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Magnetic Field Of Earth Pdf Download UPDATED


The World Magnetic Model is the standard model used by the U.S. Department of Defense, the U.K. Ministry of Defence, the North Atlantic Treaty Organization (NATO) and the International Hydrographic Organization (IHO), for navigation, attitude and heading referencing systems using the geomagnetic field. It is also used widely in civilian navigation and heading systems. The model, associated software, and documentation are distributed by NCEI on behalf of NGA. The model is produced at 5-year intervals, with the current model expiring on December 31, 2024.




Magnetic Field Of Earth Pdf Download



The December 2021 State of the Geomagnetic Field report is now available. This report provides an assessment of the performance of the WMM2020 two years after its release, and describes noteworthy changes in the Earth's main magnetic field, including magnetic pole drifts and the deepening of the South Atlantic Anomaly.


Nandini Nagarajan is a geophysicist who has spent over 40 years in studies of geomagnetism covering commissioning of observatories, low-latitude geomagnetic phenomena, magnetotelluric field campaigns and studies of electromagnetic induction for crustal structure and resource exploration. She has worked at the Indian Institute of Geomagnetism and National Geophysical Research Institute.


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In Sect. 2 we present a detailed description of the asymmetric features in the magnetic field in the two hemispheres. The subsequent sections explore the effects of these asymmetries on plasma drift (Sect. 3), thermospheric wind (Sect. 4), total electron content (Sect. 5), ion outflow (Sect. 6), currents and magnetic field perturbations (Sect. 7), and the aurora (Sect. 8). Section 9 concludes the paper.


Figure 2 shows the ground magnetic field strength (left column) and absolute inclination angle (right column) in the NH (top) and SH (middle), in the apex quasi-dipole (QD) coordinate system. The bottom row shows the inter-hemispheric difference in these quantities. The difference in magnetic field strength is quantified as the hemispheric difference divided by the flux density at the footpoint with the strongest field. Positive values signify stronger field values in the NH. The asymmetry in field inclination at conjugate points is quantified as the difference between the angles, positive where the field is closest to vertical in the NH.


We see that the flux density is more uniform in the NH than the SH. The field in the NH has two maxima, located in the Canadian and Siberian sectors (around \(-30^\circ\) and \(180^\circ\) magnetic longitude, respectively). In the SH the field has only one maximum, off the apex pole towards Australia (at \(\approx-135^\circ\) longitude), and decreases significantly towards the South Atlantic region. The difference at conjugate points at Atlantic longitudes is up to a factor of 2. In the polar cap region poleward of \(\approx \pm 80^\circ\), the field is stronger in the SH by approximately 7%. Equatorward of this, the field is strongest in the NH everywhere except for the quadrant between \(-90^\circ\) and \(180^\circ\) magnetic longitude.


The inclination or dip angle of the magnetic field is also different in the two hemispheres. The hemispheric difference follows approximately the same pattern as for the magnetic field strength, with the field lines in the NH more vertical in the regions where the field is strongest. The asymmetry reaches a peak in the \(0^\circ\mbox--90^\circ\) longitude sector, where the difference reaches more than \(10^\circ\) at latitudes just poleward of \(\pm65^\circ\).


Figure 3 illustrates the longitudinal variation of the magnetic field in both hemispheres. The left part shows the relative difference between the strongest and weakest field values along circles of constant magnetic latitude (maximum divided by minimum), given on the \(x\) axis. The dashed and dotted curves show the corresponding relative differences in Pedersen and Hall conductances, assuming that they scale as \(B^-1.6\) and \(B^-1.3\), respectively. We see that in the SH, the magnetic flux density varies by more than a factor of 2 at \(55^\circ\) latitude. The corresponding variation in daytime Pedersen conductance is approximately a factor of 3.5 and Hall conductance close to 3. In the NH, the magnetic field is much more uniform, the relative longitudinal variation in flux density at \(>50^\circ\) being approximately 1.25 at most. These inter-hemispheric differences, together with larger daily variation in solar illumination, are likely to produce larger diurnal variations in geomagnetic activity in the SH compared to the NH.


