Mysterious solar-flares first seen falling into the sun instead of bursting out of it in 1999 are finally identified as fluids of different densities that cannot mix

  • Astronomers created 3D models of solar flares to better understand them
  • Most are massive outward explosions that look like giant octopus tentacles 
  • In January 1999 astronomers saw the first downward facing solar flare outbursts
  • A study found these were caused by different densities of plasma not mixing 

Mysterious ‘finger-like’ solar-flares seen falling into the sun in 1999, rather than out of it as would be expected, have finally been explained by scientists.

Typical solar flares, which are giant eruptions from the surface of the sun, blast outward, like a giant tentacle, but in January 1999 astronomers saw one burst down, back towards our host star, something that left scientists guessing for two decades. 

In a new study, experts found that these unusual flares, dubbed ‘downward-moving dark voids’, are the result of fluids of different densities not being able to mix.  

Researchers, from Harvard University, say this discovery could help create more accurate space weather forecasts, and limit damage from eruptions reaching Earth.  

The Atmospheric Imaging Assembly (AIA) onboard NASA’s Solar Dynamics Observatory captured a supra-arcade downflow within a solar flare on June 18, 2015

Study lead author, Dr Chengcai Shen, set out to investigate what caused the strange eruptions, what was driving them and whether they were tied to magnetic reconnection, a leading theory behind their release.

Magnetic reconnection is when fast moving radiation is released after a magnetic field breaks. The sun has many magnetic fields pointing in multiple directions. 

‘Eventually the magnetic fields are pushed together to the point where they reconfigure and release a lot of energy in the form of a solar flare,’ said Dr Shen.

‘It’s like stretching out a rubber band and snipping it in the middle. It’s stressed and stretched thin, so it’s going to snap back.’

This image shows both a supra arcade downflow and a traditional solar flare within the same observation, from November 2020 

The problem is that this theory didn’t work, as the down flows were ‘puzzlingly slow’, according to Dr Shen and colleagues.

‘This is not predicted by classic reconnection models, which show the down flows should be much quicker,’ said Co-author Dr Kathy Reeves, adding that ‘It’s a conflict that required some other explanation.’

Images of the Sun captured by the Atmospheric Imaging Assembly (AIA) onboard NASA’s Solar Dynamics Observatory were analyzed by the researchers.

It takes pictures of the sun every 12 seconds in seven different wavelengths of light, allowing scientists to accurately measure variations in the star’s atmosphere. 

Harvard researchers used this to create 3D simulations of solar flares, which can be compared to actual observations – including of the 1999 downward flares. 

Still image of several supra-arcade downflows, also described as ‘dark, finger-like features,’ occurring in a solar flare. The downflows appear directly above the bright flare 

The researchers will continue studying these events and other solar phenomenon using 3D simulations creating from real world observations 

The strange downward moving flares were not the product of breaking magnetic fields after all, the researchers found.

Instead, they were formed in the turbulent environment on their own by the mixing of two fluids – plasma – with different densities.

Like with oil and water, when the two fluids are mixed together, they become unstable and separate – with oil sitting on top of water.

Dr Reeves said: ‘Those dark, finger-like voids are actually an absence of plasma. The density is much lower there than the surrounding plasma.’

The researchers will continue studying these events and other solar phenomenon using 3D simulations creating from real world observations.

Shedding light on these processes could help develop tools to forecast space weather and mitigate its impacts in the future.

The findings were published in the journal Nature Astronomy.


Solar storms, or solar activity, can be divided into four main components that can have impacts on Earth:  

  • Solar flares: A large explosion in the sun’s atmosphere. These flares are made of photons that travel out directly from the flare site. Solar flares impact Earth only when they occur on the side of the sun facing Earth.  
  • Coronal Mass Ejections (CME’s): Large clouds of plasma and magnetic field that erupt from the sun. These clouds can erupt in any direction, and then continue on in that direction, plowing through solar wind. These clouds only cause impacts to Earth when they’re aimed at Earth. 
  • High-speed solar wind streams: These come from coronal holes on the sun, which form anywhere on the sun and usually only when they are closer to the solar equator do the winds impact Earth. 
  • Solar energetic particles: High-energy charged particles thought to be released primarily by shocks formed at the front of coronal mass ejections and solar flares. When a CME cloud plows through solar wind, solar energetic particles can be produced and because they are charged, they follow the magnetic field lines between the Sun and Earth. Only charged particles that follow magnetic field lines that intersect Earth will have an impact. 

While these may seem dangerous, astronauts are not in immediate danger of these phenomena because of the relatively low orbit of manned missions.

However, they do have to be concerned about cumulative exposure during space walks.

This photo shows the sun’s coronal holes in an x-ray image. The outer solar atmosphere, the corona, is structured by strong magnetic fields, which when closed can cause the atmosphere to suddenly and violently release bubbles or tongues of gas and magnetic fields called coronal mass ejections

The damage caused by solar storms 

Solar flares can damage satellites and have an enormous financial cost.

The charged particles can also threaten airlines by disturbing Earth’s magnetic field.

Very large flares can even create currents within electricity grids and knock out energy supplies.

When Coronal Mass Ejections strike Earth they cause geomagnetic storms and enhanced aurora.

They can disrupt radio waves, GPS coordinates and overload electrical systems.

A large influx of energy could flow into high voltage power grids and permanently damage transformers.

This could shut off businesses and homes around the world. 

Source: NASA – Solar Storm and Space Weather 

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