Hi, I’m Zukky.
After reading several books on the Northern Lights, I realized something: the Kp index alone is often insufficient for accurate aurora forecasting.
More Indicators Beyond the Kp Index
Just like with weather, there are forecasts for the aurora, and the key indicator widely used is the Kp index.
The Kp index is a measure of geomagnetic disturbance. Since the aurora is an electromagnetic phenomenon, it acts like a giant magnet, disturbing the Earth’s magnetic field. The Kp index is a measurement of this disturbance, typically presented in 10 steps from 0 to 9.
However, when trying to catch the aurora in real-time, the Kp index often falls short. Because the Kp index is an average calculated over three hours, it doesn’t account for short-term changes. This is why the aurora sometimes fails to appear even with a high Kp, or surprisingly appears when the Kp is low.
This is where Solar Wind data comes in.
What is the Solar Wind?
The solar wind is a stream of charged particles (plasma) that is constantly blown out from the Sun’s corona. When this solar wind enters the Earth’s magnetosphere, it is deflected by the magnetic field and accumulates in a region on the night side called the plasma sheet.
When the plasma sheet is full, the accumulated particles are pushed out and pour into the polar regions along the magnetic field lines. When these particles collide with atoms and molecules in the Earth’s atmosphere, the aurora is created!
Simply put: The solar wind is the raw material for the aurora.
Crucially, the faster the solar wind stream blows toward Earth, the more energy is generated, leading to stronger auroras. Therefore, solar wind speed is a vital real-time indicator.
Solar Wind Speed and Aurora Activity
According to Aurora Watching Guide (2018), here is the relationship between solar wind speed and aurora occurrence:
| Solar Wind Speed (km/s) | Aurora Expectation |
| 600 or more | Excellent! |
| 500 – 600 | Highly Expected |
| 400 – 500 | Fair |
| 350 – 400 | A bit weak |
| Below 350 | Difficult |
This data is sent in real-time from the DSCOVR satellite, located about 1.5 million km from Earth.
The time it takes for the solar wind observed at the satellite to reach Earth varies, but it is generally around one hour. This means that by monitoring the solar wind speed, you can predict the real-time aurora activity within the next hour or so.
| Solar Wind Speed (km/s) | Travel Time (min) |
| 600 | 41.7 |
| 500 | 50.0 |
| 400 | 62.5 |
| 350 | 71,4 |
The Solar Wind’s Magnetic Field Strength and Direction
If you only track the Kp index and solar wind speed, you might simply conclude, “If solar activity is high and the solar wind has a lot of energy, the aurora will appear!” But there is one more critical factor: The magnetic field of the solar wind.
Earth’s Magnetic Field vs. Solar Wind’s Magnetic Field (Bz)
The Earth’s magnetic field points generally North. This magnetic field acts like a protective shield, keeping the Sun’s plasma particles at bay—like having the windows of a house closed during a storm.
However, the magnetic field carried by the solar wind, called the Interplanetary Magnetic Field (IMF), is temperamental and can point in any direction.
When the IMF’s vertical component (Bz) points South (Negative), it is opposite to the Earth’s magnetic field. This causes the Earth’s magnetic shield to temporarily “open,” allowing plasma particles to pour easily into the atmosphere—like opening the window during a storm.
Simply put: When the Solar Wind’s Magnetic Field (Bz) is strong and points South (Negative), the aurora is highly likely to occur!
Solar Wind Magnetic Field Strength (Bt)
The strength of the magnetic field (Bt) is another factor:
| Magnetic Field Strength (Bt in nT) | Aurora Expectation |
| 10 or more | Excellent! |
| 5 – 10 | Highly Expected |
| 3 – 5 | Fair |
| Below 3 | Difficult |
The stronger the magnetic field, the greater the impact on the Earth’s magnetosphere, increasing the likelihood of an aurora.
Solar Wind Magnetic Field Direction (Bz)
The direction of the magnetic field (Bz) is often the deciding factor:
| Magnetic Field Direction (Bz) | Aurora Expectation |
| South (Negative) | Highly Expected |
| North (Positive) | Difficult |
Experts suggest that if the Bz remains Southward for an extended period, or repeatedly flips between South and North, expectations are high. It is essential to check this direction along with the speed.
How to Read Real-Time Solar Wind Data
You can check real-time solar wind data on sites like the NOAA “Real Time Solar Wind“ page or Space Weather Live.
The five main graphs on the NOAA Real Time Solar Wind page are:
- Bt and Bz GSM (nT): Magnetic Field Strength (Total Bt and North-South Component Bz)
- Phi GSM (deg): Solar Wind Magnetic Field Angle
- Density (1/cm3): Plasma Density
- Speed (km/s): Solar Wind Speed
- Temperature (K°): Plasma Temperature
Focus primarily on 1. Magnetic Field Strength (Bt/Bz) and 4. Solar Wind Speed.
- Example Reading: If Bt is 6, and the Bz is fluctuating but spending time in the negative (Southward) direction, the magnetic field is promising. If the Speed is 582 km/s, the energy level is also excellent. This combination suggests a high probability of an aurora appearing in the next 40 to 50 minutes.
Conclusion: Go Beyond Kp
| Checkpoint | Metric | Target Value | Impact Time |
| Activity/Strength | Kp Index | 3 or higher (for Finnish Lapland) | Approx. 3 days |
| Intensity | Solar Wind Speed | 500 km/s or higher | Approx. 1 hour |
| Connection | Bz (Magnetic Field Direction) | Southward (Negative) | Approx. 1 hour |
While the Kp index gives you a general idea, mastering the Solar Wind Speed and Magnetic Field (Bz) provides the crucial real-time edge you need to successfully hunt the aurora.
Complete Aurora Borealis Guide
References
- 『太陽のきほん』上出洋介 2018 誠文堂新光社
- 『一生に一度は見たい絶景の楽しみ方 オーロラ・ウォッチングガイド』赤祖父俊一 監修 2018 誠文堂新光社
- 『オーロラの科学 人はなぜオーロラにひかれるのか』上出洋介 2010 誠文堂新光社
- 『極地研ライブラリー オーロラの謎 南極・北極の比較観測』佐藤夏雄・門倉昭 2015 成山堂書店
- 『オーロラ・ウォッチング オーロラに会いにいこう』上出洋介 監修 2005 誠文堂新光社
- 『磁気嵐の基礎知識』気象庁地磁気観測所HP（https://www.kakioka-jma.go.jp/knowledge/mstorm_bg.html）2025.8.11アクセス
- 『現在の宇宙天気予報 と現況』（https://www2.nict.go.jp/spe/swx/swcenter/current.html）2025.8.11アクセス
- 『Real Time Solar Wind』NOAA Space Wether Prediction Center （https://www.swpc.noaa.gov/products/real-time-solar-wind）2025.8.11アクセス
- SpaceWeatherLive（https://www.spaceweatherlive.com）2025.8.11アクセス
- 『地球磁気圏への侵入』auroranavi.com（https://auroranavi.com/aurora/magnetosphere.html）2025.8.11アクセス
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