MagWiki - Geocentric Axial Dipole Hypothesis (GAD)
The geomagnetic poles move over time because the geomagnetic field is produced by motion of the molten iron alloys in the Earth's (see ). Over the past 150 years the poles have moved westward at a rate of 0.05° to 0.1° per year, with little net north or south motion.
on the Geocentric axial dipole (GAD) hypothesis, ..
In , you learned how to calculate the dipole moments of simple diatomic molecules. In more complex molecules with polar covalent bonds, the three-dimensional geometry and the compound’s symmetry determine whether there is a net dipole moment. Mathematically, dipole moments are ; they possess both a and a . The dipole moment of a molecule is therefore the of the dipole moments of the individual bonds in the molecule. If the individual bond dipole moments cancel one another, there is no net dipole moment. Such is the case for CO2, a linear molecule (part (a) in ). Each C–O bond in CO2 is polar, yet experiments show that the CO2 molecule has no dipole moment. Because the two C–O bond dipoles in CO2 are equal in magnitude and oriented at 180° to each other, they cancel. As a result, the CO2 molecule has no dipole moment even though it has a substantial separation of charge. In contrast, the H2O molecule is not linear (part (b) in ); it is bent in three-dimensional space, so the dipole moments do not cancel each other. Thus a molecule such as H2O has a net dipole moment. We expect the concentration of negative charge to be on the oxygen, the more electronegative atom, and positive charge on the two hydrogens. This charge polarization allows H2O to hydrogen-bond to other polarized or charged species, including other water molecules. (For more information on polar bonds, see , .)
Cl2C=CCl2: Although the C–Cl bonds are rather polar, the individual bond dipoles cancel one another in this symmetrical structure, and Cl2C=CCl2 does not have a net dipole moment.
TESTING THE GEOCENTRIC AXIAL DIPOLE (GAD) HYPOTHESIS …
The Eccentric Dipole (ED) model of Earth's magnetic field is an attempt to get a better approximation Earth's geomagnetic pole locations compared to the simple dipole (i.e. bar magnet) approximation of the . The eccentric dipole approximation acknowledges that the field is slightly offset from the Earth's spin axis although it retains a dipolar character. The eccentric dipole model uses the first 8 terms of the (i.e. the Gauss coefficients up to degree 2) of the International Geomagnetic Reference Field (IGRF). These are the g10, g11, h11, g20, g21, h21, g22, h22 terms. Two different types of poles can be calculated: an eccentric dipole axial pole which is where the axis of the dipole intersects the Earth's surface and where the field is vertical at Earth's surface. See Fraser-Smith, (1987) for a full explanation. See also background information on spherical harmonics on operated by the European Space Agency.
the Earth’s magnetic field approximates a geocentric axial dipole ..
Like the , the Eccentric Dipole (CD) model of the is only an approximation of the geomagnetic field using a limited number of the coefficients in the of the IGRF model. However, these dipole models do give a general sense of the time averaged behavior of the field. It is possible to use all of the Gauss coefficients of the spherical harmonic expansion to define the field and pole locations.
匹配条件： “ axial geocentric dipole hypothesis ..
Using the most recent global database of paleomagnetic directions for the past 4 Myr we have tested whether the far-sided effect of Wilson (1970, 1971) remains a stable feature of the time-averaged field. We found out that this characteristic persists for all sub-time intervals as well as for different sites distributions. The U-shaped pattern of the mean inclination anomaly (deviation from the inclination of the axial dipole) as a function of latitude is described by a small quadrupole contribution that amounts 5% of the dipole. There is no need for other terms which in any case cannot be properly described given the overall dispersion of the data. We have analyzed the evolution of the quadrupole/dipole ratio for periods characterized by different mean axial dipole strength using composite curves of relative paleointensity. We report that periods of weaker dipole field are effectively characterized by a larger mean inclination anomaly and thus by a larger quadrupole/dipole ratio. We infer that the mean value of the inclination anomaly could potentially be an indirect indicator of the mean dipole strength.
Time-Averaged and Mean Axial Dipole Field | …
The geomagnetic poles are where the axis of a best-fitting intersects the 's surface. This dipole is equivalent to a powerful bar at the , and it is this theoretical dipole that comes closer than any other to accounting for the magnetic field observed at the Earth's surface. In contrast, the actual Earth's magnetic poles are not antipodal—that is, they do not lie on a line passing through the center of the Earth.