Move away from the fire and you’ll expect to feel cooler. But something very strange happens with the Sun. The Sun’s outer atmosphere is hundreds of times hotter than its visible surface. Where does the energy behind that difference come from? The visible surface is around 5,500 °C, while the outer atmosphere, called the corona, can exceed one million degrees Celsius. Scientists have spent decades trying to explain how this thin layer stays so hot. Researchers in China have now demonstrated a process in the laboratory that could help explain some of that heat. Growing structures in the laser-produced plasma bridged the magnetic fields, allowing the magnetic energy to be converted into heat. a comparison experiment. Coronal shower observed by NASA’s Solar Dynamics Observatory in July 2012, when cooled plasma fell toward the Sun along magnetic field lines. (NASA Goddard Space Flight Center Scientific Visualization Studio/SDO Science Team/Virtual Solar Observatory) The team, including Wei Sun of the China Atomic Energy Institute and Yongli Ping of Beijing Normal University, wanted to understand how small energy releases might begin on the Sun. Scientists suspect that countless tiny flares, called nanoflares, could collectively help maintain the temperature of the corona. The new experiment investigates a possible trigger for some of these events. The Sun is full of plasma, a state of matter that contains electrically charged particles. Magnetic fields pass through this material. When fields from opposite directions come together and change their connections, stored energy can be released, heating the particles. This process is called magnetic reconnection. To investigate how such fields might be found, the researchers fired four overlapping laser beams at a plastic target with a finely corrugated surface at the Shenguang-II laser facility in Shanghai. The pulses lasted a billionth of a second and converted the target material into plasma. As the plasma expanded into the much less dense air around it and slowed, the ripples at the boundary grew into fingers and mushroom-like structures. This behavior is known as Rayleigh-Taylor instability. The plasma also generated its own magnetic fields around the fingers, with neighboring structures carrying fields in opposite directions. Computer simulations showed how the expanding fingers bridged those fields, allowing them to reconnect and heat the surrounding plasma. For comparison, the researchers repeated the experiment with a flat plastic target. This produced a smoother plasma flow without finger-like structures. By analyzing the X-rays emitted by the plasma, they estimated the temperature of its electrons, the negatively charged particles within it. With the wave target, the maximum electron temperature reached the equivalent of approximately 820,000 °C. The value measured with the flat objective was approximately half. Small jets of plasma observed by NASA’s IRIS mission offer clues about how magnetic reconnection warms the Sun’s outer atmosphere. (NASA Goddard Space Flight Center/Scientific Visualization Studio) In a separate computer simulation that prevented the formation of magnetic fields, the additional heating largely disappeared, further supporting its role. On the Sun, similar conditions could arise where dense, relatively cold plasma falls through the surrounding atmosphere. This phenomenon, called coronal rain, could create boundaries where instability develops. The experiment offers scientists a way to investigate processes that are too small and fast to be clearly observed on the Sun. They can test what conditions bind magnetic fields and how efficiently the resulting energy release heats the plasma. The laboratory plasma was much denser and the process occurred on a much shorter time scale than in solar nanoflares. The researchers compared key physical ratios to assess which aspects could be applied to the corona, but the setup did not recreate the full buildup of the Sun’s magnetic energy. The findings support a mechanism that could help heat the corona. More observations and experiments will be needed to establish how often it operates on the Sun and how much heat it supplies. The research has been published in Physical Review Letters. This article was fact-checked by Rachel Garner and edited by Peter Dockrill. While we take pride in our process, we are human. If you spot an error, please let us know.
