Earth's magnetic field

About: Earth's magnetic field is a research topic. Over the lifetime, 20360 publications have been published within this topic receiving 446747 citations. The topic is also known as: magnetic field of Earth & geomagnetic field.

Large-Scale Flow in the Core

Abstract: We discuss the observational constraint of large-scale flow in the core. The time variation of the internal magnetic field has been attributed to internal motions in the Earth's fluid core, associated with the geodynamo. From initially simplistic models of magnetic westward drift, more detailed time-dependent models of the field have motivated the calculation of maps of core-surface flow. Advection is assumed to dominate over diffusion (the ‘Frozen-flux’ approximation), suggesting that the magnetic field provides a passive tracer for core flow. Only one equation is available to constrain two components of flow along the core surface; this leads to an unavoidable nonuniqueness in flow determination. Dynamic assumptions have been adopted to provide additional constraints, but the flows vary in morphology depending on which assumption is used. New assumptions are under development, including improved treatments of magnetic diffusion, which may help us to better understand the nature of the flow, or at least the shortcomings in the models we produce. Modern geomagnetic satellite data allow the creation of models of the geomagnetic secular variation with enhanced detail, but as yet, such models have opened up greater modeling problems rather than increased insight as to the flow. A primary difficulty is the shielding of the core field at spherical harmonic degree above 13 by the magnetic field from the Earth's lithosphere.

Martian magnetic morphology: Contributions from the solar wind and crust

Abstract: [1] Mars Global Surveyor (MGS) Magnetometer (MAG) data provide constraints on magnetic morphology at Mars, including the relative importance of the solar wind and of crustal magnetic sources. We analyze MAG data to characterize the upstream interplanetary magnetic field (IMF) and confirm trends in the magnetic field expected from the solar wind interaction with a planetary atmosphere, including increases at the shock and magnetic pile-up boundary (MPB), postshock turbulence, and field line draping around the Martian obstacle. Crustal magnetic sources locally modify the solar wind interaction, adding variability to the Martian magnetic environment that depends on planetary rotation. We identify trends in the vector magnetic field with respect to altitude, solar zenith angle, and planetary location. Crustal sources influence the magnetic field to different altitudes above different regions, and the influence of the strongest source extends to 1300–1400 km. The draped IMF partially controls the field topology above crustal sources, and crustal magnetic field lines reconnect to this field in a systematic fashion that depends upon Mars' geography, IMF strength, and IMF orientation.

Ground-based and satellite observations of traveling magnetospheric convection twin vortices

Abstract: Close inspection of observations from the International Magnetospheric Study Scandinavian Magnetometer Array revealed the existence of a peculiar type of short-period magnetic variations. In typical cases it can be described as a positive deflection of the geomagnetic D component from the quiet time level with the disturbance of the H component approximately representing the negative time derivative of the D component. Thus these events are similar to magnetic variations recently reported by Lanzerotti et al. (1986) and discussed in terms of possible signatures of flux transfer events as well as to observations by Friis-Christensen et al. (1988) in connection with readjustments of the magnetopause to a sudden change in solar wind dynamic pressure. The typical duration of such a transient magnetic perturbation is about 10 min, with amplitudes of about 20 nT. Between 1975 and 1979 about 80 of these events have been detected, with a sharp occurrence maximum at magnetic forenoon. Most events were observed during moderately quiet times (KP≃2). A more detailed analysis of two such events exhibits a two-cell ground-equivalent current vortex structure. Simultaneous magnetic field observations from GEOS 2 for one of these events support our interpretation of these events as representing pairs of downward (in the west) and upward (in the east) field-aligned currents in the magnetosphere, moving over the observer from the dayside to the nightside. The physical mechanism generating these traveling magnetospheric convection twin vortices and the associated field-aligned currents is unclear. Flux transfer events, however, are not necessarily associated with the observed short-period transient magnetic field variations.

Does the whole of the Earth's core convect?

Abstract: Higgins and Kennedy1 used data on the behaviour of iron at high temperatures and pressures to infer that the top of the Earth's liquid core is stably stratified. Attempts to confirm this result both thermodynamically2 and using seismological data3 have been inconclusive. I present here geomagnetic results which may resolve the controversy. If a stratified region exists there will be no upwelling or downwelling of core fluid at the core–mantle boundary (CMB), so there will be no horizontal divergence of velocity v, that is ▿H·v = 0, where ▿H = ▿ − r(r·▿) r denotes the unit radial vector. This hypothesis can be tested directly, using geomagnetic data, at a few isolated points on the CMB, and local averages of ▿H·v can be examined over the rest of the surface. A statistical treatment of the results strongly suggests that ▿H·v = 0, which is a consequence of a stably stratified layer.