5 December 2019

Higher resolution telescopes could solve one of the biggest mysteries in modern astrophysics

University of Sheffield scientists, as part of an international team, have moved a step closer to solving a mystery which has puzzled applied mathematicians and physicists for over a century.

The sun

The outer atmosphere of the sun, which extends outward for several million kilometers beyond the visible solar surface, is called the corona.

Although the temperature in the core of the sun is as high as 15 million degrees, it drops to about 5,700 degrees at the solar surface - however, the temperature starts to increase again with height, reaching millions of degrees or more in the corona.

What causes the coronal-temperature increase is not known and is one of the biggest yet-to-be-solved mysteries in modern astrophysics.

Solar spicules - originally discovered by Father Secchi in 1877 - are small-scale magnetised geyser-like jets observed in the solar chromosphere, an interface between the photosphere and corona.

These spicular jets are narrow columns of plasma which shoot upward to heights of about 5-8,000 km above the solar surface. It is estimated that at any given moment in time there are about a few million spicules in the solar atmosphere.

Many solar physicists suspect that spicules serve as a conduit for mass and energy to flow from the lower atmosphere to the corona but their formation process is still poorly understood mainly because their small size meant earlier telescopes were of insufficient resolution and sensitivity for scientists to observe spicules well enough.

Now, new high spatial resolution and high time-cadence observations have unveiled groundbreaking insight into the generation mechanism of many spicules, and into the possible contribution of spicules to coronal heating.

The team performed observations with the 1.6-m Goode Solar Telescope (GST) at the Big Bear Solar Observatory (BBSO), the world’s largest-aperture and highest-resolution ground-based solar telescope that is currently operational.

The telescope observed copious spicules at ultra-high spatial resolution and simultaneously measured the magnetic fields in the solar photosphere at high spatial resolution.

The team’s cutting-edge observations has provided the best evidence yet that magnetic reconnection, i.e. reorganisation of oppositely polarised magnetic field lines into lower energy state, in the lower solar atmosphere drives spicules and produces hot plasma flows into the solar corona, thus providing a direct link between magnetic activities in the lower atmosphere and coronal heating.

The researchers suggest advanced computer simulations and theoretical investigations based on the groundbreaking new observational results should be performed in the future to help solve the long-standing coronal heating problem.

Professor Robertus Erdelyi, head of the Solar Physics and Space Plasma Research Centre, at the University of Sheffield, who was among the international team of researchers working on the study and provided theoretical interpretations, said: “The current state-of-the-art observations gave an entirely new and fascinating opportunity for us to look into the origin of these plasma ejecta that puzzle astrophysicists well over a century.”

"Progress was made by bringing together some of the best observers, modellers and theoreticians. We cannot now await to move even further when ever larger telescopes, like the Danile K. Innouye Solar Telescope (DKIST) next year, or in the near future the European Solar Telescope (EST) will be available with even higher resolution capabilities. So, watch this space!"

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