(a) One example of Swarm A and B ground tracks during the period of counter-rotation in geographic coordinates. Arrows indicate the flying direction of the two satellites. (b) Relation between three selected dipole field lines (blue solid lines) and the orbits of Swarm A and B (black dashed lines). The third black line, marked by d, is a fluxtube segment bounded by the two orbits. The grey-shading segment within the highest fluxtube represents the depleted structures of EPBs with a field-aligned scale length of L, which is encountered by Swarm B but not by the lower-flying Swarm A.