While this description accounts for the dominating large-scale circulation of plasma and magnetic flux in the ionosphere and magnetosphere, large variations are observed in the global morphology of ionospheric convection. Statistical studies of ground- and space-based measurements have shown that the average patterns strongly depend on the orientation of the IMF (Heppner and Maynard 1987; Weimer 2005; Ruohoniemi and Greenwald 2005; Pettigrew et al. 2010; Haaland et al. 2007). During northward IMF, the two-cell convection pattern on average reduces, and one or two small cells appear additionally at high latitudes on the dayside (Förster et al. 2008a). The convection pattern also rotates in a systematic way with changes in the IMF Geocentric Solar Magnetic (GSM) \(y\) component. The sense of the rotation depends on the sign of the IMF \(B_y\) component, and is opposite between hemispheres. These effects can be explained in terms of different dayside magnetic field geometries (Cowley 1981), assuming that reconnection primarily occurs at the points where the IMF and the magnetosphere are most strongly anti-parallel.


The overall flux transport across the polar cap can be quantified in terms of the cross polar cap potential (CPCP), measured as the maximum electric potential difference in the polar regions. Several statistical studies have found that the CPCP is on average slightly stronger in the SH compared to the NH. Pettigrew et al. (2010), who used SuperDARN radars from both hemispheres, found a difference of \(6.5\%\). Papitashvili and Rich (2002), who used measurements from the Defense Meteorological Satellite Program (DMSP), found a difference of \(10\%\). Förster and Haaland (2015), who used Cluster electric field measurements mapped to the ionosphere, found differences of \(\sim5\%\mbox--7\%\). All these authors cite the differences in the geomagnetic field as a possible cause for the asymmetries.


On a large scale, the nightside auroral zone is characterized by enhanced outflow which largely balances the electron precipitation responsible for auroral arcs. Ionization, at least on the nightside where EUV illumination is absent, is primarily driven by the auroral precipitation (e.g Hultqvist et al. 1999). The outflow is mainly driven by strong field aligned electric fields caused by anomalous resistivity, and both \(\mathrmH^+\) and \(\mathrmO^+\) can be extracted and accelerated to escape energies. Furthermore, the nightside auroral zone is co-located with a region of Birkeland (magnetic field-aligned) currents and strong flow shears which locally tend to break up into vortices. Such small scale structures may provide an additional source of energy for plasma escape.


Both ionospheric currents and the associated magnetic disturbances depend on quantities that are best organized in different coordinate systems: The ionospheric convection (and \(\mathbfE\)), as well as the conductance produced by auroral precipitation, are organized in magnetic coordinates, while the component of the conductances that is produced by solar EUV flux is best organized in geographic coordinates. Therefore the distribution of sunlight on magnetic apex/CGM grids in the two hemispheres is never symmetrical, and perfect hemispheric symmetry in the current and magnetic disturbance fields can not be expected either.


To illustrate this point we look at the seasonal and diurnal variation in magnetic field perturbations at two pairs of nearly conjugate magnetometers. Their locations are indicated in the top left map in Fig. 4: The filled circles show the positions of the UMQ station (at \(75.6^\circ\) apex latitude, and \(41.2^\circ\) longitude in 2015) in blue and the B22 station (at \(-75.7^\circ\) and \(30.8^\circ\)) in red. The triangles mark the LYR station (at \(75.4^\circ\) and \(109.2^\circ\)) in blue and the DVS station (at \(-74.7^\circ\) and \(102.3^\circ\)) in red. They are all at nearly the same magnetic latitude, but their locations relative to the geographic poles are different. Figure 10 shows the mean magnetic perturbation at these magnetometer stations as a function of universal time hour and month. The SuperMAG baseline subtraction has been used, which is designed such that the remaining signal can be interpreted as being associated with external (solar wind/magnetospheric) drivers (Gjerloev 2012). Diurnal variations associated with the solar quiet (Sq) currents are removed. Conjugate pairs are shown in the same columns.


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