How bright are the Northern Lights?
1, The 11 year solar cycle varies the volume of charged particles emitted by the sun. In other words, the solar wind is not constant.
2, The time of year, whether the northern hemisphere is pointed toward or away from the sun. In NH summer, the NH points toward the sun so the interaction between the atmosphere is more excited but, the nights are much shorter and, in fact non-existant for several weeks. so total out put is low. In NH winter, the NH points away from the sun so the light is less bright but it lasts much longer because the sky is dark for weeks.
3. The latitude: The magnetic field is stronger closer to the poles. Sometimes the aurorae can be seen further south when solar flare activity is high.