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Abstract We present observations that show structured diffuse aurora (SDA) correlated with electron precipitation directly from the outer boundary of the outer radiation belt. The SDA maps to the nightside transition region (∼9–12 RE) in the magnetic‐equatorial plane during a substorm growth phase. The energy flux of 100‐ to 300‐keV electrons lost from the outer boundary of the radiation belt is ∼0.4 mW/m2, which is comparable to electron dropouts >100 keV during magnetic storms. The latitudinal dispersion of energetic electrons observed in the ionosphere with energetic electrons more equatorward suggests nonadiabatic scattering from a thinning current sheet. The GLobal airglOW (GLOW) model shows significant optical contributions (up to 46%) from electrons >30 keV within the SDA. Ground‐ and space‐based measurements are consistent with the conclusion that the SDA marks the outer radiation belt boundary during substorm growth phase.