The strength of the interplanetary magnetic field is given a value called Bt and is measured in nanoTesla (nT). The north-south direction of the interplanetary magnetic field (Bz) plays an important role in the formation of Northern Lights. When the north-south direction of IMF flips south, the magnetic field lines connect to the earth’s magnetosphere which faces north. A rift opens that allows the solar wind to pour into our magnetosphere. Bz becomes negative, for example –10nT, which is a good sign that auroras will start to appear.
So, when you are out chasing the northern lights, watch the orientation of the IMF closely. When it flips south, odds are that northern lights are visible overhead.