Rate of change of the H-component of magnetic field at sites close to 53° magnetic latitude, during Interval 1, i.e. spanning 7–8 September 2017. Observatory sites used are longitudinally spaced around the globe, starting with Canada at the top and progressing eastwards to New Zealand at the bottom. Times of large dH/dt are indicated by red dashed lines, and identified by (a)–(c) in the upper most panel. Vertical dotted lines indicate a time window of ±10 min around each event. The times of local magnetic midnight (MN), dawn (M₀₆), dusk (M₁₈), and midday (MD) are shown on the panels.