When you read forecasts about possible auroral chances, we speak often about the high, middle and low latitudes. But what does that mean? There is no exact definition where that boundary lies but the high latitude is situated around the 60° magnetic latitude and higher; the middle latitudes between the 50° and 60° magnetic latitude and everything below the 50° magnetic latitude is considered to be in the category of low latitude.
But wait? Magnetic latitudes? What is that? Well, when we are talking about aurora and the different latitudes, we always talk about magnetic latitudes and not geographic latitudes. You would say that when you are at a high geographic latitude (close to the north or south pole) you are more likely to see aurora. That is largely true but there is a catch: the earth's magnetic poles are not exactly in line with the geographic poles, that is why your magnetic latitude is important and not your geographic latitude. The magnetic north pole actually favours sky watchers in North America, that is why aurora there can be seen on lower geographic latitudes than in Europe. Note that the aurora is also shaped like an oval around the magnetic poles so it is even possible to be on a latitude that is too high to see aurora! It is rare for aurora to show up at the geographic north and south poles for example. Another thing to note is that the aurora is visible on the horizon even when its boundary is 4 or 5 degrees poleward of your location.
When geomagnetic activity is low, the aurora typically is located, in the hours around midnight, at about 67 degrees magnetic latitude. As activity increases, the region of aurora expands toward the equator. When geomagnetic activity is very high, the aurora may be seen at middle and low latitude locations that would otherwise rarely experience the polar lights. The map below shows if you are located on the high, the middle, or the low latitudes.
We can conclude that the middle latitudes need a Kp-index from about 4 to 7 depending on where you are. Belgium for example which is on the middle latitudes but close the lower latitudes would need a Kp-index of 7 for aurora on the horizon. The capital of France, Paris which is located on the lower latitudes would need a Kp-index of 8. Locations on even lower latitudes need a Kp-index of 9.
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