Optical Signatures of the Outer Radiation Belt Boundary

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 (similar to 9-12 R-E) 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 similar to 0.4 mW/m(2), 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. Plain Language Summary We have found that a type of aurora marks the outer boundary of the Earth's radiation belts several minutes before intense auroral activity. The aurora is diffuse and at least 2000 km long in the east-west direction, and about 100 km wide in the north-south direction. It is faint, and has multiple narrow features on its northern edge. Fast electrons that are kicked out of their orbits due to the stretching of the Earth's magnetic field lines at night contribute to the aurora's visibility. This process removes electrons from the radiation belts down into the Earth's atmosphere. We speculate that the location of such aurora can be used to help us estimate the Earth's magnetic fields with better accuracy.