Magnetic field of Nurmijärvi

The Earth has a magnetic field that resembles the field of a bar magnet located at centre of the Earth. In Finland the strength of the field is about 51000 nT (nanotesla). Magnetic field is a vector quantity which means that in addition to its strength it is important to know also its direction. The standard measurements of the geomagnetic field determine its geographic north (X), east (Y), and downward (Z) components. In Finland the field is directed almost perpendicularly downwards and thus the Z-component is much larger than the X- and Y-components.

The accompanying magnetogram show the X-, Y-, and Z-components of the magnetic field recorded today at the Nurmij&aulm;rvi geophysical observatory. At the left side of the magnetograms you can see the nanotesla scale that can be used to esimate the amplitudes of the varions in the field components.

Electric currents flowing in the upper atmosphere (in the ionosphere at 100 km altitude)cause continuous variability in the geomagnetic field. During auroral periods the strongest currents flow in the east-west direction causing variation especially in the X-component which typically decreases (i.e. the curve deviates downward).

The strength of the ionospheric currents can be easily estimated if the amplitude of the magnetic field variation is known. According to a rough thumb rule one nanotesla deviation corresponds to a current enhancement of 1000 amperes. Such currents are large when compared e.g. to the few ampere currents of domestic appliances. In Lapland during auroral periods typical measured magnetic deviations are of the order of a few hundreds of nanotesla corresponding to ionospheric currents of a few hundreds of amperes. During the strongest geomagnetic storms, like at the end of October 2003, the ionospheric currents can easily exceed million amperes.