Correction to: Nature Astronomy https://doi.org/10.1038/s41550-019-0819-7, published online 8 July 2019.

In the version of this Letter originally published, in the legends of Fig. 2b and d, in the labels ‘Bϕ: 23.0 nT’ and ‘Bϕ: 12.1 nT’, respectively, subscript ‘ϕ’ was incorrect; the labels should have read ‘Bθ: 23.0 nT’ and ‘Bθ: 12.1 nT’. These have now been corrected.