MORPHOLOGY OF NIGHTSIDE PRECIPITATION

Considerable information on the state of the magnetosphere is embedded in the structure of nightside charged particle precipitation. To redu ce ambiguity and maximize the geophysically significant information ex tracted, a detailed scheme for quantitatively classifying nightside pr ecipitation is introduced. The proposed system, which includes operati onal definitions and which has been automated, consists of boundary 1, the ''zero-energy'' convection boundary (often the plasmapause); boun dary 2e, the point where the large-scale gradient dE(e)/d lambda switc hes from positive to less than or equal to 0 (the start of the main pl asma sheet); boundary 2i, the ion high-energy precipitation cutoff (th e ion isotropy boundary or the start of the tail current sheet); bound aries 3a,b, the most equatorward and poleward electron acceleration ev ents (spectra with ''monoenergetic peaks'') above 0.25 erg/cm(2) s; bo undary 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 flux es by a factor of at least 4 to levels below those typical of the auro ral oval; and boundary 6, the poleward edge of the subvisual drizzle o ften observed poleward of the auroral oval.