What is the Kp index?

A magnetometer records the horizontal part of Earth's magnetic field. On a quiet day the trace barely moves. When energy from the solar wind pours into the magnetosphere, currents in space and in the upper atmosphere swing that trace back and forth. The K-index turns the largest swing in a 3-hour block into a single digit.

Each observatory has its own K-index. The planetary index, Kp, is the mean of standardized K values from 13 observatories. SWPC places them between 44 and 60 degrees of geomagnetic latitude, north and south.

SWPC's own wording for the scale is simple. Kp 1 is calm, and 5 or more marks a geomagnetic storm. The index dates to 1938.

For aurora watchers the useful part of Kp is its link to the oval. SWPC's tips page gives a rule of thumb: at Kp 0 the equatorward edge of the oval sits near 66 degrees geomagnetic latitude, and it moves about 2 degrees toward the equator for each Kp step. Kp 5 puts the edge near 56 degrees. Kp 9 puts it near 48.

The table applies that rule to Glow's 10 featured places. It also shows the NOAA G-scale level that matches each Kp and the SWPC Kp band it falls in.

Two cautions come with the rule. It works in geomagnetic latitude, which differs from map latitude by 5 to 9 degrees in the lower 48 states. And SWPC calls the relationship approximate: it describes averages, and a given storm can run wider or narrower. Glow's Kp to latitude chart walks through the conversion for any place.

The number most aurora apps show is SWPC's estimated planetary Kp. It updates every minute from a smaller set of magnetometers: Sitka, Meanook, Ottawa, Fredericksburg, Hartland, Wingst, Niemegk and Canberra. That is 8 stations against the 13 behind the definitive index, so the estimate can shift once the full set comes in.

Kp is also a 3-hour value. A burst of aurora can start and fade inside a single block, and the index for that block will only say that the field was disturbed somewhere in it. When you watch a live number climb, you are watching the block in progress.

SWPC ties its watches, warnings and alerts to Kp thresholds. A watch looks ahead to the highest Kp predicted for a day. A warning says a Kp level is expected. An alert says it was reached. The watches, warnings and alerts guide lists the exact thresholds.

Kp is one number for the whole planet. It cannot say whether the oval is bright over Alaska while Scandinavia sits in daylight, and it knows nothing about your cloud or your moon. That is why Glow does not print Kp as a local forecast.

For a local answer Glow reads the OVATION model instead. SWPC's OVATION product maps aurora intensity cell by cell at 1 degree spacing, driven by solar wind data measured upstream at the L1 point. The aurora chance tool reads the cell over your coordinates and sets it beside darkness and cloud.

Kp is still the right number for planning. It sets the NOAA storm scale, it drives the alerts, and it gives a quick sense of reach. Minneapolis, at 53.5 degrees on the dipole, needs about Kp 7 before the average edge sits overhead. Fairbanks, at 65.7 degrees, needs Kp 1. The Kp needed tool runs the same comparison for your own coordinates.

Seeing aurora does not require the edge overhead. SWPC says bright aurora can be seen from as far as 1000 km away with a clear view toward it, so the edge can sit well north of you. The viewing distance guide converts that into degrees.