On the Sun’s limb a flare blooms, bright as a wound, and suddenly the numbers jump. A new study from the University of St Andrews, published September 3, 2025 in The Astrophysical Journal Letters, argues that solar flare ions can reach roughly 60 million kelvin and, crucially, that ions run about 6.5 times hotter than electrons during key stages of a flare.
If right, this reframes a 50-year mystery in solar physics and helps explain why spectral lines from flares look broader than expected. It is a simple change in temperature, but it changes the story.
Solar Flare Ion Temperatures, Rewritten
For decades, textbooks and models assumed ions and electrons share a temperature in flares. That tidy equality has always sat uncomfortably with the data. X-ray and EUV lines in many events were broader than thermal models allowed, even before the impulsive flare phase, and attempts to pin the excess on hidden flows or turbulence kept running into contradictions. Center-to-limb studies showed little viewing-angle effect for line widths. Line cores looked too symmetric for stacked, unresolved upflows. The puzzle persisted.
The researchers drew on results from other space plasmas, where satellites have repeatedly measured magnetic reconnection heating ions far more than electrons. In those cases, ions heat by about 4 to 6.5 times as much. Applying the same rule to solar flares suggests electrons rise to about 10 million kelvin, while ions leap to 40–65 million. Those numbers line up with the broad spectral lines seen in flare observations, without needing to add hidden turbulence.
A Fifty-Year Mystery Narrows
There is a pleasing economy here. If ions really are super hot, then the Doppler broadening they produce can supply a substantial share of the so-called nonthermal line width that has haunted flare spectroscopy since the 1970s. The case strengthens when you consider timescales. In the onset and above-the-loop regions of a flare, densities are relatively low, which means ion-electron thermal equilibration can take hundreds to thousands of seconds. Temperature differences can be created and, importantly, can last.
That endurance matters. Historical arguments for fast equilibration leaned on densities measured in soft X-ray loops filled by chromospheric evaporation, but those measurements do not describe the earlier, more rarefied plasma above. In the places where reconnection does its heating, ions and electrons have time to diverge. The line-broadening mystery, once a story of missing motions, becomes a story of hidden heat.
A Concrete Picture, From A Familiar Star
Look at the study’s visual touchstone: a solar limb flare set against a scaled Earth for size. The image, built with open-source SunPy and data from NASA’s Solar Dynamics Observatory, drives home the scale of the physics at play. In the flare’s above-the-loop region, instruments like Hinode EIS and IRIS have repeatedly mapped line widths near or above 100 kilometers per second, while older, whole-Sun spectra caught the earliest broadening minutes before the impulsive phase. Seen through this lens, the widths cease to be a signature of invisible, chaotic flows and instead become a thermometer pointing at ions that have sprinted ahead of electrons.
There is still work to do. The current generation of 3D MHD flare simulations mostly treat a single temperature. Moving to multitemperature models, with parameterizations that capture small-scale reconnection physics, will be essential. Heavier ions may be preferentially heated, shifting the apparent nonthermal velocities by species. And turbulence does not vanish from the story, it likely still rises as outflows brake in the above-the-loop region. But if the amplitude budget changes because ions are already hot, then the whole energy ledger must be updated.
From Rule To Practice
The throughline is clean: a universal reconnection pattern seen in space plasmas, applied to the Sun, explains stubborn line widths and offers a testable forecast. Future missions like MUSE and Solar-C EUVST can map where and when ion temperatures outrun electrons. Spectroscopic techniques that partition line width into thermal and true nonthermal parts are ready to be adapted to flare conditions. And models can begin to ask a simpler question: if ions are allowed to run hot, how much mystery is left?
It is a shift in emphasis, not a revolution for its own sake. Still, there is something satisfying about a solution that does not pile new complications onto a complex system. The Sun remains unruly. The flare remains bright. The explanation, finally, runs hotter.
Explainer: What Does 60 Million Kelvin Mean?
Solar flares heat plasma, a mix of ions and electrons threaded by magnetic fields. Spectral lines, the bright features at specific wavelengths in X-ray and EUV light, broaden when particles move faster. Broader lines can come from higher temperatures (thermal Doppler broadening) or from unresolved motions like turbulence. The new study argues that in the flare onset and above-the-loop regions, reconnection heats ions 4 to 6.5 times more than electrons. That pushes ion temperatures to around 60 million kelvin, enough to explain much of the observed broadening without invoking large hidden flows. Because densities are modest in these regions, ion and electron temperatures take hundreds to thousands of seconds to equilibrate, so the temperature gap has time to matter.
Journal: The Astrophysical Journal Letters
Citation: Alexander J. B. Russell et al. 2025 ApJL 990 L39
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