How To Read An Aurora Forecast

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You’ve received an aurora alert on your phone. The Kp-index is climbing. Social media is buzzing. But what does it all actually mean? Should you rush outside right now, or wait? Is this going to be a faint grey shimmer in the clouds or the display of a lifetime?

Understanding how to read an aurora forecast is the single most useful skill you can develop as an aurora watcher. It transforms you from someone who passively hopes to see the lights into someone who can actively predict when and where they’ll appear, often hours before they do.

This guide walks you through the key data sources, explains what each number means in plain language and gives you a practical step-by-step process for assessing the conditions on any given night.

The Key Numbers

Aurora forecasting relies on a handful of measurements taken from satellites positioned between Earth and the Sun. You don’t need to understand the physics in detail, you just need to know what each number tells you and what values are favourable.

Bz: The Most Important Number

If you only look at one thing, make it Bz. This measures the north-south direction of the interplanetary magnetic field (IMF) - the magnetic field carried by the solar wind.

  • Bz positive (northward): Earth’s magnetic shield stays closed. Solar wind energy is deflected. An aurora is unlikely.
  • Bz negative (southward): Earth’s magnetic shield opens. Energy pours into the magnetosphere. An aurora becomes likely.

When the solar wind’s magnetic field points south (negative Bz), it connects with Earth’s northward-pointing field, letting energy in. When Bz is positive, the fields oppose each other and the shield holds firm.

As a rough guide:

  • Bz 0 to -5 nT: Mildly favourable. Aurora possible at high latitudes.
  • Bz -5 to -10 nT: Good. Expect visible aurora from typical viewing locations.
  • Bz -10 to -20 nT: Strong. Vivid displays likely, aurora expanding southward.
  • Bz below -20 nT: Exceptional. Major storm conditions. Aurora potentially visible at mid-latitudes.

The critical thing about Bz is that it can change rapidly. A forecast might look quiet, then Bz suddenly drops to -15 nT and the sky erupts within 30 minutes. This is why real-time monitoring matters more than long-range predictions.

Solar Wind Speed

The solar wind is a continuous stream of charged particles flowing from the Sun. Its speed determines how much energy is delivered to Earth’s magnetosphere.

  • 300-400 km/s: Normal, quiet conditions.
  • 400-500 km/s: Slightly elevated. Aurora possible if Bz cooperates.
  • 500-700 km/s: Fast. Conditions are becoming favourable.
  • Above 700 km/s: Very fast. Strong aurora likely even with moderate Bz.

Speed alone doesn’t guarantee aurora - you need negative Bz as well - but fast solar wind amplifies the effect of a southward Bz. The combination of high speed and strongly negative Bz is what produces the best displays.

Solar Wind Density

Measured in particles per cubic centimetre (p/cm3), density tells you how many charged particles are arriving. Higher density means more fuel for the aurora.

  • 1-5 p/cm3: Normal.
  • 5-20 p/cm3: Elevated. Combined with speed and negative Bz, this is promising.
  • Above 20 p/cm3: High. Often indicates a CME or shock front arriving.

A sudden spike in density is one of the earliest signs that a coronal mass ejection (CME) has arrived at Earth. When you see density jump sharply, check Bz immediately - if it goes negative at the same time, aurora is likely imminent.

Bt: Total Magnetic Field Strength

Bt measures the overall strength of the interplanetary magnetic field, regardless of direction. It tells you how much magnetic energy the solar wind is carrying.

  • Below 10 nT: Normal.
  • 10-20 nT: Elevated. Conditions are energised.
  • Above 20 nT: Strong. If Bz turns southward, expect a significant response.

A high Bt with northward Bz means the energy is there but Earth’s shield is holding. If Bz then swings south, all that stored energy can couple into the magnetosphere very quickly. Watch for this pattern - it’s often the setup for dramatic, sudden aurora.

The Kp-Index

The Kp-index is the number you’ll encounter most often. It’s a global measure of geomagnetic disturbance on a scale of 0 to 9, updated every three hours.

  • Kp 0-1: Quiet. Aurora confined to the far north (Tromsø, Fairbanks etc).
  • Kp 2-3: Unsettled. Visible from typical aurora-zone locations.
  • Kp 4-5: Active to minor storm. Aurora expanding southward. Visible from northern Scotland, southern Scandinavia, northern US states.
  • Kp 6-7: Moderate to strong storm. Visible from central UK, central Europe, central US.
  • Kp 8-9: Severe to extreme storm. Potentially visible from southern Europe and southern US. Rare events.

The Kp-index is useful for quickly gauging overall conditions, but it has limitations. It’s updated every three hours, which means it can lag behind rapidly changing conditions. A substorm might produce spectacular aurora for 45 minutes with a Kp of only 3 or 4. Conversely, a sustained Kp 5 might produce a steady but unspectacular glow.

For real-time assessment, the raw solar wind data (Bz, speed, density) is more responsive than the Kp-index. Use Kp for overall context, but use the solar wind data for minute-to-minute decisions.

Kp vs Real-Time Data

You can think of the Kp-index as like a weather summary ("today was windy") and the solar wind data as looking out of the window right now. Both are useful, but if you’re deciding whether to go outside in the next 30 minutes, the real-time data is what matters.

The Hemispheric Power Index (HPI)

The HPI estimates the total energy being deposited into the atmosphere by charged particles, measured in gigawatts (GW). It’s derived from satellite observations and updates more frequently than the Kp-index.

  • 5-10 GW: Quiet. Little auroral activity.
  • 10-20 GW: Mild. Faint aurora possible at high latitudes.
  • 20-50 GW: Active. Visible aurora at auroral latitudes.
  • 50-100 GW: Strong. Bright displays, aurora expanding southward.
  • Above 100 GW: Major storm. Intense aurora, potentially visible at mid-latitudes.

