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, physicists have known that flare plasma gets incredibly hot, but exactly how different particles heat up – and why certain spectral fingerprints look “too wide” to explain – has remained a nagging mystery since the 1970s.

A new study led by Alexander Russell at the University of St Andrews offers a clean, intuitive answer: during key phases of a flare, the ions are far hotter than the electrons.

Not just a little hotter – about 6.5 times hotter, with ion temperatures likely exceeding 60 million degrees Kelvin.

The team argues that this simple shift in perspective neatly explains why many flare emission lines appear broader than expected, without needing to invoke large, persistent turbulence.

Significance of hotter ions

Solar plasma is a soup of charged particles – lightweight, negatively charged electrons and much heavier, positively charged ions (think iron, calcium, and other elements stripped of electrons).

Historically, many solar models assumed that electrons and ions quickly share energy and settle to a common temperature. That assumption made the math tidy, but it may not reflect reality during a flare’s most dynamic moments.

Russell’s team revisited basic heating physics in flares and drew on a growing body of evidence from space plasmas closer to home. 

Hot ions and solar flares

Across the solar wind and near-Earth space, a process called magnetic reconnection – where stressed magnetic field lines snap and rapidly rejoin – has been observed to heat ions far more than electrons, following a surprisingly consistent ratio. 

“We were excited by recent discoveries that a process called magnetic reconnection heats ions 6.5 times as much as electrons,” Russell explained.

“This appears to be a universal law, and it has been confirmed in near-Earth space, the solar wind and computer simulations. However, nobody had previously connected work in those fields to solar flares.

Carry that rule of thumb to the Sun and you get a dramatic, testable prediction: early in a flare and high above the bright loop of hot plasma it creates, the ion temperature can soar past 60 million K, remaining much higher than the electron temperature for tens of minutes. 

That temperature split matters because it changes how we interpret what telescopes see. Spectral lines – bright features at specific ultraviolet and X-ray wavelengths – get wider when the particles emitting them are hotter and moving faster.

For nearly 50 years, those “too-wide” lines were usually blamed on unresolved turbulent motions. The new work suggests super-hot ions could be responsible for a big share of that extra width.

Solving an astrophysics mystery

If ions are smoking-hot, they jiggle more rapidly, and the light they emit spreads over a wider range of wavelengths.

That broadening can look exactly like turbulence, which led to decades of debate about what, physically, was stirring flare plasma so vigorously. 

Russell’s team lays out why ion-heavy heating at the onset of flares – and in the above-the-loop-top region where reconnection outflows crash into denser plasma – naturally produces the observed line widths. 

“What’s more, the new ion temperature fits well with the width of flare spectral lines, potentially solving an astrophysics mystery that has stood for nearly half a century,” noted Russell.

A key enabler of this explanation is timescale. In the dense, cooling loops that form after a flare brightens, ions and electrons collide often enough to share energy and equalize temperatures relatively quickly. 

But higher up, where densities are lower, collisions are rarer. That means the ion-electron temperature gap can persist long enough to leave a clear imprint on the flare’s spectrum. 

The paper argues that past estimates of equilibration were often based on the dense loop conditions – not the more rarefied, earlier or higher regions where reconnection does its fiercest work.

Past and future observations

The elegance of the proposal is that it doesn’t require radical new physics. It borrows a well-supported heating ratio from reconnection studies in the solar wind and Earth’s magnetosphere and simply applies it where reconnection is strongest on the Sun. 

The result is a unified picture: reconnection preferentially energizes ions; low densities up high let that temperature advantage survive; and hot ions inflate spectral lines that have long looked “non-thermal.”

If correct, the idea reshapes how researchers interpret past observations and plan future ones. Instruments that separate lines from different ions (and compare them with electron-sensitive diagnostics) could directly test whether line widths track the higher ion temperature rather than turbulence.

This new concept also encourages modelers to let ions and electrons evolve separately in the crucial early minutes of a flare, instead of forcing them to share a single temperature from the start.

Space weather, made a bit clearer

This is more than a bookkeeping fix. Space weather forecasts depend on how quickly and how hot flare plasmas get, because the resulting radiation and particle storms determine how much they can disturb Earth’s ionosphere and threaten spacecraft.

If ions get the lion’s share of heat at first, that affects energy transport, shock formation, and particle acceleration – the seeds of the most disruptive events.

The study also suggests new observational sweet spots. Look early, when the flare is just switching on. Look above the bright loop tops, where reconnection outflows crash and churn.

Furthermore, look at line widths from multiple ions, comparing them with electron-temperature diagnostics. If the widths line up with a ~6.5:1 ion-to-electron temperature split, that’s a powerful fingerprint.

A new look at solar flares

Solar physics has long assumed that ions and electrons quickly come to the same temperature in flares.

Russell and colleagues show that dropping that assumption – especially during the flare onset and in the high, thin plasma above the loops – solves a stubborn spectral riddle with one straightforward, physically motivated change: ions are simply much hotter. 

It’s a tidy answer that ties the Sun’s fiercest moments to a “universal” reconnection rule measured throughout near-Earth space, and it gives observers and modelers a clear roadmap for cracking open one of heliophysics’ longest-running mysteries.

The study is published in The Astrophysical Journal Letters.

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