Abstract We report a rare quiet‐time observation by NOAA‐18 spacecraft flying nearly along the electron isotropy boundary (IBe). Complemented by data from four nearby meridional IBe crossings of other POES spacecraft, this case study provides a novel quantitative information concerning the regular energy dependence of the loss‐cone filling ratio (Jprec/Jtrap) variations near the IBe foot. Specific strong energy‐dependent difference of loss‐cone filling rates is manifested at different spacecraft and in various events in the same way, which is consistent with the pitch‐angle scattering in the magnetotail field reversal region as confirmed by the numerical simulation. Therefore it provides a landmark to distinguish the field line curvature scattering (FLCS) mechanism from wave‐induced precipitation. In FLCS case, a strongly fluctuating loss cone filling ratio implies that equatorial magnetic field is inherently structured in the magnetotail in the IBe foot region.