The HPI is particularly useful because it responds quickly to changing conditions and gives a sense of how bright the aurora is right now, not just how disturbed the magnetic field is.

The Ovation Map

The NOAA Ovation model is one of the most practical tools for aurora watchers. It produces a real-time map showing where the aurora is likely visible right now, based on live solar wind data.

The map displays the aurora oval - the ring-shaped zone of auroral activity - overlaid on a view of the Northern (or Southern) Hemisphere. Colour intensity indicates the probability and brightness of the aurora at each location.

What makes the Ovation map more useful than the Kp-index is that it shows you where, not just how strong. You can see whether the oval extends to your location or to a webcam you’re monitoring. During a storm, you can watch the oval expand southward in near real-time.

The map predicts conditions approximately 30-40 minutes ahead, based on solar wind currently being measured at the DSCOVR satellite upstream of Earth. This gives you a short but actionable window to prepare.

Check our own aurora forecast page for an interactive aurora probability map alongside live Kp, Bz and solar wind data.

Where Does The Data Come From?

Nearly all real-time aurora data comes from the DSCOVR satellite (Deep Space Climate Observatory), positioned at the L1 Lagrange point - a gravitationally stable spot approximately 1.5 million km from Earth towards the Sun. At this location, DSCOVR sits in the solar wind upstream of Earth, measuring its speed, density and magnetic field before it arrives.

Because the solar wind takes roughly 15-60 minutes to travel from L1 to Earth (depending on speed), the DSCOVR data gives us advance warning of incoming conditions. When you see Bz plunge to -20 nT on a real-time plot, you know that same magnetic field will be hitting Earth’s magnetosphere within the hour.

The ACE satellite (Advanced Composition Explorer) serves as a backup at the same location. If DSCOVR data drops out, ACE data is used instead. You may see references to both in forecast tools.

Putting It All Together: A Step-By-Step Process

When you receive an aurora alert, or simply want to check conditions on a clear night, follow this process:

  1. Check Bz. Is it negative? If Bz is positive, aurora is unlikely regardless of other factors. If it’s -5 nT or below, keep going.
  2. Check solar wind speed. Is it elevated? Above 400 km/s is encouraging. Above 500 km/s with negative Bz is excellent.
  3. Check density. Has it spiked? A sudden increase may indicate a CME arrival and imminent activity.
  4. Check the Kp-index or HPI. Are they elevated? This confirms that the solar wind conditions are translating into actual geomagnetic activity.
  5. Check the Ovation map. Does the aurora oval extend to your latitude? If the coloured band reaches your location, aurora should be occurring overhead or to your north.
  6. Check the sky. Is it clear? Is it dark? No amount of geomagnetic activity helps if you’re under cloud or in bright twilight.
  7. Go outside and look north. Give your eyes 10-15 minutes to adjust to the darkness. If conditions are marginal, take a long-exposure photo on your phone - the camera will detect aurora before your eyes do.

The Golden Combination

The conditions most likely to produce spectacular aurora:

  • Bz strongly negative (-10 nT or below)
  • Solar wind speed above 500 km/s
  • Elevated density (above 10 p/cm3)
  • Clear, dark skies with no moonlight
  • Your location under or near the aurora oval

When all of these align, you’re looking at near-certain aurora at high latitudes and a strong chance at mid-latitudes too.

Common Patterns To Recognise

The CME Arrival

A coronal mass ejection hitting Earth has a distinctive signature in the data: a sudden jump in solar wind speed and density (the shock front), often accompanied by a spike in Bt. Bz may initially swing in either direction. If Bz goes strongly negative after the shock, expect a major storm and bright aurora within minutes. If Bz stays northward, the energy is there but the shield is holding - watch and wait, because Bz can flip south at any time during the passage.

The Coronal Hole Stream

Coronal holes produce a more gradual increase in solar wind speed over 1-2 days, with moderate but sustained activity. These are less dramatic than CMEs but more predictable - the same coronal hole can affect Earth every ~27 days as the Sun rotates, giving you recurring windows of activity to plan around.

The Substorm

Even during moderate conditions, the magnetosphere periodically releases stored energy in sudden bursts called substorms. These can produce brilliant, fast-moving aurora for 30-90 minutes, even when the Kp-index is only 3 or 4. Substorms are hard to predict in advance but easy to spot in real-time: watch for sudden negative deflections on magnetometer data from stations near your location. If you’re monitoring webcams, a substorm breakup is unmistakable - the sky suddenly erupts.

The Quiet Night That Isn’t

One of the most common mistakes is checking conditions once, seeing Kp 2 and Bz at +3 nT, and going to bed. Conditions can change within minutes. If there’s a CME in transit or a coronal hole stream expected, it’s worth checking periodically through the night. Many of the best aurora displays have caught people off guard because conditions turned favourable after midnight.

What The Forecast Can’t Tell You

No forecast can tell you exactly when the aurora will appear at your location, how bright it will be, or how long it will last. Substorms are inherently unpredictable. Bz can fluctuate rapidly. Cloud can roll in. The forecast gives you probabilities and context, not certainties.

What the forecast can do is tell you whether conditions are favourable. If Bz is -12 nT, solar wind is at 600 km/s, the Ovation map shows the oval over your latitude, and the sky is clear - the probability is high. Whether that translates into a faint arc or a full-sky corona depends on factors that are simply too dynamic to predict in advance.

This is part of what makes aurora watching compelling. You learn to read the data, stack the odds in your favour, and then let nature surprise you.

Useful Resources

For definitions of all the terms used in this guide, see our aurora glossary. For tips on when to travel and where to go, see our planning guide and best time of year guide.