Flare Ribbons Offer a Window Into Solar Explosions Hidden in the Corona

A solar flare is hard to study at its source. The explosion begins in the corona, the Sun's outermost layer, and the Sun's glare hides much of what happens there. A team of researchers has now found a way around that problem by looking at what the flare leaves behind lower down, on the Sun's surface layers.

The work, published in The Astrophysical Journal, is titled "Evolution of Spatial Complexity in Flare Ribbon Substructure and Its Relationship to Magnetic Reconnection Dynamics." Its lead author is Marcel Corchado Albelo of the U.S. National Science Foundation, who is also an ambassador for the Daniel K. Inouye Solar Telescope. Maria Kazachenko of the National Solar Observatory is a co-author and advises his graduate work.

Some background on the physics. The Sun's magnetic fields are constantly twisted and rotated. Eventually they can't take the strain and snap, then reconnect into a simpler, more stable arrangement. That process is called reconnection, and it releases energy from its magnetic restraints as a solar flare. Flares are the most powerful explosions in the Solar System, releasing energy comparable to millions or even billions of hydrogen bombs detonating together. They are brighter in X-rays in steps: classes A, B, C, M and X, each a tenfold jump in output.

The team focused on flare ribbons, which work something like a flare's footprints. They are made of plasma and high-energy particles that slam back into the Sun during reconnection, following magnetic field lines. When they hit the chromosphere, they light up brilliantly. The researchers reasoned that the fine details of those bright shapes, where energy is deposited, carry clues about what is happening far above.

They developed a way to quantify how the ribbons and their substructure evolve. They built a model, tested it against synthetic observations, and then applied it to a real event: an M6.5-class flare on June 22, 2015, observed by the Interface Region Imaging Spectrograph, or IRIS, a solar satellite launched in 2013.

The finding was that energy deposition is not smooth or uniform. Rather than one large event, the ribbon breaks into fragments of different sizes and delivers its energy in bursts. Corchado Albelo said that for the first time the team had shown experimentally a physical relationship between the irregular shapes seen and the structure of the energy conversion region. The authors write that the buildup of complexity across multiple spatial scales can serve as an observational proxy for fragmentation of the current sheet in the corona.

Kazachenko explained why this matters. Simulations have long suggested that ribbon boundaries should behave in a particular way, but there was no observational way to confirm it. Using a tool adapted from a completely different field, Corchado Albelo showed that ribbon complexity increased with reconnection rate. She called it a beautiful example of theory, observation and collaboration, driven by the persistence and creativity of a graduate student.

There are practical stakes behind the physics. Flares can cause radio blackouts when they hit the ionosphere, disrupting aviation and maritime communications. They can introduce errors in GPS, and sometimes cause malfunctions in spacecraft electronics. Their accompanying coronal mass ejections can disrupt power grids, and also produce aurorae.

The result will not forecast flares on its own. It does improve understanding of how flares release their energy, and it should inform better models. The authors describe it as a first step toward quantitatively linking ribbon shape to reconnection dynamics, and that link is what later work will have to build on.