The interplanetary
magnetic field (IMF) is the magnetic field that flows with the solar wind
from the sun through the whole heliosphere. To get good conditions for auroral
display the IMF has to point southward (Bz must be negative.). With that configuration
it has just the opposite direction of the earth's magnetic field and therefore
both show a maximum of interaction. This interaction is some kind of a coupling
between IMF and the geomagnetic field. This process leads to an erosion of the
geomagnetic field. Through this weak regions the charged particles of the solar
wind (electrons and protons) can deeply penetrated into earth's magnetic field
and spiral along the magnetic field lines down towards the surface of earth.
These particles then cause the popular and always awaited phenomenon of the
aurora borealis.
For the initiation of a geomagnetic storm and the resulting northern lights
the IMF or better the direction of the IMF is the most important parameter.