Solar flares heat ions much more strongly than electrons. The finding offers a new explanation for spectral line broadening.
Recent work from the University of St Andrews suggests that particles in solar flares can become 6.5 times hotter than earlier estimates, offering an unexpected resolution to a mystery about the Sun that has persisted for half a century.
Solar flares are powerful bursts of energy in the Sun’s outer atmosphere that raise temperatures to more than 10 million degrees. These explosive events dramatically increase the amount of solar X-rays and radiation that reach Earth, posing risks to spacecraft, astronauts, and the planet’s upper atmosphere.
The study, published in Astrophysical Journal Letters, examined how solar flares heat plasma to temperatures beyond 10 million degrees. This plasma, composed of electrons and ions, behaves in ways that differ from long-standing assumptions. The researchers discovered that the ions, which are positively charged and account for half of the plasma, can reach temperatures above 60 million degrees.
Ions heated more strongly than electrons
Drawing on findings from other fields, the research team led by Dr. Alexander Russell, Senior Lecturer in Solar Theory in the School of Mathematics and Statistics, concluded that solar flares are very likely to heat ions much more intensely than electrons.
Dr Russell, said: “We were excited by recent discoveries that a process called magnetic reconnection heats ions 6.5 times as much as electrons. 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.”
Challenging old assumptions in solar physics
“Solar physics has historically assumed that ions and electrons must have the same temperature. However, redoing calculations with modern data, we found that ion and electron temperature differences can last for as long as tens of minutes in important parts of solar flares, opening the way to consider super-hot ions for the first time.”
“What’s more,” he added, is that 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.”
There has been a long-standing question since the 1970s about why flare spectral lines, bright enhancements in the solar radiation at specific “colors” in extreme-ultraviolet and X-ray light, are broader than expected. Historically, it was believed that this could only be due to turbulent motions, but that interpretation has come under pressure as scientists have tried to identify the nature of the turbulence. After nearly 50 years, the new work argues for a paradigm shift where the ion temperature can make a large contribution to explaining the enigmatic line widths of solar flare spectra.
Reference: “Solar Flare Ion Temperatures” by Alexander J. B. Russell, Vanessa Polito, Paola Testa, Bart De Pontieu and Sergey A. Belov, 3 September 2025, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/adf74a
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