RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES4003, doi:10.2205/2007ES000258, 2008
[7] The actual magnetic field of the Earth consists of a sum of several contributions: from the nucleus (the main geomagnetic field), from the crust (the field of local anomalies) and from magnetosphere (the field of sources, external in relation to the Earth's volume). The biggest contribution into the magnetic field of the Earth is made by the main magnetic field; even if you imagine that the Earth's crust (the layer ~30 km, the lower rock doesn't get magnetized due to exceeding the Curie point) is totally composed of strongly magnetized rock (basalt and gabbro), then the contribution of the Earth crust would equal only to a few per cent of the field observed [Sharma, 1989]. The fluid nucleus of the Earth cannot have a permanent magnetization, because the latter is of a quantum nature and intrinsic only to crystals [Kitel, 1978]; the origin of the main field is attributed to the presence of circular currents in a nucleus, acting like a dynamo mechanism. The central (located in the centre of Earth) magnetic dipole, best of all approximating to the Earth's magnetic field, has a magnetic moment 8.0 1022 A m2, and its axis is tilted about the axis of Earth's rotation at approximately 11.5o. The first calculation of this magnetic moment was done by Gauss in 1835, the magnetic moment at that time was equal to 8.5 1022 A m2; since then the Earth's field is getting weaker approximately by 5% in a century. P. Sharma [Sharma, 1989] thinks that "the magnetic fields of the Earth's main dipole could disappear in two thousand years and leave researchers of geomagnetism jobless in the near future''. If the latter is correct, then the navigation systems in Russia, based on the use of magnetic field, could also stop working in the same period of time. The existence of inversions of the main magnetic field is supposed, i.e. magnetic poles change after each 2-5 thousand years. About a half of the Earth's crust is magnetized to the direction, opposite to the direction of the main magnetic field [Sharma, 1989]. The non-dipole part of the main field (average strength ~10,000 nT, at average full strength ~45,000 nT), i.e. obtained by subtracting the dipole field from the main geomagnetic field, isn't perhaps prone to inversions [Sharma, 1989]. The present main field changes unevenly in space, but has a tendency, on average, to drift to the west (western drift). In the present work the dynamics of the Earth's main magnetic field was examined only by three time series: in 1995, 2000 and 2005.
[8] The average value of the horizontal component of intensity of the main field is ~20,000 nT. The average intensity of the external field from electric currents in the ionosphere is much less, about 20 nT. Local anomalies of the field from the Earth's crust could be relatively large, reaching 190,000 nT for the Kursk magnetic anomaly [Sharma, 1989], but they aren't mapped on scale 1:25,000,000. Magnetic storms can cause changes up to 2000 nT, but they are short-lived (one or two days), and geomagnetic measurements aren't usually taken during storms. The points, where the horizontal component of the main magnetic filed is equal to zero, are called magnetic poles. The central magnetic dipole produces geomagnetic poles, not coinciding with magnetic poles (no geomagnetic poles were determined in this work). The magnetic poles aren't antipodes, they coincide with the tilted eccentric dipole, located at approximately 300 km from the Earth's center and inclined to the axis of Earth's rotation at an angle of about 11.5o. It was agreed to call the magnetic pole, situated near the northern geographical pole, the North magnetic pole, in spite of the fact that it corresponds to the southern pole of the main dipole (analogically the South magnetic pole). The line, on which the vertical component of the magnetic pole is equal to zero, is called magnetic equator. The main field to the north of this equator has a vertical component going deep into the Earth's interior, to the south - from the center of the Earth. A line, called magnetic meridian, a tangent in each point of which is parallel to the vector of horizontal component of strength of the main magnetic field.
[9] Strength of the main magnetic field is measured by satellites, airplanes, ships, observatories and is systematically recorded every 5 years (this period in application to magnetic field is called epoch), beginning from 1945. The main geomagnetic field, determined by these systematic measurements, is known as the International Geomagnetic Reference Field (IGRF). Velocity of change of the main field in each point is also recorded in IGRF. Results of these measurements are calculated into the potential of the main field and velocity of its change. The value of the field in a point between two dates of measurements in the given point within an epoch is determined by the field and velocity of its change by the linear interpolation method. The values of vector of the main field's strength are calculated as minus a gradient of potential for a selected date and a height above sea level. In the present work this height was selected equal to zero, and the data from 1995, 2000 and 2005 was used. (The last at the current moment of the epoch of IGRF measurements).
Citation: 2008), Chart-making of the Earth's main magnetic field, Russ. J. Earth Sci., 10, ES4003, doi:10.2205/2007ES000258.
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