Kp Index Thresholds by US State

The Kp index is one number for the whole planet, but the Kp you personally need is local: it depends on the magnetic latitude of where you stand. This page is that translation for the United States — one row per tracked state, sorted from the lowest threshold to the highest. In Alaska the lowest tracked threshold is Kp 1+; in Virginia it is Kp 9+.

Kp threshold by state

Lowest Kp is the threshold of the most magnetically northern city we track in that state, so it is the easiest case in the state rather than a state-wide figure. Noticeable there is the level at which the model expects a clearer display at that same city, and range shows how much the threshold varies between tracked cities inside the state. Reaching a threshold is a necessary condition for a sighting, never a sufficient one.

Full per-city values, including the bright-display band, are published as the US aurora visibility threshold reference (version 2026-09-02) in JSON and CSV.

Where the number comes from

Aurora forms in an oval centred on the geomagnetic pole, not the geographic one. Because magnetic north sits away from true north, two US cities at the same geographic latitude can have different auroral reach, which is why the table is keyed on magnetic latitude rather than the latitude on a road atlas.

NOAA SWPC describes the auroral oval expanding towards the equator as geomagnetic activity rises. The shared calculateKpBands() model in this site applies that relation — roughly magnetic latitude ≈ 67° − 2° × Kp, adjusted for the fact that a display can be seen some way north of the observer — and assigns each city discrete horizon, noticeable, and bright bands. Every value in the table above is produced by that function from the city's stored magnetic latitude.

• NOAA SWPC: Tips for Viewing Aurora
• NOAA SWPC: Aurora Tutorial
• How Aurora Forecast USA works — the wider method and its limits

What a threshold does not tell you

A threshold is a planning reference. It says what geomagnetic activity would have to do before the attempt is worth making; it does not say the attempt will succeed. Conditions the table cannot see:

• Whether Kp actually reaches the threshold on the night you go out.
• Cloud cover, which blocks a display at any Kp value.
• Darkness — the sun and long summer twilight rule out much of the year in northern states.
• Moonlight, which washes out faint aurora.
• Local light pollution and a clear view north — see dark sky sites by US state.

No threshold on this page, and no status anywhere on this site, guarantees a sighting. Check current conditions close to the time you plan to go outside.

Check tonight

• Aurora visibility tonight — current conditions against your city's threshold
• US aurora forecast
• Northern lights near me