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The main part of the Earth's magnetic field is produced in Earth's core. This part varies quite slowly, months and years are needed to see the changes. Superimposed on this quiet main field we observe complex variations on time scales from less than a second to weeks, some rather regular and others very irregular. They are all caused by electric currents flowing in the upper atmosphere and magnetosphere. At high latitudes like Norway, currents in the ionosphere at altitudes between 100 and 150 km are dominant. Their strengths may exceed a million ampères.
For each station we have subtracted the quiet internal part from the observed field.
The resulting external component has then been used to calculate a corresponding electric current in
the ionosphere.
Theoretically many different distributions of the currents will give the observed magnetic effect;
a simplifying model of the current distribution is required. We have used the simplest possible,
and still fairly realistic, model of the currents: we assume it flows horizontally along a straight line
in the ionosphere.
According to Ampère's law a current I in an infinite straight line and the magnetic field ΔF at a distance
r are connected by the relation: I = ΔF * r / 200 * (nT * m) -1 A By using also the direction of ΔF and fixing the altitude of the currents to 130 km, we compute I, its direction as well as distance and position. On the map, the current for each station is marked as an arrow on a little box with the station acronym. Pointing the cursor to the box will cause the current strength to appear. Currents calculated this way are traditionally called equivalent currents. As the term indicates, they should not be taken as a exact description of the current system, the real currents are far more complicated. Occasionally this results in unrealistic current configurations. Nevertheless, the equivalent currents are useful when assessing the ionosphere and solar terrestrial interaction, and have served as a useful concept in space physics for a century. The arrows are yellow when the geomagnetic activity is low. The currents are then dominated by the so-called Sq current system, which is basically driven by the Sun's heating of the atmosphere. At our latitudes it is weak in winter, but clearly visible in summertime. Note how the direction shifts regularly in the course of the day. At enhanced activity the arrows get pink and another system of currents, caused by interaction between the solar wind and Earth's magnetic field, appears in the polar region. During magnetic storms the arrows get red and these currents may spread far to the south, in particular during evening and night. Quite often the arrows then line up in direction east-west over northern Scandinavia. We then have an auroral electrojet, a strong narrow current nearly always associated with the presence of Northern Lights. To learn more about currents in the upper atmosphere, see for instance: J. K. Hargreaves: "The solar-terrestrial environment", Cambridge University Press 1992, and Asgeir Brekke: "Physics of the Upper Polar Atmosphere", John Wiley & Sons, Chichester 1997.
14 August 2007 |