The KP index (Planetary K-index) is the most widely used measure of geomagnetic activity and aurora strength. It's a scale from 0 to 9 that indicates how disturbed Earth's magnetic field is—higher numbers mean stronger geomagnetic storms and more intense aurora displays.
Think of the KP index as an "aurora intensity meter." When solar particles hit Earth's magnetic field, they cause disturbances that scientists can measure. The KP index quantifies this disturbance level, helping aurora hunters predict when and where northern lights will be visible.
Aurora Visibility: Minimal to none. Auroras may appear very faint and only visible far north (above 65°N latitude).
What to Expect: Faint green glow near horizon, barely visible to naked eye. Cameras may capture subtle colors. Not ideal for aurora hunting unless you're very far north.
Viewing Locations: Northern Norway, Alaska interior, northern Canada only.
Photography: Long exposures (15-30 seconds) needed to capture faint glows.
Aurora Visibility: Good. Auroras visible in typical aurora zones (60-70°N). This is the "sweet spot" for planned aurora trips.
What to Expect: Visible green auroras with some movement. Curtains and arcs clearly visible to naked eye. Colors may include green with hints of pink.
Viewing Locations: Iceland, Tromsø, Finnish Lapland, Fairbanks, Yellowknife—all prime viewing locations.
Photography: 5-10 second exposures capture vibrant colors. Tripod essential but results impressive.
Success Rate: ~70-80% chance of seeing auroras from good locations with clear skies.
Aurora Visibility: Excellent. Strong auroras visible as far south as 55-60°N. This is what aurora chasers hope for!
What to Expect: Bright, dancing auroras with rapid movement. Multiple colors (green, pink, purple). Corona displays overhead possible. Very dynamic and impressive.
Viewing Locations: Visible from southern Scandinavia, Scotland, southern Alaska, northern continental US (rare), southern Canada.
Photography: 2-5 second exposures. Fast shutter speeds capture movement. Brilliant results even with smartphone cameras.
Success Rate: ~95% chance of seeing spectacular displays from aurora zones.
Frequency: KP 5-6 occurs 50-100 times per year during solar maximum.
Aurora Visibility: Outstanding. Visible as far south as 50°N or lower. Rare and spectacular event.
What to Expect: Extremely bright, fast-moving auroras covering entire sky. Multiple colors including rare reds. Corona displays common. Pulsating and rapidly changing patterns. "Once in a lifetime" displays.
Viewing Locations: Visible from UK, northern Germany, northern US states (Montana, Minnesota, Maine), southern Canada, even sometimes Paris or New York.
Photography: 1-3 second exposures. Movement so fast you can see it changing in real-time. Smartphone cameras capture brilliant displays.
Side Effects: May disrupt GPS, radio communications, and power grids. Satellites can be affected.
Frequency: 10-30 times per year during solar maximum, rarely during solar minimum.
Aurora Visibility: Exceptional and extremely rare. Visible as far south as 40°N or even lower—Mediterranean, southern US states.
What to Expect: All-sky auroras with brilliant reds, greens, purples, and blues. Visible even in light-polluted cities. Sky appears to be "on fire." Movement so dramatic it's shocking. Historic event that makes international news.
Viewing Locations: Spain, Italy, Greece, southern US states (Texas, California), Japan, northern China. Essentially anywhere north of 35-40°N has a chance.
Photography: Sub-second exposures needed. Movement visible in video. Any camera captures stunning images.
Side Effects: Major technological disruptions possible. Power grid failures, satellite damage, communication blackouts, transformer damage.
Frequency: Extremely rare. Perhaps 1-5 times per solar cycle (11 years). Last KP 9 events: May 2024, September 2017, March 2015.
Historical Note: The 1859 Carrington Event (KP 9) caused telegraph system failures and auroras visible in the Caribbean.
The KP index is calculated from magnetometer readings at 13 geomagnetic observatories between 44-60 degrees north and south latitude. These instruments measure disturbances in Earth's magnetic field every three hours. The "P" in KP stands for "planetary," meaning it's a global average.
The data from all observatories is combined and converted to a quasi-logarithmic scale:
The KP index determines how far south auroras will be visible. Here's a practical guide:
Higher KP = visible farther south. Use this guide to determine if you'll see auroras from your location:
Understanding the KP index is only half the battle. Here's how to actually use aurora forecasts for successful viewing:
The most accurate forecasts come from monitoring solar wind as it travels from the Sun to Earth:
Because the Sun rotates every 27 days, coronal holes (sources of fast solar wind) that produced auroras 27 days ago may do so again:
Download our Aurora app for real-time forecasts, KP alerts, and cloud cover data. Get notifications when KP rises above your threshold.
The official US government source. Provides 3-day forecasts, real-time data, and geomagnetic storm warnings.
Advanced users monitor ACE satellite solar wind data for immediate predictions (Bz component is key).
KP is irrelevant if clouds block the view! Check local weather forecasts and satellite cloud imagery.
FALSE. KP 3-4 produces beautiful displays from prime viewing locations. KP 7+ is only needed if you're at lower latitudes (London, Paris, New York). If you're in Iceland, Norway, Alaska, or Finland, KP 3 is perfectly good!
PARTIALLY TRUE. While higher KP generally means more intense auroras, it also means auroras appear farther south. From far northern locations, a KP 9 storm may actually push the auroral oval south of you, making displays less impressive than KP 4-6!
FALSE. Aurora forecasting has improved dramatically but remains imperfect. Unexpected solar storms can occur with little warning. Conversely, forecasted high KP may not materialize. This is why staying multiple nights increases your chances.
FALSE. While forecasts help, auroras can surprise you! Many spectacular displays occur when forecasts predict moderate activity. If you're in an aurora zone during dark season, step outside regularly to check—you might catch an unexpected show.
Advanced aurora hunters monitor the Bz component of the Interplanetary Magnetic Field (IMF):
Faster solar wind (500+ km/s) combined with southward Bz creates the most intense auroras. Normal solar wind is 300-400 km/s. During storms, it can exceed 700 km/s.
NOAA's Ovation model provides visual maps showing predicted aurora locations and intensity for the next 30-90 minutes. More precise than KP alone.