Last night activity

Auroral light is a consequence of collision prosesses in the upper atmosphere, at altitudes 90-200 km. Energetic electrons which precipitate downward along the geomagnetic field lines collide with atmospheric oxygen and nitrogen atoms, which get excited to higher energy levels. After a while the excited states relax and the atoms emit light (typically yellow-greenish or red).

Earth's magnetic field configuration is similar to that or a large magnetized rod located at centre of the Earth. In Finland the strength of magnetic field is 50000 nT. Auroras generate electric currents which flow in the upper atmosphere and decrease the north-component of geomagnetic field (during periods of active auroras the curves deviate downward in the magnetograms shown in the right panel of this page). These auroral perturbations in the magnetic field can be even larger than 1000 nT.