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| Magnetic variations |
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The Earth’s magnetic field exhibits a variety of fluctuations. The strength of the field can suddenly decrease by up to more than 2% and than slowly increase again. These magnetic variations can be divided into two main categories.
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The first are longtime variations (10-10 years), which have their cause in the interior of the Earth. The second are variations due to electric currents in the upper atmosphere and/or in close space. This second type can last from seconds to one sunspot period.
Electric currents in the upper atmosphere and close space give the following three characteristic disturbances at the Earth’s surface:
• Daily variations due to neutral winds in the upper atmosphere
• Disturbances in the polar regions - magnetic sub storms - mainly due to electrojet
• Magnetic storms arising in near space after a fast increase of the particle population and energy
We will take a closer look at these disturbances in the next sections. |
| | | Daily disturbances |
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The daily variations arise from movements in the ionosphere which set up currents that modify the overall magnetic field, especially in the E-layer (95-150km). The movements are caused by an irregular distribution of solar heating (a sort of tide effect), and they are greatest in summer. We call this effect Sq (S for sun and q for quiet).
The Sq -effect is so small (typically ≈ 10 nT) that one normally does not take it into account. It was Kristian Birkeland who first pointed out that the main source for the magnetic disturbances are electric currents in the upper atmosphere. Sometimes at auroral latitude - during high solar activity, the daily shifts can be dramatically larger – more than 50 nT; corresponding to as much as nearly 100 km shifts in the location of the magnetic pole (see the figure to the left)
The upper part of the figure shows how a magnetic sub storm (September 19th,1977) leads to changes of the horizontal component of the earth’s magnetic field in Ny Ålesund, at Bjørnøya and in Tromsø. The lower part of the figure shows how a magnetic sub storm can come back after one or more rotations of the sun.
There is another, minor, effect following the moon phases. A very detailed analysis of the magnetic data is necessary to determine the moon’s contribution to the daily magnetic disturbances. |
| | | Disturbances in the Polar Regions | |
Disturbances concentrated in polar regions (i.e., on the pole side of ≈ 60° geomagnetic) occur due to powerful electric currents in the highest regions ≈ 100-150 km. These intense geomagnetic disturbances usually appear one or two days after large solar outbursts (see the figure below). It is also observed that these disturbances vary in accordance with the sunspot cycle. This strongly shows a connection between the Sun and the Earth. |
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The upper part of the figure shows triangulation of the electric current system by means of the magnetic registrations from Bjørnøya and Tromsø. The lower part shows the magnetic power lines around a horizontal conductor with current direction out of the paper plane.
In polar regions, the ionization rate (i.e., the relationship between the number of ionized and neutral molecules/atoms) is especially high in the upper air layers, because there, the atmosphere is also bombarded with electrons and protons, which easily penetrate into these regions. This particle flow, which causes the northern and southern lights, creates powerful electric currents in this high region. It is therefore natural to expect that these currents will cause magnetic field disturbances on the Earth’s surface. The fact that magnetic field disturbances coincide with strong northern lights activity was proved more than 100 years ago. The electric current density is given by: |
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The total current strength over a 1000 km-wide current zone can be around 1 million amperes. These currents result in powerful disturbances in the magnetic field measured at the ground, especially at night. The currents seem to be concentrated as an almost-line, forming a current element along one part of the northern light zone. Birkeland studied these simpler currents by triangulating from several observatories, after a principle sketched in the figere above.
The currents are situated at 100 km-150 km altitude, with amperage on the order of 106 - 107A.
(Bright arcs are usually associated with electrical currents flowing from Earth into space. It may be confusing, but age-old convention has it that currents flow from (+) to (-). Therefore, when negative electrons come down, the currents they carry flow upwards.)
Examples showing the variation of the horizontal component of the Earth’s magnetic field at three Norwegian observatories are shown in Figure 3.4c.
The disturbances can reach up to 100-150 nT, which is about 2% of the total field at the location, but that is seldom. The duration of such magnetic sub-storms is typically some ten minutes, up to a couple of hours. In Tromsø, which is centrally situated in the auroral zone, one or more such sub-storms are registered every evening/night. In the Oslo area, the incidence is less frequent. Sub-storms are regional, and occur almost always inside the auroral zone.
Figure 3.4c: The horizontal component of the Earth’s magnetic field at three Norwegian observatories. |
| | | Magnetic storms |
Powerful magnetic storms, global in spatial reach, occur from time to time. They appear to be due to electric storms in our near space, especially in a ring-shaped electric current located close to the equatorial plane at 4-7 RE. The effect of this ring current on the ground is therefore strongest at low latitudes; i.e., closer to the equator.
The duration of these storms can be everything from several minutes up to a maximum of two to three days. Singular powerful currents repeat after 27, 54, and 81 days. This recurrence is why it is natural to assume that these currents are related to more active regions on the Sun, because we know that the same regions point towards the Earth with a 27 day recurrence period. The total energy being dissipated in a magnetic storm can be calculated to be up to a billion kilowatt hours (109 kWh).
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