Figure 4.

Sketch illustrating the kinematics of boundaries in case of (a) pure ToF and (b) the ToF combined with the convection. The X coordinate is perpendicular to both the magnetic field and separatrix surface; it lies in the surface passed through the spacecraft position. Sep denotes the separatrix mapped to that surface, and E_i indicates the trajectory of low‐energy cutoff of specie i accelerated at separatrix; C indicates the convection trajectory, whereas SC indicates the spacecraft trajectory. Accordingly, V_S is the motion of the separatrix, V_C is the convection velocity of the plasma relative to the separatrix, and V_SC is the velocity of the spacecraft. Here T_i is the pure flight time calculated from equation (2.1), whereas apparent flight time t ₂ between separatrix crossing and particle cutoff crossings at the fast‐moving spacecraft given by equation (2.4) is much shorter (see section 2 for details).