Lab Laser Plasma Recreates Solar Nanoflare Heating Process
Physicists in China have reproduced in the laboratory a process that may help explain why the Sun's outer atmosphere is hundreds of times hotter than its surface: magnetic fields in laser-made plasma came together and turned magnetic energy into heat.
The work, by a team including Wei Sun of the China Institute of Atomic Energy and Yongli Ping of Beijing Normal University, was published online on 20 October in Physical Review Letters.
The puzzle of the hot corona
The visible surface of the Sun is around 5,500 °C, while its outer atmosphere, the corona, can exceed a million degrees Celsius. Scientists have spent decades trying to explain how this thin layer stays so hot. One suspect is countless tiny flares, called nanoflares, that could collectively supply the heat. The new experiment tests a possible trigger for some of them.
The Sun is full of plasma, a state of matter made of charged particles, threaded by magnetic fields. When oppositely directed fields meet and reconnect, stored magnetic energy is released and heats the particles. This process is called magnetic reconnection.
How the experiment worked
At the Shenguang-II laser facility in Shanghai, the researchers fired four overlapping laser beams at a plastic target with a finely rippled surface. Each pulse lasted a billionth of a second and turned material from the target into plasma. As the plasma expanded into the much less dense surroundings and slowed, the ripples at its boundary grew into fingers and mushroom-shaped structures, a behaviour known as the Rayleigh–Taylor instability.
The plasma generated its own magnetic fields around the fingers, and neighbouring structures carried fields pointing in opposite directions. Computer simulations showed that the expanding fingers pushed these fields together so that they reconnected and heated the plasma around them.
What the measurements showed
The team repeated the experiment with a flat plastic target, which gave a smoother flow without fingers. From the X-rays the plasma emitted, they estimated the temperature of its electrons. With the rippled target the peak electron temperature reached roughly 820,000 °C; with the flat target it was about half as high. In a separate simulation in which magnetic fields were prevented from forming, the extra heating largely disappeared, which supports the role of the fields.
On the Sun, similar conditions could arise where relatively cool, dense plasma falls through the surrounding atmosphere, a phenomenon called coronal rain, creating boundaries where the instability can develop.
Limits of the comparison
The laboratory plasma was much denser than in solar nanoflares, and the process unfolded on a much shorter timescale. The setup also did not recreate the Sun's full buildup of magnetic energy. The findings support a mechanism that could help heat the corona; further observations and experiments are needed to establish how often it operates on the Sun and how much of the heating it supplies. More in our science section.