Evolution with time of the flare index, of the total flare index ratio, and free energy ratio. Panel a: variation of the flare index during January 2010 and December 2014: total flare index (solid black line), the contribution from the NH (dashed red line), and from the SH (dash-dotted blue line). Panel b: ratio of the total flare index to its maximum (solid black line) and the ratio of the free energy to its maximum (dash-dotted black line).