(a) Schematic of a cylindrical MC (solid red curve), the ecliptic plane (grey), and the position of the Sun S and the spacecraft E at L1. The AW, η, Sun-centre O_c and the CME source location O_s on the solar disk are labelled in the figure. The FR axis is shown as the dash-dotted red line. The Sun-Earth line O_cE is shown by the dashed black line and is on the ecliptic plane. The lines O_sX and PQ indicate the heliocentric distance r and L_MC, respectively. The perpendicular distance p is denoted by the violet line BX. The line BX is perpendicular to O_cE. (b) The expanding circular cross-section of a cylindrical MC. X is the centre of the cross-section. At t = 0, the MC cross-sectional circumference is denoted by the red circle. The radial expansion of the MC with speed V_ρ is in the direction of the yellow arrows, and in the FR frame of reference, the spacecraft propagates with a speed V_pro towards the path indicated by black arrows.