When the Earth’s magnetic field is active, the aurora lights up and can be seen farther away from the poles. Geomagnetic activity is influenced by solar activity and coronal holes on the Sun, which change over time. We use a measurement called the planetary K index or Kp to determine the level of geomagnetic activity. The Kp index ranges from 0 to 9.
If the Kp index is between 0 and 2, the aurora will be located far north, appearing faint and not very active. It won’t be as impressive to observe.
When the Kp index falls between 3 and 5, the aurora moves away from the poles, becomes brighter, and displays more activity such as movement and formations. If you are in the right place, you can enjoy a beautiful view of these auroras.
For Kp values from 6 to 7, the aurora moves even further from the poles, becomes significantly brighter, and shows increased activity. At this level of geomagnetic activity, it might be possible to see the aurora from the northern edge of the United States.
When the Kp index reaches 8 to 9, the aurora moves even closer to the equator, becoming very bright and active. These are the most spectacular auroras, and they can be seen by a larger number of people. At these levels, people in the northern states of the USA may even witness the aurora directly above them.
It’s important to note that the relationship between the Kp index and the latitude where the aurora is visible is not exact. The given ranges represent averages, but there may be instances when these relationships don’t precisely hold true.
There is an approximate connection between the Kp index and the southernmost extent of the auroral oval. This relationship is based on geomagnetic latitude, not geographic latitude. When the Kp index is 0, the auroral oval extends approximately to 66 degrees. For each level increase in Kp, the aurora moves about 2 degrees closer to the equator. For example, at Kp = 1, the aurora would move to around 64 degrees, and at Kp = 2, it would reach about 62 degrees. This pattern continues until a Kp index of 9, where the aurora can be seen as far south as 48 degrees magnetic latitude.
This information and image is rephrased and referenced from an official SWPC article.
Click here to view the NOAA/SWPC article or visit https://www.swpc.noaa.gov/content/tips-viewing-aurora
