The Kp index explained: from quiet nights to G5 storms

What Kp is

The planetary K-index, Kp, summarises how disturbed the Earth's magnetic field is. Magnetometers around the world measure the deviation from a quiet day. The deviation is converted to a number from 0 to 9 for each three-hour block of the UTC day, and the stations are combined into one planetary value.

Kp is reported in thirds: 0, 0.33, 0.67, 1.0 and so on up to 9.0, which gives 28 possible values. Written the old way, 4.67 is 5− and 5.33 is 5+. NOAA's real-time series on the Aurorix chart uses the decimal form.

What NOAA says Kp means for aurora

NOAA's viewing guide groups Kp into bands. At Kp 0 to 2 the aurora is far north, quite dim and not very active. At Kp 3 to 5 it moves further from the poles, gets brighter and shows more motion. At Kp 6 to 7 it is quite bright and active and can be seen from the northern edge of the United States. At Kp 8 to 9 it is very bright and the oval is visible to the most people, with aurora possible directly overhead in the northern states of the USA.

NOAA adds that the relationship between Kp and auroral latitude is approximate and represents averages. There are nights when it does not hold.

Storm levels G1 to G5

NOAA also publishes the geomagnetic storm scale. A storm starts at Kp 5, which is G1. Kp 8 including 9− is G4 and only Kp 9 is G5. The table below is NOAA's own, with the typical frequency over an 11-year solar cycle.

Those frequencies matter for planning. A G1 storm happens on about 900 days of an 11-year cycle, which is roughly a fifth of all days. A G5 storm happens on about 4 days in a cycle. If you live in the Nordic countries you do not need one, as the next section shows.

Where the oval sits at each Kp

NOAA's rule of thumb is that the equatorward edge of the oval is near 66 degrees geomagnetic latitude at Kp 0 and moves about 2 degrees toward the equator for each Kp step, reaching 48 degrees at Kp 9. The table below applies that rule. The destination pages convert each place to geomagnetic latitude and read off the Kp at which the edge reaches it.

The scale page and the 2-degree rule are not identical. NOAA lists aurora "typically" at 55 degrees for G2, while the rule gives 54 degrees for Kp 6. Both are averages; use them as a guide to the order of magnitude.

What to do with Kp on a trip

• Check Kp for the dark hours of the night you are there, not for now. The forecast chart shows three-hour blocks.
• Compare it with the Kp your place needs. Several of the places on Aurorix need Kp 0 to 2, so a "quiet" Kp is not a reason to stay in.
• Treat Kp as the activity level and the OVATION value as the position of the oval at this moment. They answer different questions.

NOAA geomagnetic storm scale

Source: NOAA SWPC space weather scales, https://www.swpc.noaa.gov/noaa-scales-explanation

Where the oval reaches at each Kp

NOAA's rule of thumb: the equatorward edge is near 66 degrees geomagnetic latitude at Kp 0 and moves about 2 degrees toward the equator for each step.

Lähteet

• NOAA SWPC: Tips on viewing the aurora
• NOAA SWPC: Space weather scales
• NOAA SWPC: Planetary K-index