A strong geomagnetic storm in spring 2024 brought the northern lights unusually far south, as the auroral oval expanded well beyond its typical position. "I am surprised at how sparse the measurement network is, even though we know that the impacts of space weather can vary greatly from one area to another," says Doctoral Researcher Otto Kärhä from the University of Oulu, Finland.
"For safety reasons, it is important to expand measurement instrumentation also in southern Finland and across Arctic sea regions—areas where the network is currently sparse or non-existent—in order to better understand how disturbances are distributed," Kärhä continues. He will defend his doctoral thesis on 28 November 2025.
Space weather refers to the interaction between the solar wind and various solar eruptions with Earth’s magnetic field. The most intense space weather events can generate large-scale disturbances in the geomagnetic field, known as magnetic storms. These disturbances may interfere with power transmission, communication systems, and navigation. “The regional nature of space weather can be compared to ordinary weather—such as differences in temperature or cloud cover,” Kärhä explains.
Geomagnetic storms are monitored using magnetometers, which are primarily installed within the auroral zone, where the strongest field variations usually occur. Station networks can detect disturbances and provide information on how the most intense variations distribute across different latitudes. Local disturbance data is especially important for assessing space-weather-related risks to infrastructures and technologies. However, the current network is considered too sparse in Fennoscandia.
In his dissertation, Kärhä examines how large momentary differences in geomagnetic storm-time variations can become between stations—that is, how local disturbances truly are. In addition to the 2024 storm, earlier cases were studied. During a storm recorded in Fennoscandia in October 1977, the disturbance in the northward magnetic field component differed by over 500 nanoteslas between stations only 170 km apart. During the so-called Halloween superstorm in October 2003, the difference reached 1,200 nanoteslas over 160 km. The strongest variation was detected on Earth’s night side.
"Subsurface conductivity structures also guide the flow of currents and create areas with higher disturbance risk," Kärhä notes. "Future magnetic storms cannot be predicted precisely, but their probability increases as the solar cycle declines over the coming years. A deeper understanding of space weather is vital for global safety."
"This dissertation shows that the magnitude of space-weather-induced geomagnetic disturbances can vary significantly within just a few tens of kilometres. Natural geomagnetic disturbances may resemble human-made interference, and distinguishing between them is increasingly important in the current geopolitical climate," emphasises dissertation supervisor Professor Eija Tanskanen, Director of the Sodankylä Geophysical Observatory at the University of Oulu.
The study utilised both modern digital measurements and extensive historical material originally recorded on 35 millimetre film in the 1970s. The dissertation presents a digitisation method through which nearly 40 kilometres of magnetic field variation film records can be converted into digital data for future research use.
Master of Science (Technology) Otto Kärhä will defend his doctoral thesis at the University of Oulu on Friday 28 November 2025. The dissertation, in the field of physics, is titled From strong to superstorms: regional effects and spatial geomagnetic gradients driven by extreme space weather. The opponent will be Professor Pieter Kotzé (North-West University), and the custos Professor Eija Tanskanen (University of Oulu). The public examination will take place on the Linnanmaa campus, room TA105, starting at 12:00, and can also be followed remotely.
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Solar storms hit more locally than expected - current instrument network too sparse University of Oulu
Safe Earth research programme