Aurora Forecast and the Kp Index Explained

The Kp index is a 0 to 9 number that measures how disturbed Earth's magnetic field is, updated in three-hour blocks. A reading of 5 or more means a geomagnetic storm, which NOAA labels G1 (Kp 5) through G5 (Kp 9). The higher the number, the farther from the poles the aurora has been seen: NOAA's scale cites northern Michigan and Maine at G1, New York and Idaho at G2, and as low as Florida and southern Texas at G5. For a shorter-term view, NOAA's 30-minute aurora forecast predicts where the aurora will be 30 to 90 minutes ahead.

What the Kp index measures#

According to NOAA's Space Weather Prediction Center (SWPC), the K-index quantifies disturbances in the horizontal component of Earth's magnetic field with an integer from 0 to 9, where 1 is calm and 5 or more indicates a geomagnetic storm. It's derived from the largest fluctuations a magnetometer sees during a three-hour interval. The planetary version, Kp, is the mean standardized K-index from 13 geomagnetic observatories between 44 and 60 degrees geomagnetic latitude, north or south. The "K" comes from the German word Kennziffer, "characteristic digit", and the index was introduced by Julius Bartels in 1938.

SWPC's estimated 3-hour Kp is derived from ground-based magnetometers including those at Sitka (Alaska), Meanook, Ottawa, Fredericksburg (Virginia), Hartland (UK), Wingst and Niemegk (Germany) and Canberra, with data from the US Geological Survey, Natural Resources Canada, the British Geological Survey, the German Research Centre for Geosciences (GFZ) and Geoscience Australia. The chart updates every minute.

Kp, the G-scale and how far south the aurora has been seen#

NOAA's Space Weather Scales turn Kp into a storm scale, G1 to G5. The table below uses NOAA's own wording for where the aurora has been seen at each level; these are historical observations, not guarantees.

NOAA's averages show how rare the high levels are: 900 days of G1 per 11-year cycle (roughly 80 days a year, our arithmetic), against about 4 days of G5 per cycle. A forecast of "Kp 5" in your town in the middle of the US doesn't mean you'll see anything; it means the aurora oval has pushed south toward you.

The 30-minute aurora forecast#

SWPC's aurora forecast is a different tool from Kp. It uses the OVATION model, built at Johns Hopkins Applied Physics Laboratory, and gives a 30 to 90 minute forecast of where the aurora will be and how intense. The lead time is how long the solar wind takes to travel from the L1 observation point, about 1.6 million km upstream of Earth, to Earth.

NOAA's notes for reading it:

• The maps show the aurora as a green oval around each magnetic pole; the oval turns red where the aurora is forecast to be more intense.
• The aurora isn't visible in daylight, and can often be observed somewhere on Earth from just after sunset or just before sunrise.
• You don't need it overhead: it can be seen from as much as 1,000 km away when it's bright and conditions are right.
• When the solar wind data aren't available, the model is driven from the current Kp instead, and there's no forecast lead time.

Why storms matter to satellites, not just photographers#

SWPC says storms heat the upper atmosphere and can add drag on low-Earth-orbit satellites, create errors in GPS positioning and harm power grids. The best-known case for the space industry came in February 2022. SpaceX launched 49 Starlink satellites on February 3 into a low insertion orbit; a geomagnetic storm then heated and expanded the atmosphere, the extra drag prevented the satellites from leaving safe mode to raise their orbits, and SpaceX said up to 40 of them would re-enter. NASA researchers have published an analysis of the event, The Loss of Starlink Satellites in February 2022, calling it a case of how moderate storms can affect assets in low-Earth orbit.

For the constellation side of that story, see our guides to how Starlink satellite counts work and what investors should track in satellite constellations.

Read it yourself: a quick routine#

1. Check Kp and the G level for the next 24 hours; Kp of 5 or more means a storm is forecast.
2. Open the aurora forecast near sunset; look for the oval reaching your latitude.
3. Check the sky: dark site, clear weather, away from city light, facing the magnetic pole.
4. Re-check every 30 minutes; the forecast lead time is short.
5. For satellites or radio, look at the NOAA R, S and G scales together.

Where Martian Alpha fits#

Martian Alpha's free Space Weather dashboard collects live space weather from official government forecasters in one place: radio blackout, solar radiation and geomagnetic storm levels, the Kp index and forecast, solar X-ray flux, solar wind and the aurora forecast, with a plain-English summary of what it means for satellites, GPS and radio. Create a free account to follow it from your own dashboard; aurora and geomagnetic storm alerts by email are part of the Space Enthusiast plan ($5 a month), listed on the plans page. The underlying numbers come from NOAA, so for the official product go to SWPC.

FAQ#

What Kp do I need to see the aurora?#

It depends on your location. NOAA's scale lists, as examples, northern Michigan and Maine at G1 (Kp 5), New York and Idaho at G2 (Kp 6) and Florida and southern Texas at G5 (Kp 9). Closer to the poles you can see it at lower Kp.

What is the Kp index?#

A 0 to 9 measure of disturbance in Earth's magnetic field over a three-hour period. SWPC calls 1 calm and 5 or more a geomagnetic storm.

How accurate is the aurora forecast?#

NOAA describes it as a short-term 30 to 90 minute forecast of the location and intensity of the aurora based on the OVATION model, with lead time set by solar wind travel from L1. It's a forecast of probability and intensity, not a guarantee of what you'll see.

Can a geomagnetic storm affect satellites?#

Yes. NOAA says storms can increase drag on low-Earth-orbit satellites. In February 2022 a moderate storm contributed to the loss of Starlink satellites shortly after launch.

Is this a recommendation about space stocks?#

No. It explains how to read space-weather data and isn't investment advice.

Sources#

• NOAA SWPC, Planetary K-index: https://www.swpc.noaa.gov/products/planetary-k-index
• NOAA SWPC, NOAA Space Weather Scales: https://www.swpc.noaa.gov/noaa-scales-explanation
• NOAA SWPC, Geomagnetic Storms: https://www.swpc.noaa.gov/phenomena/geomagnetic-storms
• NOAA SWPC, Aurora, 30 Minute Forecast: https://www.swpc.noaa.gov/products/aurora-30-minute-forecast
• NASA NTRS, "The Loss of Starlink Satellites in February 2022: How Moderate Geomagnetic Storms Can Adversely Affect Assets in Low-Earth Orbit": https://ntrs.nasa.gov/citations/25843440650796
• SpaceX, Starlink update of February 8, 2022 (spacex.com/updates)