the spacecraft position, give the positions of the boundary at a given time.
Abstract: A magnetic cloud associated with a 2N flare on January 1, 1978 was observed by IMP-8, Helios A, Helios B, and Voyager 2 The variation of the magnetic field observed at each spacecraft is represented to good approximation by Lundquist's solution for a cylindrically symmetric force-free magnetic field with constant alpha A least-squares fit of Lundquist's solution to the data from each spacecraft gives the local orientation of the axis of the magnetic cloud The times of the estimated boundaries of the magnetic cloud at each spacecraft, together with the speeds of the boundaries and the spacecraft position, give the positions of the boundaries at a given time From these results the magnetic cloud is determined to resemble a flux rope whose minor radius is approximately 015 AU at 1 AU, and whose radius of curvature at 1 AU is approximately 1/3 AU
137 citations