The Nordic Dynamo08 project was able to verify the generation process of solar magnetic fields, one of the most important physical phenomena in the cosmos. Maarit Korpi, Academy Research Fellow, and Adjunct Professor (Docent) of Astronomy at the University of Helsinki, was the lead researcher in the Dynamo08 project.

“Planets, stars, galaxies and groups of galaxies all have magnetic fields, so magnetism is truly a universal phenomenon. Research into magnetic activity is essentially associated with the Sun and stars. Planets and galaxies are magnetic but passive. The most important modeling target is the Sun, and in the future we hope to be able to make forecasts of its behavior, in the same way as meteorologists forecast the weather on Earth.”

According to current understanding, long-term changes in the magnetic activity of the Sun affect the climate on Earth. Although solar activity is only one factor amongst many causing changes in global climate, one of the objectives for the researchers participating in Dynamo08 is to gain better understanding of the solar influence on the climate change over a long time range. Solar activity has also several effects that can be observed over shorter time periods.

“The Sun’s magnetic activity causes variation in, for example, the strength of the solar wind and in space weather, causing phenomena like the aurora borealis. These factors have to be taken into account whenever space flights and satellite missions are being planned. Changes also affect telecommunication connections on Earth,” says Korpi.

Studies have shown that space weather may also increase the radiation risk in aircraft and disrupt radio connections. Additionally, it may cause problems in electricity transmission systems and natural gas pipelines.

Significance of sun spot number

The Sun is the only star for which we can observe the surface directly.

“The other stars are visible to us only as point objects, the surface of which can be observed and mapped only indirectly. Studies have revealed that stars that rotate faster than the Sun are magnetically much more active and that spots cover most of the surface of such stars.”

For centuries, sun spots have been studied to determine variation in the Sun’s magnetic field, or magnetic activity. The sun spot number shows both regular behavior over a roughly 11-year period and irregular long-term variation. For example, between 1645 and 1715 (during the period known as the Maunder Minimum) hardly any sun spots were observed. This was roughly the time when the Earth experienced the Little Ace Age.

“From the 1950s to the present day, solar activity has been high. The current solar cycle 24 seems to be exceptional; it started late and so far, the activity level has been much lower than predicted. Predicting solar activity is important not only for studying its effects on global climate but also for forecasting space weather. Magnetic eruptions of the Sun cause storms in space, affecting, for example, the function of satellites.”

Dynamo mechanism and the magnetic field

The Sun’s magnetic field has not existed from the very beginning, but it has been generated by something, and something sustains it. The key is electromagnetic induction, which is the ability of charged objects to generate a magnetic field when they move. Just as an example, this is why electric cables collect dust.

“Nuclear reactions occurring inside the Sun and other stars heat matter to extreme temperatures, which means that free electronic charges are abundant due to ionization. Stars also rotate and due to convection, their surface layers bubble like boiling water in a kettle. This keeps electronic charges on the move, making the conditions ideal for electromagnetic induction,” Korpi explains. Convection is heat transfer in gas by the heated flow regimes.

In fact, the view that the magnetic fields of the Sun and stars were generated and sustained as a result of the dynamo mechanism based on these phenomena has been accepted since the 1950s. So why is complex computation necessary?

“Reproducing the Sun’s magnetic cycle using theoretical models has proven to be a challenge. Models that work almost correctly do exist, but usually details, such as the irregular part of the solar cycle, are poorly modeled.”

Researchers have tried to perform direct numerical simulations of the dynamo process for two decades, ever since computers became robust enough to make this type of computation possible. It is known that in theory a large-scale hydromagnetic dynamo involving convective turbulence rotation and large-scale shear due to solar differential rotation should be able to generate magnetic fields.

“The results have raised concern regarding dynamo theory: large-scale dynamos have not been detected in models that have included all the basic physics essential for the dynamo function, like convection and rotation. Models have continuously been improved in pace with the development of computers. For example, the number of grid points in the computing grid has been doubled several times since parallel computers have become commonly used, and this has meant major improvements in the accuracy of modeling small-scale motions. Despite all this, dynamos have continued to hide from us – until today,” Korpi smiles.

Images:

Top: Maarit Korpi, adjunct professor (docent) of astronomy at the University of Helsinki, was the lead researcher in the Dynamo08 project. © Seppo Saarentola.

Middle: Energy density of the Sun’s magnetic field within the turbulent solar convection zone. The energy density of the magnetic field is approximately equal to that of the heat flow, which means that the magnetic field is dynamically significant, yet not strong enough to dominate plasma motions. © Jyrki Hokkanen

Bottom: Magnetic field at two different times of the simulation. The first image (top) shows a disorganized early-phase magnetic field. The large-scale magnetic field grows at a much slower rate, but at later phases of the simulation, organised field structures can be clearly seen (bottom). The image was published in Käpylä, Korpi & Brandenburg, 2008: Large-scale dynamos in turbulent convection with shear, A&A, 491, 353.

A breakthrough for Dynamo08

Researchers have worked on computer models for almost two decades hoping to establish the existence of large-scale dynamos. A breakthrough was made in the Dynamo08 project by supplementing the model with differential rotation.

“It was a breakthrough for us to add just one essential factor to our computer model, namely differential rotation. Based on observations, it is known that the Sun’s rotation speed is not constant at its surface and not beneath the surface either. From established theory, we know that changes in this type of rotational speed boost the dynamo action, and this occurred also in our direct simulations: when combined, convection, rotation, and changes in the rotational speed were able to produce a large-scale dynamo. However, the computing grid we used in these calculations was not accurate enough to guarantee that the dynamo would work in a model in which turbulent convection is modeled more accurately,” Korpi recalls.

In addition to Maarit Korpi, other members in the Dynamo08 project were Petri Käpylä, PhD, Academy of Finland Postdoctoral Researcher, University of Helsinki, and Professor Axel Brandenburg, Nordita, Nordic Institute for Theoretical Physics, Stockholm.

The Dynamo08 project was classified as one of CSC’s Computational Grand Challenge projects. The Grand Challenge projects are aimed at high-impact scientific research, and the GC calls are arranged twice a year. More than 1.5 million central processor hours were used in the Dynamo08 computer runs during the project; this equals roughly 190 central processor years over a few months. This was possible when as many as 2000 computing cores were used simultaneously, i.e. parallel computing on CSC’s Louhi supercomputer.

“This is an important physical question and we had a highly developed model that had to be verified by numerical computing. We were looking for a yes or no answer, and we got it. The dynamo worked. Getting the yes answer was a truly great moment!"

More information on Grand Challenge proposals