Abstract Low‐altitude measurements of energetic electron fluxes offer insight into the dynamics of the radiation belts and plasma sheet. However, distinguishing between key magnetospheric regions–such as inner belt, slot region, outer belt, and plasma sheet–based on low‐altitude data remains challenging, particularly for missions lacking pitch‐angle resolution. A commonly used boundary indicator, the flux anisotropy (i.e., precipitating‐to‐trapped flux ratio), is unavailable in such cases. In this study, we propose an alternative approach to identify the plasma sheet—radiation belt interface, commonly associated with the isotropy boundary, using high energy‐resolution measurements from the Colorado Inner Radiation Belt Experiment (CIRBE) CubeSat. We demonstrate that this boundary is marked by a distinct spectral change: a power‐law form in the plasma sheet transitions to an exponential form within the outer radiation belt. These findings are supported by comparisons with Polar‐orbiting Operational Environmental Satellites (POES) observations, and we discuss the potential mechanisms responsible for the spectral transition.

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