Moving radio bursts associated with solar flares detected by XSM onboard Chandrayaan-2

Imagine the Sun not as a static, glowing ball, but as a restless, magnetic giant that constantly throws tantrums. When its tangled magnetic fields snap and reconnect, they unleash massive explosions called solar flares. Think of these flares as the Sun's version of a firecracker popping, but on a scale that can fry satellites and scramble radio signals on Earth. While we can see the "flash" of these explosions in X-rays, the Sun also screams in radio waves. These radio bursts are like the sonic booms or the crackling static that happens right after the pop. Scientists have long known that these radio signals are clues to how fast particles are being accelerated and how shockwaves are rippling through the Sun's atmosphere. Understanding the timing between the flash (the flare) and the boom (the radio burst) is like trying to figure out exactly how fast a shockwave travels after a bomb goes off; it helps us predict when the Sun might send a dangerous storm our way.

This paper is a detective story written by Abhishek Potdar and Anshu Kumari, who acted as cosmic timekeepers. They used a special X-ray camera called the XSM, riding on India's Chandrayaan-2 spacecraft, to watch solar flares, while simultaneously listening to the radio "screams" from ground-based telescopes. Their goal was to see how closely the timing of the radio bursts matched the timing of the flares. They focused on two specific types of radio bursts: "Type II" and "Type IVm." You can think of Type II bursts as the shockwaves moving through the solar atmosphere, and Type IVm bursts as the lingering glow of trapped energy. By looking at 36 specific events between late 2019 and late 2022, they discovered a very tight relationship between the flares and the bursts, but with some fascinating twists depending on how big the explosion was.

The researchers found that the radio bursts and the flares are best friends in terms of timing. When they plotted the start times of the radio bursts against the start times of the flares, the connection was incredibly strong, with a correlation score of about 0.99. This is almost a perfect match, suggesting that whenever a flare starts, a radio burst is almost guaranteed to follow quickly. However, the speed at which the radio burst starts after the flare depends heavily on the size of the flare.

Here is where the story gets interesting. When the Sun throws a small, weak tantrum (a "B class" flare), the radio shockwave (Type II) takes its time, showing up about 27.6 minutes later. It's like a slowpoke who takes a long walk to the party. But when the Sun throws a massive, energetic tantrum (a "C class" or "M class" flare), the radio burst arrives much faster, showing up in just about 5 to 6 minutes. This suggests that bigger, more energetic flares create shockwaves that form and race outward almost instantly.

The story gets even stranger when looking at when the radio bursts stop. For the Type II bursts, they actually ended before the X-ray flare finished fading away. The bigger the flare, the earlier the radio burst stopped. For the strongest "M class" flares, the radio burst ended a full 26.8 minutes before the flare itself was done. It's as if the shockwave finishes its job and disappears, leaving the flare to fizzle out on its own. This implies that the most energetic flares create shockwaves that are so powerful and fast they do their work and vanish quickly, perhaps because the environment they travel through changes as they move away from the Sun.

The Type IVm bursts, which are more like the lingering glow of trapped energy, behaved a bit differently. They started about 14.4 minutes after the flare, regardless of whether the flare was a medium "C class" or a strong "M class." This suggests that the process of trapping electrons to create this glow takes a consistent amount of time. However, once they started, the stronger flares kept this glow going for much longer. The "M class" flares kept the radio signal alive for about 68.8 minutes after the flare started, while the "C class" flares only held on for about 31.2 minutes. This tells us that bigger flares create better "containers" to hold onto the energetic electrons for a longer time.

Finally, the authors looked at how long it took for the flares to reach their peak brightness. They found that most flares hit their peak within 20 minutes of starting. This is a crucial finding for prediction. It suggests that if we see a solar flare start, we have a roughly 20-minute window to predict if it will be followed by a major eruptive event, like a coronal mass ejection (a giant cloud of solar plasma). The paper concludes that while the energy of the flare is important, the timing isn't just about power; it's also influenced by the local magnetic environment and the conditions of the plasma around the Sun. So, while we can't predict the Sun's mood swings perfectly, we now have a better stopwatch to measure how fast its tantrums unfold.