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Non-adiabatic motion of plasma sheet protons causespitch-angle scattering and isotropic precipitation to theionosphere, which forms the proton auroral oval. This mechanismrelated to current sheet scattering (CSS) provides a specificenergy–latitude dispersion pattern near the equatorward boundaryof proton isotropic precipitation (isotropy boundary, IB), withprecipitation sharply decreasing at higher (lower) latitude forprotons with lower (higher) energy. However, this boundary maps tothe inner magnetosphere, where wave-induced scattering mayprovide different dispersion patterns as recently demonstrated byLiang et al. (2014). Motivated by the potential usage of the IBsfor the magnetotail monitoring as well as by the need to betterunderstand the mechanisms forming the proton IB, we investigatestatistically the details of particle flux patterns near theproton IB using NOAA-POES polar spacecraft observations madeduring September 2009. By comparing precipitated-to-trapped fluxratio (J0/J90) at >30 and >80 keV proton energies, wefound a relatively small number of simple CSS-type dispersion events(only 31 %). The clear reversed (wave-induced) dispersion patternswere very rare (5 %). The most frequent pattern had nearlycoinciding IBs at two energies (63 %). The structured precipitation with multiple IBs was very frequent (60 %), that is, with twoor more significant J0/J90 dropouts. The average latitudinalwidth of multiple IB structures was about 1°. Investigationof dozens of paired auroral zone crossings of POES satellites showedthat the IB pattern is stable on a timescale of less than 2 min (a fewproton bounce periods) but can evolve on a longer (severalminutes) scale, suggesting temporal changes in some mesoscalestructures in the equatorial magnetosphere. We discuss the possible role of CSS-related and wave-inducedmechanisms and their possible coupling to interpret the emergingcomplicated patterns of proton isotropy boundaries

Energy–latitude dispersion patterns near the isotropy boundaries of energetic protons