Candles flickering on the solar surface

At high spatial resolution, Ellerman Bombs appear as candles flickering on the solar surface. Despite all we have learned about their nature in the past few years, a lot still stays unclear, writes Dr. Sanja Danilovic, from Stockholm University (Sweden). The European Solar Telescope will enable us to look into these features in unprecedented detail, she adds.


High-resolution observations and simulations of Ellerman Bombs. / Credit: Observations by Gregal Vissers (ISP, Stockholm University), animation by NCAR Vapor software


On September 21, 1915, while observing the Sun at Mount Wilson Observatory (USA), Ferdinand Ellerman saw an intense brightening in solar spectra. He wrote: "On the first occasion the appearance was so extraordinary that it seemed hardly real; after the second observation, however, the existence of such phenomena as part of the solar activity seemed established." Because these features were so prominent in spectra of hydrogen lines, he named them "solar hydrogen bombs". Today we call them after the discoverer himself - "Ellerman bombs" or just "EBs".

These features are common part of flux emergence. They occur repetitively in young emerging active regions, last for a few minutes and then disappear. When we look at them with today's instrumentation and sample the wings of the H-alpha spectral line, they appear as candles that flicker at the solar surface, sometimes with multiple flames. They are as long as 1000 km and almost always point towards the solar limb.

It took us exactly 100 years to explain what they really are. Thanks to numerical simulations, we can now say with certainty that these are locations where magnetic field reconnects and magnetic energy is converted to kinetic and thermal energy. During the reconnection, hot pockets of plasma are formed, which we then see as elongated flame-like features when we look at them from certain perspectives. They mark the location where heavy material that emerges from inside the Sun, gets rid of much of its mass and becomes free to expand further towards the upper solar atmosphere.

Although we have learned a lot about the nature of EBs in the past few years, a lot still stays unclear. The European Solar Telescope will enable us to look at these features in more detail and discover more about their properties and the underlying physical process.

The first half of the movie shows EBs as flames close to the solar limb in the red wing of the H-alpha spectral line as observed with the CRISP instrument at the Swedish 1-m solar Telescope on La Palma (Spain). The second half presents simulations showing the emergence of a magnetic sheet to the solar surface. Color lines denote magnetic field lines as they emerge and reconnect. The green areas are hot regions which correspond to the flames we observe. The animation was made by using the NCAR Vapor visualization software.


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