Solar flares are a fascinating thing and have a profound effect on what astronomers refer to as “space weather.” These events vary with the Sun’s 11-year solar cycle, releasing immense amounts of radiation across the electromagnetic spectrum (from extreme ultraviolet to X-rays) into space. The effects of flares have been observed since time immemorial, which include aurorae at high latitudes (Aurora Borealis and Australis), but have only been the subject of study and prediction for about a century and a half. Still, there is much that remains unknown about these dramatic events.
For instance, flares are known to affect the Sun’s atmosphere, from the visible surface (photosphere) to its outermost layer (corona). However, there are still questions about how these events influence the lower layers of the atmosphere. In a recent study led by the University of Colorado, Boulder, a team of researchers documented the rotation of two very small sunspots of the Sun’s surface (pores) using the Daniel K. Inouye Solar Telescope (DKIST) at Mauna Kea. These pores were linked to a less powerful flare and moved in a way that has never been observed, suggesting that the dynamics of the Sun’s atmosphere are more complex than previously thought.
The study was led by Rahul Yadav, a Research Scientist from the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado, Boulder (UC Boulder). He was joined by colleagues from UC Boulder’s Department of Astrophysical and Planetary Sciences, the U.S. National Science Foundation’s (NSF) National Solar Observatory (NSO), and the Institute of Solar-Terrestrial Physics of SB RAS. The paper that details their findings, “Photospheric Pore Rotation Associated with a C-class Flare from Spectropolarimetric Observations with DKIST,” recently appeared in the Astrophysical Journal Letters.
Solar flares are thought to occur when stored magnetic energy in the Sun’s atmosphere accelerates charged particles in the surrounding plasma. They occur in active regions and are often accompanied by a significant amount of plasma being ejected into space – a Coronal Mass Ejection (CME) – and the release of accelerated particles – a Solar Particle Event (SPE). These can play havoc with satellites in Earth’s orbit, and interfere with radio antennas and electronic grids on the surface, which is why scientists are interested in learning more about them.
Flares are classified according to their strength: B-class is the weakest, C and M-class are slightly more energetic, and X is the strongest. Previous studies have shown how intense solar flares can lead to large sunspots rapidly rotating and distorting active regions on the Sun’s surface. But as Dr. Yadav explained in an NSO press release, what they observed was quite unexpected. “[T]his study marks the first time that such rotation has been observed on a smaller scale—less than 2,000 kilometers [~1,245 mi] across—associated with a less intense C-class flare,” he said.
In addition, previous observations have found that rotational movements of sunspots occur directly at the flare ribbon, where the most intense emissions occur during a flare event. This time, the team observed a pre-flare rotation located a short distance from the flare ribbon, which suggests that the coupling between different layers of the Sun’s atmosphere during flares may be more complex than previously thought. Yadav and his colleagues suggest that the process they observed is driven by changes in the Lorentz force caused by interactions between solar charged particles (aka. solar wind) and its magnetic fields.
As Prof. Maria Kazachenko, an NSO scientist and co-author of the study, explained:
“As the magnetic field lines in the corona reorganize, they could induce changes in the lower atmosphere, leading to the observed rotation. This discovery adds a new dimension to our understanding of the complex magnetic interactions that occur during solar flares.”
The unique observations the team made using the Inouye telescope offer new insights into the mechanisms through which solar flares influence the lower layers of the Sun’s atmosphere. For example, past observations have revealed much about sunspot rotations that occurred during more powerful flares (M—or X-class). However, the Inouye data revealed that similar rotational movements can occur with less intense flares and on smaller scales. These findings could lead to new research avenues and help refine our models of solar activity.
This will have implications for the growing constellations of telecom, research, internet, and Earth observation satellites in Earth’s orbit. Predicting space weather, which affects everything in the Solar System to the very edge of the Heliosphere, is also important for long-duration missions in space. For astronauts working on the Moon and Mars and transiting through deep space, knowing more about flare activity will help mitigate the risk of radiation exposure.
Further Reading: NSO, AJL