Solar flares over 6 times hotter than previously thought

New research from the University of St Andrews has proposed that particles in solar flares are 6.5 times hotter than previously thought and provided an unexpected solution to a 50-year-old mystery about our nearest star. 

Solar flares are sudden and huge releases of energy in the Sun’s outer atmosphere that heat parts of it to greater than 10 million degrees. These dramatic events greatly increase the  solar X-rays and radiation reaching Earth and are hazardous to spacecraft and astronauts, as well as affecting our planet’s upper atmosphere. 

The research, published today in Astrophysical Journal Letters, looked at evidence of how flares heat solar plasma to greater than 10 million degrees. This solar plasma is made up of ions and electrons. The new research argues that solar flare ions, positively charged particles that make up half of the plasma, can reach over 60 million degrees. 

Looking at data from other research areas, the team, led by Dr Alexander Russell, Senior Lecturer in Solar Theory from the School of Mathematics and Statistics, realised that solar flares are very likely to heat the ions more strongly than the 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.” 

 “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 “colours” 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.