Correction to: Nature Astronomy https://doi.org/10.1038/s41550-018-0442-z, published online 6 June 2018.

In the version of this Letter originally published, in the second sentence of the last paragraph before the Methods section the word ‘altitudes’ was mistakenly used in place of the word ‘latitudes’. The sentence has now been corrected accordingly to: ‘Low-dispersion class 1 events indicate that low-density ionospheric regions predominantly occur in the northern hemisphere at latitudes between 20° and 70°.’