Evolution of B_z in the heliosphere as simulated in Run4. The co-latitudinal component in the spherical coordinate system is B_clt, which is equivalent to −B_z on the ecliptic plane. The red and blue spectra of the colour bar correspond to positive and negative B_z, respectively. Each of the panels (a) to (d) shows the view of the equatorial (X–Y) plane and the meridional (X–Z) plane at a particular time, mentioned at the top of each. (a) CME1 in the pre-interaction phase is identified schematically with a dashed ellipse; (b) CME2 is shown evolving behind CME1 in the early stage of interaction; (c) CME2 is seen compressing the interval of the trailing negative B_z component of CME1 to create CE1a (darker blue region) in the sheath ahead of itself; and (d) CE1a is seen to be further compressing the interval of the leading positive B_z component of CME1 to create CE1b (darker red region) during the interaction phase of the event. The animation of this figure can be found online.