Solar flares are the Sun at full volume—sudden bursts of magnetic energy that can supercharge Earth’s upper atmosphere, disrupt radio signals, and threaten satellites and astronauts. For decades, scientists have known that flare plasma reaches extraordinary temperatures, but how different particles heat up—and why spectral fingerprints often look “too wide” to explain—has remained a mystery since the 1970s.
Now, a new study led by Alexander Russell at the University of St Andrews offers a clear and elegant solution: during key phases of a flare, ions are much hotter than electrons.
Not just a little hotter—about 6.5 times hotter, with ion temperatures soaring past 60 million degrees Kelvin.
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A Simpler Explanation for a Longstanding Mystery
The research team argues that this imbalance neatly explains why many flare emission lines appear broader than expected, without requiring large amounts of turbulence. Solar plasma is a mix of lightweight, negatively charged electrons and much heavier, positively charged ions, such as iron and calcium stripped of their electrons.
For years, models assumed these particles quickly reached the same temperature. While mathematically convenient, this may not reflect what really happens during the explosive moments of a flare.
Lessons from Near-Earth Space
Russell’s team revisited the basic physics of flare heating, drawing on evidence from closer to home. Across the solar wind and near-Earth space, a process called magnetic reconnection—where magnetic field lines snap and reconnect—has been shown to heat ions far more than electrons, consistently following a 6.5-to-1 ratio.
“We were excited by recent discoveries that magnetic reconnection heats ions 6.5 times as much as electrons,” Russell explained. “This appears to be a universal law, confirmed in near-Earth space, the solar wind, and simulations. But nobody had previously applied it to solar flares.”
Carry that principle to the Sun, and the result is dramatic: during the flare onset and high above the bright loops of plasma they generate, ion temperatures can soar far beyond those of electrons, staying hotter for tens of minutes.
Rethinking Spectral Line Broadening
This matters because it changes how telescopes interpret flare light. Spectral lines—bright features at ultraviolet and X-ray wavelengths—get wider when emitting particles are hotter and move faster. For nearly 50 years, their unexpected width was blamed on turbulence.
The new study suggests that super-hot ions could explain much of that broadening. Faster-moving ions spread the light they emit across a wider wavelength range, mimicking turbulence. Russell’s team showed that ion-heavy heating during reconnection naturally produces the observed line widths.
Why the Temperature Gap Lasts
The key is density. In the dense loops that form after a flare, ions and electrons collide frequently enough to quickly share energy. But higher up, where the plasma is thinner, collisions are rare. This allows the ion-electron temperature gap to persist long enough to leave a visible fingerprint on the flare’s spectrum.
Past estimates often assumed dense loop conditions, overlooking the thinner, earlier regions where reconnection dominates.
A Unified Framework for Solar Flares
The beauty of this explanation is its simplicity. It doesn’t require exotic new physics—only applying a well-supported heating ratio from magnetic reconnection studies to the Sun’s most violent moments.
The result is a unified picture: reconnection preferentially energizes ions, low densities preserve their heat, and those hot ions broaden spectral lines once attributed to turbulence.
Implications for Space Weather
This discovery has real-world importance. Space weather forecasts rely on accurate models of flare heating and particle acceleration. If ions initially absorb the lion’s share of heat, it changes how shocks form, how particles are accelerated, and how much energy storms can deliver to Earth.
Future observations will be crucial. By comparing emission lines from different ions with electron-sensitive diagnostics, astronomers can test whether line widths match the predicted 6.5-to-1 temperature split.
If correct, this insight reshapes how researchers interpret past flare data and guides the design of future telescopes and models.
A Half-Century Puzzle, Solved?
For decades, solar physics assumed that ions and electrons must quickly reach the same temperature. Russell and colleagues show that letting them evolve separately—especially during flare onset and in high, thin plasma—solves a stubborn spectral riddle with one straightforward change: ions are simply much hotter.
It’s a tidy solution that connects solar flares to a universal rule of magnetic reconnection observed across space, offering researchers a new roadmap for cracking one of heliophysics’ longest-running mysteries.
