x component of the curl of electromotive force [∇ × (u × B)] _x (black solid line) together with its main contributions, advection (blue), compression (red) and shear (orange) plotted over height z in the location of high induced magnetic field (x = 75 Mm, y = 30 Mm, also indicated in Figs. 9a, c as vertical lines); we refer also to Eqs. (3)–(6) for the contributions. Additionally we over-plot the same component of induced magnetic field (dashed black). The horizontal black dotted line represents the zero value. Data from of Chen et al. (2014, 2015). We refer to Sect. 3.4.