Variations in the Geomagnetic Field
|
Magnetic Declination for London: 1600 6 degrees East 1800 75 degrees West 2004 6 degrees West. |
||
|
The geomagnetic field varies in time and space
Polar wandering
BBC News, Dec 31, 2003: Earth loses its magnetism… Magnetic field 10% fading …
|
This map shows the geographic variation of magnetic declination over a period of 400 years. The time is indicated by the changing number above |
"Magnetic north remains elusive..." Ned Rozell in Heartland Feb 16, 2003
Secular variations
Reversals Magnetic Anomalies Westward drift |
|
We know that the intensity of the field underwent a decline in strength since it was first mathematically analyzed and mapped in detail by Carl Friedrich Gauss in 1835. Changes in the geomagnetic field can occur rapidly within seconds as well as over a long period of time. Scientists differ between changes to the internal dipole field and the fluctuations displayed by the non-dipole field. Periods that take longer than a few years to evolve are thought to originate deep within the Earth. Variations in the Main Field. Apart from the fact that the magnetic poles can 'jump' and be here today and there tomorrow, the poles have also undertaken a great journey circumscribing the polar regions. The North Pole is currently located in the Canadian Arctic at 82.7 degrees N, 113.4 degrees W (National Geophysical Data Center, 2004), and it is on a north-westward move of about 10 kilometers per year (Rodzell, 2003). When it was discovered in 1831, James Clark Ross found it at 70 degrees N, 96.5 degrees W (Merrill & McElhinney, 1983). The deviation from true north changes therefore all the time. But in addition to the movement of the poles, the field's intensity has been waxing and waning (National Geophysical Data Center, 2004). Scientists say that ancient pottery and sediment tell us that it declined on average about 0.2% for the last 1000 years (Backus et al., 1996). Since the field has been mathematically analyzed in the mid-nineteenth century, it has declined 10% (1996). The phenomenon is closely watched as a weakened field could be a prelude to a reversal. Reversals are random, however and no one knows for sure how to predict them yet (Glatzmaier, 1997). The average period between the more recent reversals for the past 15 Ma years is about 100 000 years (Backus et al., 1996), yet the current period exists approximately for 780 000 years. Times of transition are estimated to have lasted from 2 000 to 10 000 years in the past (1996). For possible causes and a simulated model for a reversal, move here source: http://www.phy6.org/earthmag/reversal.htm Paleomagnetization, the alignment of ferric minerals inside rocks such as basalts at mid-ocean ridges schematically here pictured above, has become a useful source of information for the behavior of the main magnetic field. It has led to the discovery of the polar wandering and in conjunction helped solve the phenomenon of lithospheric plate movement. Paleomagnetization still plays a major role in the determination of paleogeographic locations of continents or terranes and the temporal resolutions for former magnetic field reversals and field intensities. |
Variations in the non-dipole field. The non-polar field or lithospheric field is caused by the magnetization of the crust. It has a depth of 10 to 50 km, and its magnetic properties are characterized by very short wavelengths (Holme, 2002). It varies in that it exhibits a slow westward drift. This drift occurs at a rate of 0.3 degrees in a year (Merrill & McElhiney, 1983). Slow changes for the magnetic field are termed secular variations. But not all of the regions of the field strength distributions move at the same rate, and some of them are stationary and are called standing parts. The westward drift is recognized by the westward movement of isolated pockets (Holme, 2002). These isopores of maximum or minimum field intensities are known as magnetic anomalies. See the low intensity isopore of the South Atlantic below.
This map delineates the magnetic field into regions of different strength. Note the high and low patches in the North and South called isopores. The units are nanoTesla. These lines of field strength tend to move in a western direction in what has been termed westward drift. Isolated norm deviations have been recognized in many places around the world: in Central Africa, along the South Atlantic coasts, in Jemen, Siberia, Minnesota and in Mongolia. Some of these such as Mongolian Anomaly are stationary and cannot be explained by the westward drift. (Merrill & McElhinney, 1983). Anomalies occur in an area the size of hundreds of kilometers across, others are just a few meters wide (Backus et al., 1996). They can be the result of strong clockwise and counterclockwise eddy currents in the inner core (Holme, 2002); they can also be caused by subsurface conditions like a fault, crustal thinning or duplication, and differential magnetization such as adverse polarity. The movement of the westward drift is analyzed to study the flow of circulating convection currents (Holme, 2002). See a geological cross-section diagram for a faulted subsurface anomaly. |
Finally, there are variations in an external field measured at the surface and directly above Earth in space. They are due to electrical currents in the ionosphere relating to the sun's activity. The fluctuations occur as a result of the diurnal exposure to the sun (geomag.usgs.gov). According to these writers, particles in the ionosphere are ionized by the sunlight and generate their own magnetic fields, which add slightly to the horizontal intensity of the geomagnetic field. The magnetic force on the side facing away from the sun weakens slightly, but as the sun returns, the strength recovers. It varies in intensity up to 60 000 nT at the poles and 25 000 to 42 000 nT in lower latitude regions (Stern, 2001). source: http://geomag.usgs.gov/intro.html An example of diurnal variations measured at 11 observatory stations ranging from Alaska in the North to Guam in the South. Observe the high activity in the North More severe fluctuations in the magnetic field are registered during magnetic storms when solar wind feeds the auroral electrojet currents. These storms occur quite regularily, and slight variations for the surface field are the norm. But when the magnetic storm in space is severe, then the affects on the magnetic field are much more chaotic, leading to power outages, global communication disruptions and navigational equipment failure (Scientific American, 2001). The surface field is said to 'jump' at the onset of a large solar wind shockwave (Russell & Luhmann, 1997). A drop in surface field intensity of about 50 nanoTesla or more classifies a geomagnetic storm (1997). The field recovers usually within hours or days. For more on magnetic storms travel here The main field is decribed by non-linear mathematical models such as the International Geomagnetic Reference Field and charts are updated every five years. For a computation of magnetic field values for your area, to go National Geophysical Data Center |
