Solar flares are one of the most closely watched processes in solar physics. Partly that’s because they can prove hazardous both to life and equipment around Earth, and in extreme cases even on it. But also, it’s because of how interestingly complex they are. A new paper from Pradeep Chitta of the Max Planck Institute for Solar System Research and his co-authors, available in the latest edition of Astronomy & Astrophysics, uses data collected by ESA’s Solar Orbiter spacecraft to watch the formation process of a massive solar flare. They discovered the traditional model used to describe how solar flares form isn’t accurate, and they are better thought of as being caused by miniaturized “magnetic avalanches.”
On September 30th, 2024, Solar Orbiter was watching a particular patch of the Sun where a solar flare eventually formed. The first hint that anything was happening occurred about 30 minutes before the flare itself formed. The High-Resolution Imager of the EUI telescope onboard the Orbiter detected “relentless” weak magnetic reconnection events in that particular part of the Sun’s corona.
The camera was set to a 2-second capture cycle, and every two seconds new magnetic strands of plasma began forming and dissipating. Strands also appeared to wind and unwind rapidly, living up to what solar physicists have called them for a long time - ropes. But, at this stage at least, there was a notable lack of high energy particles detected by one of the other instruments onboard - STIX, a hard X-ray monitor. In other words, a flare had not yet formed.
Fraser discusses some more of the science we hope to do with Solar Orbiter.As time went on, and Solar Orbiter continued observing, the magnetic strands began to build in energy, into what the authors call a “Self-Organized Criticality”. As an analogy, think of a pile of sand. You can build a sand pile up grain by grain, until eventually it gets to a point where one more grain (or magnetic stress in this case) causes a small slide on one of the sides, which triggers slides in its neighboring pieces of sand, eventually leading to an avalanche and the collapse of the sand pile.
Something similar appears to happen with these magnetic “ropes” of plasma. After enough energy has built up in the area in the form of connected magnetic threads, one particular break, no different from any that had been happening all along, causes a collapse of the main flux rope, which then causes the eruption of a solar flare. Traditionally, the model for this process involves a “current sheet” where the stretched magnetic field lines behind the flux rope drop back down to magnetically reconnect. That reconnection was thought to be the place where the energy powering the flare was released. The new study shows that the avalanche of tiny, individual threads breaking inside the rope itself reconnect, which causes the avalanche, and therefore the energy release powering the flare.
Once the flare formed, the new Solar Orbiter data showcased another interesting phenomenon - plasma blobs that looked like rain. When the flare happened, it created snake-like lines on the surface of the Sun called flare ribbons, which were clearly visible in the Solar Orbiter data. But since this data was such higher resolution than previous observations of a flare, they were also able to see small, super-bright dots moving down the magnetic loops forming the rope and hitting the ribbons. They appeared like glowing raindrops, but only lasted a few seconds, and were on the edge of what the Orbiter could detect - only a few hundred kilometers wide.
Fraser discusses the potential danger of solar flares.Instead of physical chunks of plasma falling back to the Sun’s surface, the paper concludes that these are actually impact zones of high energy particles being shot down the loop by the magnetic disconnects happening above. So instead of a blob of physical material, instead these are localized explosions of heat and light where the high-energy electrons hit the Sun’s surface after being propelled forward from the magnetic chaos going on above.
One particular point from that last section shows how much more we have to learn about the processes on the Sun. Solar Orbiter, which is one of our most powerful Sun observing platforms, is still only able to resolve features on the scale of hundreds of kilometers. That leaves a lot of room for improvement on finer features in our giant stellar neighbor, and the authors acknowledge that more data is needed to truly track how these disconnect and reconnect cycles truly cause the avalanche process. But, at least for now, our understanding of solar physics has taken another step forward thanks to an incredible dataset and the scientists and engineers that managed to collect and analyze it.
Learn More:
ESA - Magnetic avalanches power solar flares, finds Solar Orbiter
L.P. Chitta et al. - A magnetic avalanche as the central engine powering a solar flare
UT - ESA's Solar Orbiter Takes a Ludicrously High Resolution Image of the Sun
