'Focused' solar explosions get hotter

Using data from the Hinode and RHESSI solar observatories, astronomers have discovered that solar flares - explosions in the atmosphere of the sun - get much hotter when they stay 'focused.'

Dr Ryan Milligan of Oak Ridge Association of Universities, Tennessee, who is stationed at NASA's Goddard Space Flight Centre in the US, presented his result on Wednesday 2 April at RAS National Astronomy Meeting in Belfast.

Solar flares are caused by the sudden release of magnetic energy. The largest can release as much energy as a billion one-megaton nuclear bombs. However, the flare observed in this study was a much more common 'micro' flare. Researchers at space agencies like NASA and ESA want to understand flares because they generate radiation that can be hazardous to unprotected astronauts, like those walking on the surface of the Moon.

'A flare typically divides its energy between directly heating the solar atmosphere and accelerating particles,' said Dr Milligan. 'These unusual flares seem to focus on one task, devoting all their energy to heating. This allows them to become millions of degrees hotter than their multi-tasking cousins.'

Flares normally occur above loops of electrically conducting gas, called plasma, in the Sun's atmosphere. When a typical flare goes off, it heats the plasma and sends beams of electrons racing down the sides of the loops. The electron beams evaporate more plasma from the Sun's visible surface, which expands back up the loops.

'This evaporated plasma has traditionally been believed to be the source of the hottest temperatures seen in solar flares,' said Dr Milligan. 'However, the flare in this new observation reached a temperature of 15 million degrees Celsius - some five million degrees hotter than expected for a flare of this size - without any evidence for beams of accelerated electrons.'

Dr Milligan used the RHESSI and Hinode spacecraft to make his observation of the microflare on 7 June 2007. RHESSI revealed that the flare had a peak temperature of 15 million degrees, and also that the flare showed no evidence for high energy electrons. Hinode was able to show the effects of the energy released at various layers in the solar atmosphere. In particular, the Extreme ultraviolet Imaging Spectrometer (EIS) instrument was used to detect signatures of plasma evaporation from the Sun's surface. The low velocities observed confirmed the RHESSI observation that high energy electrons were not present.

'If our assumption is correct, then this result tells us that the energy released during a solar flare is more efficient at achieving a higher temperature if the energy is used to directly heat the plasma in the Sun's atmosphere, instead of being divided between heating and particle acceleration. This very effect has recently been shown in simulations of energy release during microflares,' said Dr Milligan.