Answer by Veronica Ng, Founder of ChasingLights.com, on Quora:
The Aurora Borealis is on all year round (it's caused by solar activity and the Sun doesn't much care which month it is on Earth), and the only reason why it's better to see them between September to March is because the Northern latitudes get more darkness during these months.
To elaborate further:
In order to maximize your chances of seeing the Aurora Borealis, you should get as close to the center of the auroral belt as possible. The auroral belt is typically 3° to 6° wide in latitude and between 10° and 20° from the geomagnetic poles, putting the auroral band at a latitude between 65° and 72° North most of the time. The place with the highest chance of seeing the Northern Lights is therefore somewhere around 69°N, right in the center of the auroral belt.
The KP-index, which is commonly used in "aurora forecasts", is a scale from 0 to 9 and an indicator of geomagnetic activity. The higher the geomagnetic activity, the further South the Northern Lights can be seen, since aurora belt stretches and becomes wider when geomagnetic activity is high. This means that if we have a KP-index of 7 for instance, you can see the Northern Lights as far South as London, and if you have a KP-index of 0, you can only see the Northern Lights around 69°N.
Now, the areas at 69°N experiences the midnight Sun in summer. This means that in Summer, for around 6 weeks, the Sun remains above the horizon without setting even once. Since we need darkness to see the Northern Lights, you would not be able to see the Northern Lights during these months at high Northern latitudes like 69°N. You would, however, be able to see the Northern Lights even during these months in areas much further South, say, London, assuming we had a geomagnetic storm and the KP-index goes up to 6 or 7 or more.
If you would like to maximize your chances of seeing the Northern Lights, your absolute best bet would be to go to Tromsø, Norway (located 69°N) between the months of September and March, and go out on at least 2 or 3 proper Northern Lights chases (that means chasing after clear sky, and choosing locations with clear and dark views of the horizon in the necessary directions). I run Northern Lights tours from Tromsø and our success rate is easily in the high 90-percentile because of the fantastic geographical location.
If you absolutely must travel in late July or early August, then you have to go further South since it's too bright in the Northern latitudes to see the Northern Lights. However, your chances of seeing the lights would also be much lower since you're much further from the auroral belt.
I hope this helps!
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