Phys. Rev. Lett. (in the press)

DON PETTIT/ISS EXPEDITION 6/NASA

Streams of charged particles blown from the Sun's corona — known as the solar wind — can produce extreme space weather, such as geomagnetic storms and aurorae. The solar wind is not uniform: there are different velocity flows with speeds of several hundreds of kilometres per second and large temperature variations. Intriguingly, heavy ions in the solar wind — helium in particular — tend to be much hotter than the dominant component of ionized hydrogen. Justin Kasper and colleagues explain that a certain type of plasma wave is the cause of this preferential heating.

Ions are heated through a resonant interaction with oscillations in the plasma called ion cyclotron waves. Kasper et al. suggest that ions heavier than hydrogen can also interact with counter-propagating waves and therefore experience stronger heating. These predictions agree well with the seventeen years' worth of observational data collected by NASA's Wind spacecraft, and explain why helium can be up to seven times hotter than hydrogen.

As the distribution of plasma waves near the corona is not yet fully understood, it is hard to pinpoint the exact location where ion heating occurs, but new clues are expected from observations made by spacecraft flying close to the Sun.