George Ellery Hale Prize for MPS Director

Prof. Dr. Sami K. Solanki receives the George Ellery Hale Prize of the American Astronomical Society.

The American Astronomical Society (AAS) has honored Prof. Dr. Sami K. Solanki, Director at the Max Planck Institute for Solar System Research (MPS) in Germany, with a high distinction: the AAS Solar Physics Division (SPD) has named the Göttingen researcher this year's recipient of the George Ellery Hale Prize, which distinguishes selected scientists for their long-standing and ground-breaking contributions to solar research. The award ceremony took place yesterday at the Triennial Earth-Sun Summit (TESS2022) symposium in Seattle, USA. With this prize, the SPD recognizes Solanki's contributions to understanding how the magnetic properties of our star determine its dynamic and variable nature and how this affects Earth's climate. In addition, the SPD praised Solanki's work developing space instruments and missions to study the Sun.

In cosmic comparison, the Sun doesn’t particularly stand out: compared with other stars, its size, surface temperature, and age are quite average. Prof. Dr. Sami K. Solanki's take on his research object, however, is a different one. "The Sun is fascinating, beautiful, dynamic ? and fun," he said in the opening remarks of his talk yesterday at the awards ceremony in Seattle.

For Solanki and other astronomers, the star at the center of our Solar System also offers another key advantage over its cosmic peers: its proximity. The "mere" 150 million kilometers separating the Earth from the Sun allows to observe individual structures and processes on the Sun with Earth-based telescopes or space probes. These observational data can be used to verifiy computer simulations – making it possible to understand our star better than any other.

Magnetic star

For example, researchers can precisely study the dynamic and capricious nature of the Sun. This is at the heart of Solanki's research. What are the dynamic processes that enable the Sun to heat its outmost layer, the corona, to more than a million degrees and to repeatedly emits particles and radiation into space in violent eruptions? How do dark and bright regions form and develop on the solar surface and how do they influence the Sun's brightness variations? And how have these brightness variations affected Earth's climate in the past?

"The answers to all these questions are found in the Sun's complex and variable magnetic field," Solanki says. It originates deep within its interior. From there, the magnetic fields wash up to the Sun's visible surface and extend into its chromosphere and corona.

The four scientific instruments on ESA's Solar Orbiter spacecraft, to which the MPS has significantly contributed under Solanki's leadership, peer into this highly dynamic region between the surface of the Sun and its outermost layer. The spacecraft reached its closest point to the Sun in March of this year, gathering unique observational data from a distance of only 48 million kilometers.

This allowed Solar Orbiter’s instrument EUI (Extreme-Ultraviolet Imager) to capture the highest-resolution images of the solar corona to date and, among other things, to detect tiny solar flares, so-called campfires, that may contribute to the incredibly high temperatures in the corona, which can exceed one million degrees. Clues to the magnetic processes underlying such campfires are provided by observational data from Solar Orbiter's Polarimetric and Helioseismic Imager (PHI), which was developed and built at MPS. Solanki is the instrument’s Principal Investigator. Investigations are still in the early stages, but there is much to suggest that restructurings of the small-scale magnetic fields of the solar surface release the energy necessary for the campfires.

The balloon mission Sunrise also looks into the region between the Sun’s surface and outer atmosphere. The balloon-borne, more than seven meter high solar observatory observes the Sun from the stratosphere, giving it access to its ultraviolet radiation. "Sunrise constitutes a completely new way to study the Sun," Solanki says. The concept has already proven successful twice: in 2009 and 2013, the observatory, equipped with a telescope, three scientific instruments and an image stabilization system, was able to elicit unique information from the Sun during stratospheric flights lasting several days.
The third flight, scheduled to take place this summer, had to be terminated a few hours after launch due to technical difficulties. The team is now looking for a possibility to launch again and thus exploit Sunrise's unique potential once more. Compared to its predecessors, Sunrise III carries newly developed instruments that can observe processes in the chromosphere with unprecedented altitude resolution. 

Probing solar history

Another focus of Solanki's research is the Sun’s influence on Earth's climate. To understand this influence, it is necessary to reconstruct the Sun's brightness variations over as long a period as possible and compare them with terrestrial temperatures over the same period. Historical records and observational data from the Sun as well as natural evidence of solar activity, "archived" for example in the concentration of radioactive elements in tree rings or ice cores, facilitate a look into the Sun’s past. Studies of this kind show that the fluctuations in the Sun's brightness over the past millennia were quite small, averaging less than a tenth of a percent, but never the less were reflected in the temperatures on Earth. The rise in terrestrial temperatures seen in the last decades, however, is not the Sun’s doing.

"The time for which we can reconstruct solar brightness variations is very small compared to the Sun’s overall age," Solanki cautions. What was the Sun like before? In a high-profile study, a team of researchers led by Solanki was able to compare the Sun's behavior with that of hundreds of similar stars. The scientists found that the brightness of other stars varies up to five times as much as that of the Sun. It is therefore conceivable that our star is capable of much greater fireworks and has only been going through an unusually quiet phase for a few millennia. Or is the Sun in principal a rather inactive star? This question remains a subject of intensive research.

Further information

Prof. Dr. Sami K. Solanki studied at the Swiss Federal Institute of Technology (ETH) in Zurich (Switzerland), where he received his PhD. After research stays in Zurich and at the University of St. Andrews in Scotland, he was appointed a professorship at the University of Oulu in Finland in 1998. A year later, he moved to Germany as director at the Max Planck Institute for Solar System Research (then called the Max Planck Institute for Aeronomy). Between 2009 and 2021, he was also a professor at the School of Space Research at Kyung Hee University in South Korea. Among Solanki's many awards are the Georg Bartels Medal of the European Geoscience Union (2015) and an honorary doctorate from the University of Oulu (2017). Professor Solanki is Editor-in-Chief of the open access journal Living Reviews in Solar Physics.

The American Astronomical Society (AAS) has more than 8000 members and is organized into six divisions. The Solar Physics Division (SPD), the division dedicated to solar research, annually awards the George Ellery Hale Prize to scientists who have made outstanding contributions to solar research over an extended period of time. The award is named after the American astronomer George Ellery Hale, who in the early 20th century discovered, among other things, that the dark sunspots on the Sun are accompanied by exceptionally strong magnetic fields.