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 dE(e)/d lambda switches from positive to less than or equal 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/cm(2) s; boundary 4s, the transition of electron precipitation from unstructured on a greater than or equal to 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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