Field-of-view particle counting schemata. Panel a shows the sphere method, panel b shows the omnidirectional method, and panel c shows the FOV method. The purple line shows an arbitrary spacecraft trajectory, r_sat is the base circle to count incident particles, γ is the cone opening angle, and d_max is the maximum counting distance from the base circle. The dots show example particles with their velocity vectors. While the green particles were counted, the red ones were neglected, as (1) they are located outside the cone or their distance to the base circle is larger than d_max, (2) they are inside the cone but do not intersect the base circle, or (3) they are inside the cone pointing toward the base circle, but their energy and distance do not correspond to the criterion mentioned in the text. The coordinates are given in MASO (explained in the text) shifted to the center of each trajectory point.