Time series of variance in J_z (5-min values) (gray line) from MR on 24th and 25th October 2011, plotted alongside variance in z component of interplanetary magnetic field (IMF B_z) measured by the ACE spacecraft. B_z time data was corrected to account for propagation time from the location of the ACE spacecraft to Earth (using a distance of 1.5 × 10⁶ km (L1 point) and the observed solar wind speed).