At the morning of 12.5.2021, at 9am Central European time, GFZ observatories showed a clear and sudden magnetic field pulse. At the same time, the magnetic field in the solar wind was time and again aligning southwards, which may indicate a subsequent geomagnetic storm. When charged particles penetrate deeper into the Earth's magnetic field and atmosphere than usual, satellites in particular, and in extreme cases our technical infrastructure on Earth, can suffer great damage. We spoke with Prof. Claudia Stolle, who classifies this geomagnetic storm for us and explains basic backgrounds.


Prof. Stolle, there was a solar wind alert on 12.5.2021. Could you briefly explain what the solar wind is and why we need to worry about it on Earth?

C. Stolle: The solar wind describes a continuous flow of particles emitted by the sun, mainly protons and electrons, propagating into space. During normal solar activity not much variability is noticeable in the Earth's magnetic field and on Earth, just a few auroras can be seen now and then in the polar regions. However, during eruptions on the Sun’s surface, the solar wind becomes denser and faster and strong, irregular interaction with the Earth's magnetic field occurs. Electric and magnetic fields in near-Earth space are amplified and particles penetrate deeper into the Earth's magnetic field and into the atmosphere than usual. This can potentially damage satellite and ground-based infrastructure that is important for our society.


How particular was this solar wind event of 12.5.2021 for you as an expert? And how threatening? With this in mind, almost exactly 100 years ago, on 13.5.1921, the strongest solar storm of the 20th century, the so-called New York Railroad Storm, happened. It lasted three days and caused some serious damage to the infrastructure of that time. Are these two events comparable?

C. Stolle: The storm on 12 May 2021 was particular in the sense that it occurred during a solar minimum, thus at a time when usually very few geomagnetic storms are expected. The last storm of similar strength was in August 2018. In contrast 4 to 5 storms per year are very common in a waning solar maximum. However, the current storm was not particularly strong, as it was the case for the last superstorm, the so-called Halloween Storm in October 2003, which developed highest Kp values and lasted almost three days, similar to the New York Railroad Storm. The current storm therefore had little impact on technological infrastructure in space and on Earth. Nevertheless, it has been important for us; we already have specific plans to study this particular storm with satellite and ground-based data. A storm like the one in 1921 would be of very different matter today, as our society now builds on satellite- and ground-based infrastructure that could be severely affected.


Speaking of 100 years ago: What basic regularities or cycles do you observe in solar eruptions that can eventually grow into solar and geomagnetic storms?

C. Stolle: The solar cycle takes about eleven years and most storms occur in the waning phase, shortly after the sunspot maxima. An interesting point is: a hundred years ago, there were no satellites with sensors that could measure the solar wind. But ground-based magnetic observatories started measuring such storms much earlier. Then, magnetograms were registered on photographic paper and can still be analyzed today. By studying them, we learn about space weather and space climate.


The geomagnetic storm at 12.5.2021 only lasted a few hours and did not develop into a severe event, although this could not be ruled out at the time of alert. What does it ultimately depend on whether such a storm develops into a larger and possibly more dangerous event?

C. Stolle: First of all, the magnetic field in the solar wind must frequently turn southwards for hours or days so that the solar wind can couple well into the Earth's magnetic field. A severe event is likely when several eruptions occur on the sun that propagate at different speed in the solar wind and hit Earth simultaneously or shortly after each other, thus adding up their effects.


It is particularly difficult to predict whether and with what intensity a solar storm will couple into the Earth's magnetic field. What specific information helps you in this estimation, and how early can you warn, if necessary?

C. Stolle: Observations of the sun and the solar wind play an important role. If the sun is quiet, we usually have a certainty for at least one day that near-Earth space is quiet, too. However, if a solar eruption propagates in the solar wind, it cannot be predicted exactly whether it will hit the Earth or not. If warnings were given each time for an eruption on the sun, it would create to many false warnings. You can't shut down the whole infrastructure like power grids and satellites several times a year.


What are the main differences between the approaches by the colleagues in Japan, the experts of the American weather and oceanography agency NOAA and the methods that we employ here at the German Research Centre for Geosciences? What are our strengths at the GFZ?

C. Stolle: Our specialty is the observation and analysis of the Earth's magnetic field through a global network of observatories. This is a clear advantage. Since this method is well established, GFZ’s data have a huge user group. Many models and real-time warning systems, some also at the GFZ, are based on them, and it makes our work important. At the same time, there is a lot of expertise at GFZ in analyzing a multitude of data taken on the ground and on satellites. This doesn't always happen in real time, but it provides crucial knowledge to prepare for future space weather events and to better understand the system Earth.

Further Information:

• Sources of the Earth's magnetic field: Link to FAQs
• Indices of Global Geomagnetic Activity: Link
• Satellite mission and ground observations: Link