Our planet has a magnetic field which helps keep us protected from the otherwise devastating radiation of the Sun. This magnetic field acts like a defensive shield to keep the majority of the suns particles out thus keeping us safe. To overcome this enough that we see Auroras, a few things need to happen. This is a simplified view but it will help us to understand the forces at work that create Auroras.
First, the solar wind has to be strong enough to push the particle deep enough into our atmosphere to have a significant impact. This force is measured in Wind Speed and a good baseline for Auroras in Fairbanks is 350 km/s or stronger. Second, the particles being pushed by the solar winds need to be dense enough to overload the defensive shield and thus we look at particle density. A good baseline here is around 2 ions/cm3 or better for mild Aurora activity. Finally, we need a favorable IMF Bz component or a weak shield if you will. This is measured in nanoteslas and for good Aurora we need the Bz to be negative or "south." A good starting point here is anything below 0nT. All three of these forces interact with one another to determine our likelihood of seeing Northern Lights.
Think of it like this, If we are hammering a nail into a piece of wood, the wind speed is the driving force or the hammer. The bigger the hammer the more force. The nail size is the particle density, the bigger the nail the more impact it has. The Bz is the hardness of the wood. The more positive the harder the wood and the more force required to hammer the nail in. The more negative or "south" the Bz the softer the wood.