Watching Solar Flares Heat Up

What happens just as a solar flare begins to produce X-rays? New research leverages data from an Earth-orbiting X-ray telescope to examine the early stages of solar flares.

A Common Occurrence

With the help of Sun-gazing observatories in space and on Earth, researchers have witnessed thousands of solar flares. Solar flares are powered by the explosive rearrangement of magnetic fields, accelerating charged particles to breakneck speeds and generating a brilliant flash of X-rays.

While the overall picture of flare production is clear, the details are still under scrutiny. To understand how solar flares happen, researchers must study each stage of the process closely. Today’s article examines the onset phase, which happens just before a flare ramps up to maximum brightness.

A Three-Phase Process

Solar flares tend to happen in three stages:

1. The precursor phase, in which pent-up magnetic energy is released and the first signs of the flare appear in the form of non-thermal particle motions
2. The impulsive phase, in which charged particles get accelerated to extremely high energies, triggering the release of radio waves, hard (high-energy) X-rays, and gamma rays
3. Finally, the decay phase, in which soft (low-energy) X-rays gradually brighten and then fade

Anant Telikicherla (Laboratory of Atmospheric and Space Physics, University of Colorado Boulder) and coauthors focused on the onset phase, which refers to the moments just before the impulsive phase, when the first soft X-rays are emitted but before the hard X-ray emission begins. Using data from the Dual-zone X-ray Solar Spectrometer (DAXSS), the team analyzed the soft X-ray spectra of six solar flares. The sampled flares took place in 2022 and had a range of characteristics, from average-intensity C-class flares to more powerful M-class flares. Most of the flares in the sample were eruptive, meaning they were accompanied by explosions of plasma into space, while one was confined, lacking a plasma explosion.

Flare and Back Again

The observations revealed that during the onset phase, when the first soft X-rays are produced, the solar plasma is already extremely hot, in the range of 10–15 million kelvin. The temperature then dips before rising again during the impulsive phase. In this way, the onset phase parallels the rising and falling temperatures of the main flare phase, leading Telikicherla’s team to suspect that the onset phase acts as “preconditioning” for the main event.

The authors also examined extreme-ultraviolet images of the flares to understand the underlying motions of the plasma. They saw two general types of behavior, which they call one-loop and two-loop onset flares. For a one-loop onset flare, a single arc of coronal plasma brightens during the onset phase. This coronal loop then interacts with a second loop that brightens during the impulsive phase. During a two-loop onset flare, two coronal loops brighten, merging into a single loop during the impulsive phase.

Remarkably, the observed behavior appears to mimic the evolution of the main phase of a solar flare, suggesting that the onset phase offers a preview of what’s to come. Future work analyzing hard X-ray emission will explore the connection further, helping to understand whether the characteristics of the onset phase can be used to predict the properties of the main flare stage.

Citation

“Investigating the Soft X-Ray Spectra of Solar Flare Onsets,” Anant Telikicherla et al 2024 ApJ 966 198. doi:10.3847/1538-4357/ad37f6