Observations of an extreme space weather storm caused by two successive solar eruptions are reported in Nature Communications this week. The findings may help understand how such incidents are caused by combinations of events.

Coronal mass ejections are large-scale eruptions of plasma and magnetic field from the outer layer of the Sun. They are known to be a major influence on space weather, which can affect spacecraft and satellites around the Earth. Most coronal mass ejections have effects that are equivalent to the normal solar wind by the time they reach us, but how they evolve as they travel from the Sun is not clearly understood. Ying Liu and colleagues use multi-point sensing satellites to study the rare case of two consecutive eruptions on 23 July 2012. This situation resulted in an extreme storm with a high solar wind speed and magnetic field at around Earth radius. Their observations show how the two coronal mass ejections interact as they travel, producing a change in direction of propagation, an enhancement of their magnetic field, and only a modest slowing of their speed. The study suggests that current modelling of solar wind speed and magnetic field may be underestimated if such interactions are not properly accounted for.