Neutral particles play a crucial role in the creation of mysterious jets of plasma called spicules that burst from the surface of the Sun. Computer simulations done by researchers in the US and Norway suggest that an interplay between neutral particles and plasma in the Sun's atmosphere allow tangled magnetic fields to launch the jets.
The middle layer of the Sun's atmosphere – the chromosphere – is permeated by about 10 million spicules at any given time. These jets travel at speeds of 50–150 km/s and reach lengths of 10,000 km before collapsing. Rather than pointing straight out of the Sun, they tend to flop back towards the surface – giving the chromosphere the appearance of a lawn in need of cutting. Spicules could be providing hot plasma to the Sun's outer atmosphere – the corona – and a better understanding of the jets could help solve the long-standing puzzle of why the corona is millions of degrees hotter than the surface of the Sun.
However, understanding what drives the emergence of spicules has been a difficult task. They are tricky to observe because they move very quickly, with each jet lasting only 5–10 min. This means that it has been difficult to improve computer simulations of spicules by comparing them to observations of the real thing. Indeed, scientists have been working on one particular computer model of the chromosphere for 10 years without being able to simulate the emergence of spicules.
Now, Juan Martínez-Sykora and colleagues at the Lockheed Martin Solar and Astrophysics Laboratory and the University of Oslo have found a missing ingredient that appears to have been holding back the success of the computer model – neutral particles.
Solar physicists believe that spicules are created when tangled magnetic fields from within the Sun emerge into the chromosphere and straighten out like a snapping whip. Previous models had not been able to reproduce this behaviour as Martínez-Sykora explains: "Usually magnetic fields are tightly coupled to charged particles. With only charged particles in the model, the magnetic fields were stuck, and couldn't rise beyond the Sun's surface. When we added neutrals, the magnetic fields could move more freely."
Previous simulations had ignored these neutral particles because it is computationally very expensive to include them. Indeed, the team's new version of the computer model that includes neutral particles took a year to run on NASA's Pleiades supercomputer.
Worth the wait
The long wait was worth it because the model was able to simulate spicules for the first time. Furthermore, the output of the model was a close match to spicules observed by NASA's Interface Region Imaging Spectrograph space telescope and the Swedish 1-m Solar Telescope in the Canary Islands.
The simulation also revealed that the snapping magnetic fields create Alfvén waves. These are strong magnetic waves that physicists believe are responsible for heating the Sun's atmosphere and driving the solar wind of charged particles towards Earth.