Radial evolution profile of the CME1 and CME2 along the Sun–Earth line at different times of propagation and interaction extracted from Run4. Top to bottom (in each plot): speed (v), proton number density (n_p), z-component of magnetic field (B_z), total magnetic field (B), and proton plasma beta (β_p). The yellow and blue shaded areas depict the extent of magnetic ejecta of CME1 and CME2, respectively, extracted using the criterion β_p < 1 (β_p < 5 in (a)). All physical quantities except the speed are scaled by (D/1 au)² where D is the radial distance from the Sun. The CME shocks (S1 and S2) are marked in yellow lines. The colourful vertical lines in the B_z panel correspond to different radial distances, which help to explain the various phases of Run4 (detailed in Sect. 6.3): (a) Propagation of CME1 alone in the heliosphere; (b) formation of a compressed negative B_z ejecta CE1a (shown with the magenta arrow); (c) further compression of CE1a; (d) development of a compressed positive B_z ejecta, CE1b (shown with a green arrow), ahead of CE1a; (e) further compression of CE1b; and (f) diffusion of CE1a (∼0 nT) upon reaching 1.2 au while CE1b undergoes further enhancement.