The Sun doesn’t just fuel almost all life on Earth. It’s also our astrophysical laboratory, the only star close enough to observe in sufficient detail.
Author
- Hannah Schunker
ARC Future Fellow, Physics, University of Newcastle
This enormous ball of hydrogen and helium is made of plasma – extremely hot, electrically charged gas.
The churning of plasma within the Sun produces an invisible magnetic field that envelops our entire Solar System.
Up close, at the Sun’s surface, the plasma is turbulent, dragging the magnetic field with it. It roils and bubbles and occasionally bursts out clouds of material known as coronal mass ejections .
Over the years, scientists have made many close-ups of the Sun’s surface with various telescopes and observatories. Now, we have the highest-resolution images yet, thanks to the Daniel K. Inouye Solar Telescope ( DKIST ) on Maui, Hawai’i.
In a study published today in Nature , a team led by researchers from the US National Solar Observatory and Max Planck Institute for Solar System Research delivers new observations of the surface of the Sun with unprecedented precision – revealing a feature never seen before.
A solar mystery
The Sun’s magnetic field drives what we call space weather. Explosive flares in the outer atmosphere of the Sun – the corona – can cause radiation storms on Earth , disrupting satellites, power grids and communications.
And there’s a major mystery concerning the corona. The Sun’s surface is about 5,500°C . That’s much cooler than its core, about 27 million degrees Celsius. But the corona, rising above the surface, reaches millions of degrees.
To heat it, energy must pass through cooler intermediate layers. Why does the temperature, after dropping, suddenly rise again? To pin down which mechanism is responsible, we need to observe physical processes on the Sun’s surface that unfold over short times and small distances.
DKIST’s four-metre mirror, advanced optics and high-speed cameras resolve features as small as 20 kilometres across, which is tiny compared to the Sun’s radius of 696,000km. They’ve captured subtle brightness variations and magnetic structures with unmatched clarity.
Connecting the loops
On the Sun’s surface, the roiling flows of plasma drag magnetic fields with them. Sometimes they form coherent magnetic sunspots; at others times they twist and tangle.
These surface magnetic fields extend upward into the corona, and the mechanics at the surface cause magnetic fields aloft to snap and reconnect. This reconfiguration of the magnetic field can release stored energy into heat and fast flows, sometimes triggering explosive solar flares.
The key challenge is to connect these small, short events at the surface to the large-scale heating and eruptions in the Sun’s atmosphere above.
Solving that link would make a huge advance towards understanding why the corona is so hot. It would also improve our ability to predict space weather and how it might affect Earth, as well as interpret the magnetic activity and space weather of distant stars.
In the new study, the team presents the first clear evidence on the Sun of one of nature’s most striking patterns – the Kelvin-Helmholtz instability.
These rolling, wave-like billows form when fast fluid slides past slower fluid. Sometimes you can see it along the edges of windswept clouds.
This instability was theoretically predicted to exist in the Sun’s plasma, but it was unknown at what scales and what role it might play.
To turn observations into insight, the team paired DKIST data with sophisticated computer simulations, creating “synthetic observations” to match what the telescope saw.
This revealed these instabilities are likely key in mixing plasma and braiding the magnetic fields. It means small-scale billows and vortices near the Sun’s surface pump energy upward, priming the corona for its extreme temperatures and setting the stage for explosive flares.
Making a simulation always involves some approximations. The models can’t reproduce the Sun’s extreme physics, but they do closely match up to the observations, which is really impressive.
The simulation results also suggest that despite looking coherent on the surface, the magnetic fields beneath features like dark pores are fragmented.
This is super exciting, because we know that the magnetic field is generated somewhere inside the Sun – but we don’t know how or where. The fragmentation beneath suggests these magnetic features are only formed once they reach the surface, not deeper below. If correct, this will really challenge existing models of how sunspots and magnetic regions form.
The Sun is the only star we can study up close. By connecting its smallest magnetic twists to its largest outbursts, we unlock the key to interpreting every other star.
