Fast and periodic propagating disturbances along coronal loops detected with EUI on board Solar Orbiter
Using high-resolution observations from the Solar Orbiter/EUI instrument, this study identifies two distinct types of propagating disturbances along coronal loops: slow, damped disturbances consistent with magneto-acoustic modes or upflows, and newly detected "fast" disturbances with high velocities (500–2000 km/s) and minimal damping that likely originate from magnetic reconnection or Alfvénic waves.
Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
The Sun's Hidden Highways
Imagine the Sun not as a static, burning ball of gas, but as a bustling, chaotic city where energy is constantly being built, transported, and sometimes lost. At the very top of this city, in a layer called the corona, the temperature is millions of degrees hotter than the surface below—a mystery that has baffled scientists for decades. How does this upper layer get so hot? One leading theory suggests that the heat comes from tiny, invisible explosions or waves traveling up from the surface, acting like a million tiny heaters turning on and off. To solve this puzzle, astronomers need to see these tiny energy packets as they travel. For a long time, our telescopes were like old binoculars; they could see the big buildings (large solar loops) but missed the tiny details of the traffic moving between them.
Recently, however, we got a new pair of super-powerful glasses: the Solar Orbiter, a spacecraft that gets closer to the Sun than any before it. This mission allows us to zoom in on the "coronal loops"—giant arches of magnetic gas that connect different parts of the Sun's surface. By watching these loops with extreme clarity, scientists hope to catch the "propagating disturbances" (PDs). Think of these disturbances as ripples or pulses of energy moving along the loops. Some of these ripples are slow and gentle, like a wave rolling onto a beach, and we've seen them for years. But the big question is: are there other, faster, more energetic ripples that we've been missing because they were too small or too fast for our old tools to catch? Finding them could be the key to understanding how the Sun's upper atmosphere stays so incredibly hot.
The Paper's Discovery: Catching the Sun's "Fast Lane" Traffic
In this new study, a team of astronomers used the Solar Orbiter's high-resolution camera, called EUI, to take a very close look at these coronal loops. They didn't just look; they set up invisible "slits" (like cutting a thin slice of a loaf of bread) across 13 different loops to watch what happened over time. What they found was a traffic jam of two very different types of disturbances moving along the same road.
First, they confirmed the "slow" traffic. These are the disturbances we already knew about. They move at about 80 to 105 km/s (roughly the speed of sound in the Sun's atmosphere) and are usually seen near the bottom of the loops, close to the surface. As they travel upward, they get weaker and fade away, much like a shout that gets quieter as you walk away from it. The paper confirms these behave like slow sound waves or gentle upflows.
But the real excitement comes from the "fast" traffic. The team discovered a new type of disturbance zipping along the upper parts of these loops at speeds between 500 and 2,200 km/s. To put that in perspective, these fast pulses are moving so quickly they could circle the Earth in less than a minute! These "fast PDs" are invisible in the lower parts of the loops, likely because the background noise is too loud there, but they light up the upper arches. Unlike the slow ones, these fast pulses don't fade away as they travel; they keep their strength for a long distance. They also appear to be rhythmic, showing up in a pattern roughly every 2 minutes.
The authors are careful to say they haven't solved the mystery of exactly what these fast pulses are yet, but they have ruled out some possibilities. They argue these are definitely not the slow sound waves we've seen before, because those are too slow and fade too quickly. They also suggest these aren't just random glitches in the camera or simple thermal heating, because the timing and speed are too specific.
Instead, the paper suggests a few exciting possibilities for what these fast pulses might be. They could be:
- Magnetic Reconnection Jets: Tiny, periodic explosions where magnetic field lines snap and reconnect, shooting out fast flows of plasma.
- Fast Magnetohydrodynamic (MHD) Waves: Powerful waves that travel through the magnetic field, similar to how a sound wave travels through air, but much faster.
- Alfvén Waves with a Twist: A type of wave that usually doesn't change the density of the gas, but in this case, might be creating a "density front" (a pile-up of gas) that looks like a bright pulse.
The team even ran computer simulations to test the Alfvén wave idea. Their results showed that if a wave has a specific speed (between 440 and 620 km/s), it could create the exact kind of bright, fast-moving pulse they observed. This suggests that these fast pulses might be the "smoking gun" of energy being transported from the Sun's surface to its hot upper atmosphere.
Interestingly, the fast pulses seem to come from only one side of the loop (one footpoint) rather than both, which is a bit like seeing cars only enter a highway from one specific on-ramp. This hints that the "engine" creating these pulses is located in a very specific spot on the Sun's surface, possibly driven by the rhythmic churning of the Sun's surface (granulation) or magnetic shocks.
While the paper doesn't claim to have definitively identified the cause, it provides strong evidence that these fast, high-energy events are real, common, and distinct from the slow waves we've studied for decades. By catching these "fast lane" disturbances, the study opens a new window into how the Sun might be heating its own atmosphere, suggesting that the answer lies in a mix of magnetic reconnection, fast waves, and the complex dance of plasma at the smallest scales we can now see.
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