Considerable information on the state of the magnetosphere is embedded in the structure of nightside charged particle precipitation. To reduce ambiguity and maximize the geophysically significant information extracted, a detailed scheme for quantitatively classifying nightside precipitation is introduced. The proposed system, which includes operational definitions and which has been automated, consists of boundary 1, the “zero‐energy” convection boundary (often the plasmapause); boundary 2e, the point where the large‐scale gradient dEe/dλ switches from positive to ≤0 (the start of the main plasma sheet); boundary 2i, the ion high‐energy precipitation cutoff (the ion isotropy boundary or the start of the tail current sheet); boundaries 3a,b, the most equatorward and poleward electron acceleration events (spectra with “monoenergetic peaks”) above 0.25 erg/cm2 s; boundary 4s, the transition of electron precipitation from unstructured on a ≥10‐km spatial scale (spectra have 0.6–0.95 correlation coefficients with neighbors) to structured (correlation coefficient usually 0.4 and below); boundary 5, the poleward edge of the main auroral oval, marked by a spatially sharp drop in energy fluxes by a factor of at least 4 to levels below those typical of the auroral oval; and boundary 6, the poleward edge of the subvisual drizzle often observed poleward of the auroral oval.

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