Northern lights forecasts: when it's worth looking north

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Northern lights are fascinating. But while they are more common in northern regions such as Scandinavia or Alaska, they are rarely seen in Germany, as was the case in May this year. From the 7th to the 11th, there was a series of solar flares and coronal mass ejections (CME), which triggered a record-breaking geomagnetic storm of the highest level G5 (more on this below). The result was not only countless pretty photos on social media, but also technical challenges. Satellites had to correct their orbits to avoid losing control, as happened to Intelsat in 2022.

Since then, various websites have been promising the chance to see auroras in Germany again at regular intervals. But even if the sun is currently at the peak of its eleven-year cycle, you can only expect to see the natural spectacle in Germany under certain conditions. Right now, the chances are actually very good. On Monday in the morning and evening hours, it was not only possible to photograph northern lights at Cap Arkona, in Hooksiel or in Vechta, but also to catch a glimpse of them in Hanover:

Northern lights occur when charged particles from the solar wind hit the earth's atmosphere. These particles are deflected by the Earth's magnetic field and enter the upper atmosphere in a ring around the magnetic poles. There they excite oxygen and nitrogen atoms to glow, which produces the characteristic green and red hues of the aurora borealis.

Kp index - the planetary index

The Kp index

The Kp index (planetary index) is a measure of the strength of disturbances in the earth's magnetic field. It is calculated every three hours from measurements taken by magnetometers distributed around the world. The index was developed in 1949 by Julius Bartels at the Geophysical Institute in Potsdam (now the GeoForschungsZentrum, GFZ):

• Kp 0-3: calm
• Kp 4: active
• Kp 5: smaller storm
• Kp 6-7: major storm
• Kp 8-9: severe storm

The higher the Kp index, the further the auroral zone (also known as the aurora oval) extends around the geomagnetic North Pole. Even if this is not at the geographic North Pole, it can be simplified to say that the higher the Kp index, the further south auroras can occur.

Reliable data and forecasts of the Kp index can be found, for example, on the GFZ website and at the Space Weather Prediction Center of the National Oceanic and Atmospheric Administration (NOAA)

The frequency of northern lights in different regions of the world is closely linked to the position of the magnetic pole, which is not located at the geographic North Pole. In fact, the magnetic pole is currently located at approximately 85.75° north latitude and 144.46° west longitude. Canada and the United States are further north in relation to the magnetic pole than their geographical position would suggest, whereas Germany is further south. This is why the fascinating light phenomena can be seen more often in North America than in Central Europe.

The NOAA scale for classifying solar storms

Solar storms are divided into three categories according to their effects on the Earth :

• R-scale: Radio disturbances caused by X-ray flashes
• S-scale: Radiation effects caused by high-energy particles
• G-scale: Geomagnetic effects due to plasma clouds

Each scale ranges from 1 (weak) to 5 (extreme). A high G value is particularly important for auroras in Germany:

G1 ("slight") - Kp = 5 - Frequency: 1700 per solar cycle

G2 ("moderate") - Kp = 6 - 600 per solar cycle

G3 ("severe") - Kp = 7 - 200 per solar cycle

G4 ("severe") - Kp = 8 - 100 per solar cycle

G5 ("extreme") - Kp = 9 - 4 per solar cycle

As a reminder: A solar cycle lasts eleven years. As the strength of geomagnetic storms increases, more and more electrical devices are affected.

Basic requirements for auroras over Germany

Several factors have to come together for auroras to reach Germany:

1. high solar activity: the sun must be particularly active and emit large quantities of charged particles.

2. strong geomagnetic storm: the emitted particles must be so numerous and energetic that they trigger a violent geomagnetic storm.

3. expansion of the auroral zone ("aurora valley"): Normally, auroras are only visible in high northern latitudes. In order for auroras to be seen further south, they must expand due to a geomagnetic storm.

4. clear night sky: The local weather conditions must be favorable, the sky cloudless.

5. low light pollution: The observation site should be as far away as possible from disturbing artificial light sources.

At what storm strength are auroras likely in Germany?

Specifically, auroras can occur in Germany from a geomagnetic storm of strength G3 on the five-level scale of the National Oceanic and Atmospheric Administration (NOAA). This corresponds to a Kp index of 7.

The Kp index is directly related to the geomagnetic latitude up to which auroras can be visible. The higher the Kp value, the further south auroras can be observed. For Germany, this means roughly the same:

• During a G3 storm, northern lights can be visible up to a geomagnetic latitude (not to be confused with geographical latitude!) of around 50°. However, they can then usually only be seen photographically as a faint greenish or violet-colored curtain on the northern horizon.
• A G4 storm (Kp 8 to 9) is usually required for clearer sightings up to the center of Germany. However, such strong storms are rare and only occur on average around 100 times in an eleven-year solar cycle.
• G5 storms such as the one in May 2024 are exceptionally rare, in which case auroras can even occur as far south as southern Germany and beyond.

Predictions and observation

Scientists use observations of the sun and measurements of the solar wind to make short-term predictions. Interested parties can find current data and forecasts on the pages of the NOAA's Space Weather Prediction Center, where the current Kp index and warnings of geomagnetic storms are published. But even when severe storms do occur, they often only last a few hours. So it may well be that there are good conditions for auroras in the afternoon, but the geomagnetic storm subsides towards evening and you are still standing outside for nothing. It is extremely rare for the storm to last all night. This is why such events can usually only be seen in certain regions.

Even if the prediction of auroras is a complex undertaking that depends on many factors, it can be summarized as follows: From a strength of G3, auroras can at least be seen photographically over northern Germany in optimal weather conditions. For sightings further south, you need a G4 storm (Kp 8 to 9) or a very long exposure time. Only with G5 storms can you hope to see auroras in southern Germany and beyond.

If conditions are favorable, it is advisable to look north during the night hours, with the best chances often being in the second half of the night just before dawn. It is best not to rely on your eyes alone, but to use a camera or smartphone with a tripod, as this can "see" the Northern Lights much better than the human eye with a long exposure. Under good conditions, you can sometimes take spectacular photos:

(vza)